Course motor 1-an introduction to electrical motors basics

2012

Ali Hassan

Certified Energy Manager – AEE ‐USA

Course Motor-1: An Introduction to Electrical Motors Basics

Copy Rights to: alihassanelashmawy.blogspot.com

of 127

About Author

Hi, I'm Ali Hassan el‐Ashmawy, I began my career from 1999 as a

site electrical engineer then as area manager from 2001 then as

electrical designer from 2003 then as senior electrical designer

from 2006 and up to date.

In my past experience, I designed and construct about 100 projects in different

countries like Egypt, Kuwait, Indonesia, KSA, Gabon and Iraq.

My designs were approved by many international authorities like USA corps of

engineers and USA ministry of exterior – OBO Office.

I'm certified energy manger CEM from AEE – USA since 2006 and I hope to become a

well‐known designer in the field of electrical design.

To contact me please email to [email protected]


of 127

Course Description:

This course is intended to prepare the target persons with the ability to understand

and recognize different types, components, theory of operation and applications of

All Electrical Motors.

The target Persons:

Design engineers, new graduate engineers, under graduate engineering students,

site field engineers, maintenance engineers and technicians.

Skills Development:

On completion of this course the target person will be able to:

1. Understand the technology, concepts and terminology of Electrical Motors.

2. Recognize different types, theory of operation, components and accessories

of Electrical Motors.

3. Specify correctly any type of Electrical Motors for certain applications.


of 127

Table of Contents

S/N Item

Page No.

1 Introduction 6 2 Principle of How Motors Work 6 3 Motor Basic Parts 7 3.1 AC Motor Basic Parts 7 3.2 DC Motor Basic Parts 21 4 Types of Motor 25 4.1 DC Motors 26 A 1. A Brush DC motors 28 A.1 Permanent Magnet 34 A.2 Shunt‐Wound 35 A.3 Series‐Wound 36 A.4 Compound‐Wound 37 A.5 Separately excited DC motor 38 A.6 Universal Motor 39 A.7 Servo Motors 40 B 1. B Brushless DC motors 41 4.2 Ac Motors 48 4.2.1 Induction motor 48 A Single Phase, Squirrel Cage, Induction Motor 55 A.1 Shaded‐Pole Induction Motors 55 A.2 Split‐Phase AC Induction Motor 56 a Capacitor‐Start 58

b Permanent Split Capacitor (Capacitor Run) AC Induction Motor

59

c Capacitor Start/Capacitor Run AC Induction Motor 60 d Resistance‐Start 61 A.3 Universal motor 63 B Three Phase, Squirrel Cage, Induction Motor 64 C Single Phase, Wound Rotor, Induction Motor 65 C.1 Repulsion motor 65 a Repulsion‐start Induction‐run motor. 68 b Repulsion Induction motor 68 D Three Phase, Wound Rotor, Induction Motor 69

4.2.2 Synchronous motor 71 A Non‐excited motors. 77 A.1 Reluctance motors. 77 A.2 Hysteresis motors. 80 A.3 Permanent magnet motors. 82 B DC‐excited motors. 83 B.1 Brush type Synchronous motors 84 B.2 Brushless type Synchronous motors 84


of 127

S/N Item

Page No.

C Stepper motor 86 C.1 Variable‐Reluctance Step Motors 88 C.2 Permanent‐Magnet‐Rotor Step Motors 89 C.3 Hybrid Permanent‐Magnet Step Motors 90 4.2.3 Linear Motors 91 A Linear induction motor (LIM) 92 B Linear synchronous motor (LSM) 95 5 Motors Selection Procedures 96 5.1 Ac Motors Selection Procedures 96 A The power supply 96 B System requirements 101 C Motor class 106 D Motor insulation type 110 E Motor Duty Cycle Applications 112 F Bearing type 114 G Method of mounting the motor 115 H The cost and size of the motor 119 I Method of speed control 119 J Environmental conditions 121 5.2 DC Motor Selection Procedures. 125


of 127

1‐ Introduction

Electric motors defined as electromechanical devices that convert electrical energy to mechanical energy; they are the interface between the electrical and mechanical systems of a facility.

Electric motors are an important part of any electrical system. They used throughout every manufacturing plant, office, and home consuming about 64% of all electricity generated.

There are numerous ways to design a motor, thus there are many different types of motors and each type possess different operating characteristics (that will be listed later). Based on these characteristics the motor can be chosen for a specified application.

2‐ Principle of How Motors Work:

Principle of How Motors Work

1. Electrical current flowing in a loop of wire will produce a magnetic field across the loop.

2. When this loop is surrounded by the field of another magnet, the loop will turn, producing a force (called torque) that results in mechanical motion


of 127

3‐ Motor basic parts: Electric machines are classified into two categories D.C. and A.C. motors, the basic parts for each type will be different for each type as follows: 3.1‐ AC Motor Basic Parts:

AC Motor Basic Parts

The basic parts for AC motors are as follows:

1. Enclosure. 2. Stator. 3. Rotor. 4. Bearings. 5. Conduit Box. 6. Eye Bolt.


of 127

1‐ Enclosure

Enclosure

The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). A motor's enclosure not only holds the motor's components together, it also protects the internal components from moisture and containments. The degree of protection depends on the enclosure type. In addition, the type of enclosure affects the motor's cooling. There are two categories of enclosures as follows: • Open Enclosure.

• Totally enclosed Enclosure.


of 127

Open and Enclosed Types

A‐ Open Enclosure

open drip proof (ODP) enclosure

Open enclosures permit cooling air to flow through the motor. One type of open enclosure is the open drip proof (ODP) enclosure. This enclosure has vents that allow for air flow. Fan blades attached to the rotor move air through the motor when the rotor is turning. The vents are positioned so that liquids and solids falling from above


of 127

at angles up to 15° from vertical cannot enter the interior of the motor when the motor is mounted on a horizontal surface. When the motor is mounted on a vertical surface, such as a wall or panel, a special cover may be needed. ODP enclosures should be used in environments free from contaminates. B‐ Totally enclosed Enclosure This category will include the following three types:

1. Totally Enclosed Non‐Ventilated Enclosure. 2. Totally Enclosed Fan‐Cooled Enclosure. 3. Explosion‐Proof Enclosure.

a‐ Totally Enclosed Non‐Ventilated Enclosure (TENV)

Totally Enclosed Non‐Ventilated Enclosure (TENV)

In some applications, the air surrounding the motor contains corrosive or harmful elements which can damage the internal parts of a motor. A totally enclosed non‐ventilated (TENV) motor enclosure limits the flow of air into the motor, but is not


of 127

airtight. However, a seal at the point where the shaft passes through the housing prevents water, dust, and other foreign matter from entering the motor along the shaft. Most TENV motors are fractional horsepower. However, integral horsepower TENV motors are used for special applications. The absence of ventilating openings means that all the heat from inside the motor must dissipate through the enclosure by conduction. These larger horsepower TENV motors have an enclosure that is heavily ribbed to help dissipate heat more quickly. TENV motors can be used indoors or outdoors.

b‐ Totally Enclosed Fan‐Cooled Enclosure (TEFC)

Totally Enclosed Fan‐Cooled Enclosure (TEFC)

A totally enclosed fan‐cooled (TEFC) motor is similar to a TENV motor, but has an external fan mounted opposite the drive end of the motor. The fan blows air over the motor's exterior for additional cooling. The fan is covered by a shroud to prevent anyone from touching it. TEFC motors can be used in dirty, moist, or mildly corrosive environments.


of 127

c‐ Explosion‐Proof Enclosure (XP)

Explosion‐Proof Enclosure (XP)

• Hazardous duty applications are commonly found in chemical processing, mining, foundry, pulp and paper, waste management, and petrochemical industries. In these applications, motors have to comply with the strictest safety standards for the protection of life, machines and the environment. This often requires use of explosion proof (XP) motors.

• An XP motor is similar in appearance to a TEFC motor, however, most XP enclosures are cast iron.


of 127

• Division I locations normally have hazardous materials present in the atmosphere.

• Division II locations may have hazardous material present in the atmosphere under abnormal conditions.

• Locations defined as hazardous, are further defined by the class and group of hazard. For example,

‐ Class I, Groups A through D have gases or vapors present. ‐ Class II, Groups E, F, and G have flammable dust, such as coke or grain dust. ‐ Class III is not divided into groups. This class involves ignitable fibers and lint. 2‐ Stator

Stator


of 127

The motor stator consists of two main parts: A‐ Stator Core The stator is the stationary part of the motor's electromagnetic circuit. The stator is electrical circuit that performs as electromagnet. The stator core is made up of many thin metal sheets, called laminations. Laminations are used to reduce energy losses that would result if a solid core were used. B‐ Stator (Windings) Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core. When the assembled motor is in operation, the stator windings are connected directly to the power source. Each grouping of coils, together with the steel core it surrounds, becomes an electromagnet when current is applied. Electromagnetism is the basic principle behind motor operation. 3‐ Rotor

Rotor

The rotor is the rotating part of the motor's electromagnetic circuit. Magnetic field from the stator induces an opposing magnetic field onto the rotor causing the rotor to “push” away from the stator field. There are a lot of rotor types like Squirrel cage rotor and wound rotor, they will be explained later.


of 127

4‐ Bearings

Bearings, mounted on the shaft, support the rotor and allow it to turn. Not all bearings are suitable for every application; a universal, all‐purpose bearing does not exist. The choice of bearing arrangement is based on the following qualities:

• Load carrying capacity in the axial and radial direction. • Overspeed and duration. • Rotating speed. • Bearing life.

The size of the bearing to be used is initially selected on the basis of its load carrying capacity, in relation to the load to be carried, and the requirements regarding its life and reliability. Other factors must also be taken into consideration, such as operating temperature,


of 127

dirty and dusty environmental conditions, and vibration and shocks affecting bearings in running and resting conditions.

Bearings Types: There are many types of bearings on the market, each with different characteristics and different uses, these types are as follows: A‐ Deep groove ball bearings

Deep groove ball bearings are the most common type of bearing, and can handle both radial and thrust loads. Due to their low‐frictional torque, they are suitable for high speeds. In a ball bearing, the load is transmitted from the outer race to the ball and from the ball to the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it to spin very smoothly. This also means that there is not very much contact area holding the load, so if the bearing is overloaded, the balls can deform, ruining the bearing.


of 127

B‐ Cylindrical roller bearings

These roller bearings are used in applications where they must hold heavy radial loads. In the roller bearing, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load out over a larger area, allowing the bearing to handle much greater radial loads than a ball bearing. However, this type of bearing is not designed to handle much thrust loading. C‐ Angular contact ball bearings

Angular Contact ball bearings have raceways in the inner and outer rings which are displaced with respect to each other in the direction of the bearing axis. This means that they are suitable for the accommodation of combined loads such as simultaneously acting radial and axial loads in vertical machines.


of 127

D‐ Spherical roller thrust bearing

In Spherical Roller thrust bearings, the load is transmitted from one raceway to the other at an angle to the bearing axis. They are suitable for the accommodation of high axial loads in addition to simultaneously acting small radial loads. Spherical roller thrust bearings are also self‐aligning. E‐ Sleeve Bearings

Sleeve bearings have no moving parts, they rely on a thin film of oil to reduce friction and allow the motor shaft to turn freely. This film of oil is critical to the life of a sleeve bearing. When properly lubricated, there is actually no physical contact between the bearing and the shaft. If for some reason the oil film breaks down, metal‐to‐metal contact


of 127

between the shaft and the bearing will cause the bearing to wear very quickly and soon fail Sleeve bearings are often chosen because of their relatively quiet operation and lower cost compared to ball bearings. Sleeve bearings can be divided to: a‐ Flange mounted sleeve bearings They are used for machines with a shaft height of up to 1120mm. Machines with bearings of this type are quick and easy to align. The air gap between stator and rotor comes from the factory already adjusted, and does not need any further adjustment on site during installation. b‐ Foot mounted sleeve bearings They are mounted on a pedestal. The pedestal can either be integrated in the stator frame, or can be mounted separately. If it is integrated with the stator frame it is easy and fast to align. 5‐ Conduit Box

Conduit Box

Point of connection of electrical power to the motor’s stator windings.


of 127

6‐ Eye Bolt

Eye Bolt

Used to lift heavy motors with a hoist or crane to prevent motor damage.


of 127

2‐ DC Motor Basic Parts:

DC Motor Basic Parts

The basic parts for DC motors are as follows: 1‐ Stator The stator carries the field winding and Poles. The stator together with the rotor constitutes the magnetic circuit or core of the machine. It is a hollow cylinder. 2‐ Rotor It carries the armature winding. The armature is the load carrying member. The rotor is cylindrical in shape.


of 127

3‐ Armature Winding This winding rotates in the magnetic field set up at the stationary winding (Field winding). It is the load carrying member mounted on the rotor. An armature winding is a continuous winding; that is, it has no beginning or end. It is composed of a number of coils in series. 4‐ Field Winding This is an exciting system which may be an electrical winding or a permanent magnet and which is located on the stator. Note: DC Motors are generally classified by how their Armature & Field windings are connected to their DC power supply. 5‐ Commutator

Commutator Winding

The coils on the armature are terminated and interconnected through the commutator which comprised of a number of bars or commutator segments which are insulated from each other. The commutator rotates with the rotor and serves to rectify the induced voltage and the current in the armature both of which are A.C.


of 127

6‐ Brushes

Brushes

These are conducting carbon graphite spring loaded to ride on the commutator and act as interface between the external circuit and the armature winding. 7‐ Poles

Poles

The field winding is placed in poles, the number of which is determined by the voltage and current ratings of the machine.


of 127

8‐ Slot/Teeth For mechanical support, protection from abrasion, and further electrical insulation, non‐conducting slot liners are often wedged between the coils and the slot walls. The magnetic material between the slots is called teeth. 9‐ Motor Housing The motor housing supports the iron core, the brushes and the bearings.


of 127

Classification of Electric Motors Main Types of Motor Electric motors are broadly classified into two categories as follows:

1. AC Motors. 2. DC Motors.

Within those two main categories there are subdivisions as shown in the below image.

Motor Types

Notes: Recently, with the development of economical and reliable power electronic components, there are numerous ways to design a motor and the classifications of these motors have become less rigorous and many other types of motor have appeared. Our classification of motors will be comprehensive as can as possible.


of 127

First: DC motors

DC motors

DC power systems are not very common in the contemporary engineering practice. However, DC motors have been used in industrial applications for years Coupled with a DC drive, DC motors provide very precise control DC motors can be used with conveyors, elevators, extruders, marine applications, material handling, paper, plastics, rubber, steel, and textile applications, automobile, aircraft, and portable electronics, in speed control applications.

Advantages of DC motors:

1. It is easy to control their speed in a wide range; their torque‐speed characteristic has, historically, been easier to tailor than that of all AC motor categories. This is why most traction and servo motors have been DC machines. For example, motors for driving rail vehicles were, until recently, exclusively DC machines.

2. Their reduced overall dimensions permit a considerable space saving which let the manufacturer of the machines or of plants not to be conditioned by the exaggerated dimensions of circular motors.

Disadvantages of DC motors

1. Since they need brushes to connect the rotor winding. Brush wear occurs, and it increases dramatically in low‐pressure environment. So they cannot be used in artificial hearts. If used on aircraft, the brushes would need replacement after one hour of operation.


of 127

2. Sparks from the brushes may cause explosion if the environment contains explosive materials.

3. RF noise from the brushes may interfere with nearby TV sets, or electronic devices, Etc.

4. DC motors are also expensive relative to AC motors.

Thus all application of DC motors have employed a mechanical switch or commutator to turn the terminal current, which is constant or DC, into alternating current in the armature of the machine. Therefore, DC machines are also called commutating machines. Types of DC motors:

Types of DC motors


of 127

The DC motors are divided mainly to:

1. Brush DC motors (BDC). 2. Brushless DC motors (BLDC).

1. A Brush DC motors

Brush DC motors

A brushed DC motor (BDC) is an internally commutated electric motor designed to be run from a direct current power source. Applications: Brushed DC motors are widely used in applications ranging from toys to push‐button adjustable car seats. Advantages: Brushed DC (BDC) motors are inexpensive, easy to drive, and are readily available in all sizes and shapes


of 127

Construction:

Brushed DC motor Construction

All BDC motors are made of the same basic components: a stator, rotor, brushes and a commutator. 1‐ Stator The stator generates a stationary magnetic field that surrounds the rotor. This field is generated by either permanent magnets or electromagnetic windings.


of 127

2‐ Rotor

Rotor (Armature)

The rotor, also called the armature, is made up of one or more windings. When these windings are energized they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the opposite poles generated by the stator, causing the rotor to turn. As the motor turns, the windings are constantly being energized in a different sequence so that the magnetic poles generated by the rotor do not overrun the poles generated in the stator. This switching of the field in the rotor windings is called commutation.


of 127

3‐ Brushes and Commutator

Commutator Example

Segments and Brushes


of 127

Unlike other electric motor types (i.e., brushless DC, AC induction), BDC motors do not require a controller to switch current in the motor windings. Instead, the commutation of the windings of a BDC motor is done mechanically. A segmented copper sleeve, called a commutator, resides on the axle of a BDC motor. As the motor turns, carbon brushes (ride on the side of the commutator to provide supply voltage to the motor) slide over the commutator, coming in contact with different segments of the commutator. The segments are attached to different rotor windings, therefore, a dynamic magnetic field is generated inside the motor when a voltage is applied across the brushes of the motor. It is important to note that the brushes and commutator are the parts of a BDC motor that are most prone to wear because they are sliding past each other.

How the Commutator Works:

How the Commutator Works

As the rotor turns, the commutator terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.


of 127

Types of BDC motors:

Types of DC motors

The different types of BDC motors are distinguished by the construction of the stator or the way the electromagnetic windings are connected to the power source. These types are:

1. Permanent Magnet. 2. Shunt‐Wound.


of 127

3. Series‐Wound. 4. Compound‐Wound. 5. Separately excited DC motor. 6. Universal Motor. 7. Servo Motors.

A‐ Permanent Magnet

Permanent Magnet Motor

A permanent magnet DC (PMDC) motor is a motor whose poles are made out of permanent magnets to produce the stator field.


of 127

Advantages:

1. Since no external field circuit is needed, there are no field circuit copper losses.

2. Since no field windings are needed, these motors can be considerable smaller.

3. Widely used in low power application. 4. Field winding is replaced by a permanent magnet (simple construction and

less space). 5. No requirement on external excitation.

Disadvantages:

1. Since permanent magnets produces weaker flux densities then externally supported shunt fields, such motors have lower induced torque.

2. There is always a risk of demagnetization from extensive heating or from armature reaction effects (Some PMDC motors have windings built into them to prevent this from happening).

B‐ Shunt‐Wound

Shunt‐Wound Motor

Shunt‐wound Brushed DC (SHWDC) motors have the field coil in parallel (shunt) with the armature. The speed is practically constant independent of the load and therefore suitable for commercial applications with a low starting load, such as centrifugal pump, machine tools, blowers fans, reciprocating pumps, etc.


of 127

Advantages:

1. The current in the field coil and the armature are independent of one another. as a result, these motors have excellent speed control.

2. Loss of magnetism is not an issue in SHWDC motors so they are generally more robust than PMDC motors.

3. Speed can be controlled by either inserting a resistance in series with the armature (decreasing speed) or by inserting resistance in the field current (increasing speed).

Disadvantages:

1. Shunt‐wound Brushed DC (SHWDC) motors have drawbacks in reversing applications, however, because winding direction relative to the shunt winding must be reversed when armature voltage is reversed. Here, reversing contactors must be used.

C‐ Series‐Wound

Series‐Wound Motor

Series‐wound Brushed DC (SWDC) motors have the field coil in series with the armature. These motors are ideally suited for high‐torque applications such as traction vehicles (cranes and hoists, electric trains, conveyors, elevators, electric cars) because the current in both the stator and armature increases under load. Advantages:

1. The torque is proportional to I2 so it gives the highest torque per current ratio over all other dc motors.


of 127

Disadvantages:

1. A drawback to SWDC motors is that they do not have precise speed control like PMDC and SHWDC motors have.

2. Speed is restricted to 5000 RPM. 3. It must be avoided to run a series motor with no load because the motor will

accelerate uncontrollably.

D‐ Compound‐Wound

Compound‐Wound Motor

Compound Wound (CWDC) motors are a combination of shunt‐wound and series‐wound motors. CWDC motors employ both a series and a shunt field. The performance of a CWDC motor is a combination of SWDC and SHWDC motors. CWDC motors have higher torque than a SHWDC motor while offering better speed control than SWDC motor. It is used in Applications such as Rolling mills, sudden temporary loads, heavy machine tools, punches, etc. Advantages:

1. This motor has a good starting torque and a stable speed.

Disadvantages:

1. The no‐load speed is controllable unlike in series motors.


of 127

E‐ Separately excited DC motor

Separately excited DC motor

In a separately excited DC motor the field coils are supplied from an independent source, such as a motor‐generator and the field current is unaffected by changes in the armature current. The separately excited DC motor was sometimes used in DC traction motors to facilitate control of wheel slip.


of 127

F‐ Universal Motor

Universal Motor

The universal motor is a rotating electrical machine similar to DC series motor, designed to operate either from AD or DC source. The stator & rotor windings of the motor are connected in series through the rotor commutator. The series motor is designed to move large loads with high torque in applications such as crane motor or lift hoist.


of 127

G‐ Servo Motors

Servo Motors

Servo Motors are mechanical devices that can be instructed to move the output shaft attached to a servo wheel or arm to a specified position. Servo Motors are designed for applications involving position control, velocity control and torque control.

Servo Motors Components


of 127

A servo motor mainly consists of a DC motor, gear system, a position sensor which is mostly a potentiometer, and control electronics.

Servo Motors Applications

2‐ Brushless DC motors

Brushless DC motors

In brushes DC motors, the mechanical commutator and associated brushes are problematical for a number of reasons as follows:

1. Brush wear occurs, and it increases dramatically in low‐pressure environment.

2. Sparks from the brushes may cause explosion if the environment contains explosive materials.

3. RF noise from the brushes may interfere with nearby TV sets, or electronic devices, etc.


of 127

Brushless Direct Current (BLDC) motors are one of the motor types rapidly gaining popularity. BLDC motors are used in industries such as Appliances, Automotive, Aerospace, Consumer, Medical, Industrial Automation Equipment and Instrumentation. As the name implies, BLDC motors do not use brushes for commutation; instead, they are electronically commutated. BLDC motors have many advantages over brushed DC motors and induction motors, a few of these are:

1. Better speed versus torque characteristics. 2. High dynamic response. 3. High efficiency. 4. Long operating life. 5. Noiseless operation. 6. Higher speed ranges.

In addition, the ratio of torque delivered to the size of the motor is higher, making it useful in applications where space and weight are critical factors. Construction BLDC motors are a type of synchronous motor. This means the magnetic field generated by the stator and the magnetic field generated by the rotor rotates at the same frequency. BLDC motors come in single‐phase, 2‐phase and 3‐phase configurations. Corresponding to its type, the stator has the same number of windings. Out of these, 3‐phase motors are the most popular and widely used.

1‐ Stator

Stator of a BLDC Motor


of 127

The stator of a BLDC motor consists of stacked steel laminations with windings placed in the slots that are axially cut along the inner periphery. Most BLDC motors have three stator windings connected in star fashion. Each of these windings is constructed with numerous coils interconnected to form a winding. One or more coils are placed in the slots and they are interconnected to make a winding. Each of these windings is distributed over the stator periphery to form an even numbers of poles. Depending upon the control power supply capability, the motor with the correct voltage rating of the stator can be chosen. Forty‐eight volts, or less voltage rated motors are used in automotive, robotics, small arm movements and so on. Motors with 100 volts, or higher ratings, are used in appliances, automation and in industrial applications. 2‐ Rotor

Rotor of a BLDC Motor

The rotor is made of permanent magnet and can vary from two to eight pole pairs with alternate North (N) and South (S) poles.


of 127

BLDC Rotor Magnet Positions

Based on the required magnetic field density in the rotor, the proper magnetic material is chosen to make the rotor. Ferrite magnets are traditionally used to make permanent magnets.

3‐ Hall Sensors

BLDC Hall Sensors

• Unlike a brushed DC motor, the commutation of a BLDC motor is controlled electronically. To rotate the BLDC motor, the stator windings should be energized in a sequence. It is important to know the rotor position in order to understand which winding will be energized following the energizing sequence. Rotor position is sensed using Hall Effect sensors embedded into the stator.

• Most BLDC motors have three Hall sensors embedded into the stator on the non‐driving end of the motor.


of 127

• Whenever the rotor magnetic poles pass near the Hall sensors, they give a high or low signal, indicating the N or S pole is passing near the sensors. Based on the combination of these three Hall sensor signals, the exact sequence of commutation can be determined.

• Based on the physical position of the Hall sensors, there are two versions of output. The Hall sensors may be at 60° or 120° phase shift to each other. Based on this, the motor manufacturer defines the commutation sequence, which should be followed when controlling the motor.

• Note: The Hall sensors require a power supply. The voltage may range from 4 volts to 24 volts. Required current can range from 5 to 15 mAmps.

Theory of Operation

• Each commutation sequence has one of the windings energized to positive power (current enters into the winding), the second winding is negative (current exits the winding) and the third is in a non‐energized condition.

• Torque is produced because of the interaction between the magnetic field generated by the stator coils and the permanent magnets of the rotor.

• In order to keep the motor running, the magnetic field produced by the windings should shift position, as the rotor moves to catch up with the stator field. What is known as “Six‐Step Commutation” defines the sequence of energizing the windings.

• In six‐step commutation, only two out of the three Brushless DC Motor windings are used at a time. Steps are equivalent to 60 electrical degrees, so six steps make a full, 360 degree rotation. One full 360 degree loop is able to control the current, due to the fact that there is only one current path. Six‐step commutation is typically useful in applications requiring high speed and commutation frequencies. A six‐step Brushless DC Motor usually has lower torque efficiency than a sine‐wave commutated motor.

Typical BLDC Motor Applications We can categorize the type of BLDC motor control into three major types:

1. Constant load. 2. Varying loads. 3. Positioning applications.

1‐ Applications with Constant Loads:

These are the types of applications where a variable speed is more important than keeping the accuracy of the speed at a set speed. In addition, the acceleration and


of 127

deceleration rates are not dynamically changing. In these types of applications, the load is directly coupled to the motor shaft. For example, fans, pumps and blowers come under these types of applications. These applications demand low‐cost controllers, mostly operating in open‐loop. 2‐ Applications with Varying Loads:

These are the types of applications where the load on the motor varies over a speed range. These applications may demand high‐speed control accuracy and good dynamic responses.

For example,

• In home appliances: washers, dryers and compressors. • In automotive, fuel pump control, electronic steering control, engine control

and electric vehicle control. • In aerospace, there are a number of applications, like centrifuges, pumps,

robotic arm controls, gyroscope controls and so on. • These applications may use speed feedback devices and may run in semi‐

closed loop or in total closed loop. 3‐ Positioning Applications: Most of the industrial and automation types of application come under this category. The applications in this category have some kind of power transmission, which could be mechanical gears or timer belts, or a simple belt driven system. In these applications, the dynamic response of speed and torque are important. Also, these applications may have frequent reversal of rotation direction. These systems mostly operate in closed loop.


of 127

Finally, a comparison between Brushed DC motor (BDC) and Brushless DC motor (BLDC) is as shown in the below image.


of 127

Second: AC Motors Alternating current (AC) motors use an electrical current, which reverses its direction at regular intervals. The main advantage of DC motors over AC motors is that speed is more difficult to control for AC motors. To compensate for this, AC motors can be equipped with variable frequency drives but the improved speed control comes together with a reduced power quality. Types of AC Motors:

AC motors in common use today may be divided into two broad categories:

1. Induction (asynchronous) motors. 2. Synchronous motors. 3. Linear Motors.

These two types of motors differ in how the rotor field excitation is supplied as follows: For induction motors, there is no externally‐applied rotor excitation, and current is instead induced into the rotor windings due to the rotating stator magnetic field. For synchronous motors, a field excitation is applied to the rotor windings. This difference in field excitation leads to differences in motor characteristics, which leads in turn to different protection and control requirements for each motor type. 1‐ Induction motor Induction motors are the most common motors used for various equipments in industry.


of 127

Induction Motor: So called because voltage is induced in the rotor (thus no need for brushes), but for this to happen, the rotate than rotor must at a lower speed the magnetic field to allow for the existence of an induced voltage. Therefore a new term is needed to describe the induction motor which is the slip. The slip: A driving torque can only exist if there is an induced current in the shading ring. It is determined by the current in the ring and can only exist if there is a flux variation in the ring. Therefore, there must be a difference in speed in the shading ring and the rotating field. This is why an electric motor operating to the principle described above is called an “asynchronous motor”. The difference between the synchronous speed (Ns) and the shading ring speed (N) is called “slip” (s) and is expressed as a percentage of the synchronous speed. S= (Nsyn – Nm)/ Nsyn Where s is the slip. Slip is one of the most important variables in the control and operation of induction machines. s = 0 : if the rotor runs at synchronous speed. s = 1 : if the rotor is stationary. s is –ve : if the rotor runs at a speed above the synchronous speed. s is +ve : if the rotor runs at a speed below the synchronous speed. Advantages:

1. Simple design, rugged, low‐price, easy maintenance. 2. Wide range of power ratings: fractional horsepower to 10 MW. 3. Run essentially as constant speed from no‐load to full load. 4. Its speed depends on the frequency of the power source. 5. Most popular motor today in the low and medium horsepower range. 6. Very robust in construction. 7. Have replaced DC Motors in areas where traditional DC Motors cannot be

used such as mining or explosive environments Of two types depending on motor construction; Squirrel Cage or Slip Ring.


of 127

Disadvantages:

1. Not easy to have variable speed control. 2. Requires a variable‐frequency power‐electronic drive for optimal speed

control. 3. Most of them run with a lagging power factor.

Principle of operation:

• The stator is usually connected to the grid and, thus, the stator is magnetized.

• Stator magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings.

• Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound‐rotor, and induced current flows in the rotor windings.

• The rotor current produces another magnetic field. • A torque is produced as a result of the interaction of those two magnetic

fields.

Construction: An induction motor has two main parts 1‐ Stator

Induction Motor Stator


of 127

This is the immobile part of the motor. A body in cast iron or a light alloy houses a ring of thin silicon steel plates (around 0.5mm thick). The plates are insulated from each other by oxidation or an insulating varnish. The “lamination” of the magnetic circuit reduces losses by hysteresis and eddy currents. The plates have notches for the stator windings that will produce the rotating field to fit into (three windings for a 3‐phase motor). Each winding is made up of several coils. The way the coils are joined together determines the number of pairs of poles on the motor and hence the speed of rotation. 2‐ Rotor This is the mobile part of the motor. Like the magnetic circuit of the stator, it consists of stacked plates insulated from each other and forming a cylinder keyed to the motor shaft. Types of Induction Motors

Types of Induction Motors


of 127

Induction motors are classified according to the Rotor Type as follows:

A‐ Squirrel‐Cage Rotor:

Squirrel‐Cage Rotor


of 127

It consists of thick conducting bars embedded in parallel slots. These bars are short‐circuited at both ends by means of short‐circuiting rings. B‐ Wound Rotor:

Wound Rotor


of 127

It has a three‐phase, double‐layer, distributed winding. It is wound for as many poles as the stator. The three phases are wired internally and the other ends are connected to slip‐rings mounted on a shaft with brushes resting on them. Each of the two types of Induction motors above can be classified into two main groups as follows: I‐ Single‐phase induction motors: These only have one stator winding, operate with a single‐phase power supply, have a squirrel cage rotor, and require a device to get the motor started. This is by far the most common type of motor used in household appliances, such as fans, washing machines and clothes dryers, and for applications for up to 3 to 4 horsepower. Single phase induction motors come also with wound rotor which has excellent starting and accelerating characteristics, and they are ideal for Value Operators, Farm Motor Applications, Hoists, Floor Maintenance Machines, Air Compressors, Laundry Equipment and Mining Equipment. II‐ Three‐phase induction motors: The rotating magnetic field is produced by the balanced three‐phase supply. These motors have high power capabilities, can have squirrel cage or wound rotors (although 90% have a squirrel cage rotor), and are self‐starting. It is estimated that about 70% of motors in industry are of this type, are used in, for example, pumps, compressors, conveyor belts, heavy‐duty electrical networks, and grinders. They are available in 1/3 to hundreds of horsepower ratings.

Now, let us see the first classification of induction motors based on the above types:


of 127

1‐ Single Phase, Squirrel Cage, Induction Motor: This category have many types as shown in the below image.

A‐ Shaded‐Pole Induction Motors Construction and operation principle:

Shaded‐Pole Induction Motors


of 127

Shaded‐pole motors have only one main winding and no start winding. Starting is by means of a design that rings a continuous copper loop around a small portion of each of the motor poles. This “shades” that portion of the pole, causing the magnetic field in the shaded area to lag behind the field in the unshaded area. The reaction of the two fields gets the shaft rotating. Advantages:

1. Because the shaded‐pole motor lacks a start winding, starting switch or capacitor, it is electrically simple and inexpensive.

2. The speed can be controlled merely by varying voltage, or through a multi‐tap winding.

3. Mechanically, the shaded‐pole motor construction allows high‐volume production.

4. These are usually considered as “disposable” motors, meaning they are much cheaper to replace than to repair.

Disadvantages:

1. It’s low starting torque is typically 25% to 75% of the rated torque. 2. It is a high slip motor with a running speed 7% to 10% below the synchronous

speed. 3. Generally, efficiency of this motor type is very low (below 20%).

Applications:

The low initial cost suits the shaded‐pole motors to low horsepower or light duty applications. Perhaps their largest use is in multi‐speed fans for household use. But the low torque, low efficiency and less sturdy mechanical features make shaded‐pole motors impractical for most industrial or commercial use, where higher cycle rates or continuous duty are the norm.

B‐ Split‐Phase AC Induction Motor Construction and operation principle: The split‐phase motor is also known as an induction start/induction run motor. It has two windings: a start and a main winding. The start winding is made with smaller gauge wire and fewer turns, relative to the main winding to create more resistance, thus putting the start winding’s field at a different angle than that of the main winding which causes the motor to start rotating. The main winding, which is of a heavier wire, keeps the motor running the rest of the time.


of 127

Advantages and disadvantages:

1. The starting torque is low, typically 100% to 175% of the rated torque. 2. The motor draws high starting current, approximately 700% to 1,000% of the

rated current. 3. The maximum generated torque ranges from 250% to 350% of the rated

torque.

Applications: Good applications for split‐phase motors include small grinders, small fans and blowers and other low starting torque applications with power needs from 1/20 to 1/3 hp. Avoid using this type of motor in any applications requiring high on/off cycle rates or high torque. Types: Split‐phase motors are designed to use inductance, capacitance, or resistance to develop a starting torque and so, they have many types as follows:

1. Capacitor‐Start. 2. Permanent Split Capacitor (Capacitor Run) AC Induction Motor. 3. Capacitor Start/Capacitor Run AC Induction Motor. 4. Resistance‐Start.


of 127

1‐ Capacitor‐Start Construction and operation principle:

Capacitor‐Start Split‐Phase AC Induction Motor

The stator consists of the main winding and a starting winding (auxiliary). The starting winding is connected in parallel with the main winding and is placed physically at right angles to it. A 90‐degree electrical phase difference between the two windings is obtained by connecting the auxiliary winding in series with a capacitor and starting switch. (because X C about equals R). The main winding has enough resistance‐inductance (referred to as inductive reactance and expressed as XL) to cause t When the motor is first energized, the starting switch is closed. This places the capacitor in series with the auxiliary winding. The capacitor is of such value that the auxiliary circuit is effectively a resistive‐capacitive circuit (referred to as capacitive reactance and expressed as XC). In this circuit the current leads the line voltage by about 45ºout of phase ‐ so are the magnetic fields that are generated. The effect is that the two windings act like a two‐phase stator and produce the rotating field required to start the motor. (because X L about equals R). The currents in each winding are therefore 90º he current to lag the line voltage by about 45º When nearly full speed is obtained (75% of Rated speed), a centrifugal device (the starting switch) cuts out the starting winding. The motor then runs as a plain single‐phase induction motor. Since the auxiliary winding is only a light winding, the motor does not develop sufficient torque to start heavy loads. Split‐phase motors, therefore, come only in small sizes.


of 127

Advantages and disadvantages:

1. Since the capacitor is in series with the start circuit, it creates more starting torque, typically 200% to 400% of the rated torque.

2. The starting current, usually 450% to 575% of the rated current, is much lower than the split‐phase due to the larger wire in the start circuit.

3. Sizes range from fractional to 10 hp at 900 to 3600 rpm.

2‐ Permanent Split Capacitor (Capacitor Run) AC Induction Motor Construction and operation principle:

Permanent Split Capacitor (Capacitor Run) AC Induction Motor

A permanent split capacitor (PSC) motor has a run type capacitor permanently connected in series with the start winding. This makes the start winding an auxiliary winding once the motor reaches the running speed. Since the run capacitor must be designed for continuous use, it cannot provide the starting boost of a starting capacitor. The typical starting torque of the PSC motor is low, from 30% to 150% of the rated torque. PSC motors have low starting current, usually less than 200% of the rated current, making them excellent for applications with high on/off cycle rates.


of 127

Advantages:

1. The motor design can easily be altered for use with speed controllers. 2. They can also be designed for optimum efficiency and High‐Power Factor (PF)

at the rated load. 3. They’re considered to be the most reliable of the single‐phase motors, mainly

because no centrifugal starting switch is required.

Applications: Permanent split‐capacitor motors have a wide variety of applications depending on the design. These include fans, blowers with low starting torque needs and intermittent cycling uses, such as adjusting mechanisms, gate operators and garage door openers. 3‐ Capacitor Start/Capacitor Run AC Induction Motor

Construction and operation principle:

Capacitor Start/Capacitor Run Split‐Phase AC Induction Motor

This motor has a start type capacitor in series with the auxiliary winding like the capacitor start motor for high starting torque. Like a PSC motor, it also has a run type capacitor that is in series with the auxiliary winding after the start capacitor is switched out of the circuit. This allows high overload torque.


of 127

Advantages

1. This type of motor can be designed for lower full‐load currents and higher efficiency

Disadvantages

1. This motor is costly due to start and run capacitors and centrifugal switch.

Applications It is able to handle applications too demanding for any other kind of single‐phase motor. These include woodworking machinery, air compressors, high‐pressure water pumps, vacuum pumps and other high torque applications requiring 1 to 10 hp. 4‐ Resistance‐Start Construction and operation principle:

Resistance‐Start Split‐Phase AC Induction Motor

A modified version of the capacitor start motor is the resistance start motor. In this motor type, the starting capacitor is replaced by a resistor. This motor also has a starting winding in addition to the main winding. It is switched in and out of the circuit just as it was in the capacitor‐start motor. The starting winding is positioned at right angles to the main winding. The electrical phase shift between the currents in the two windings is obtained by making the impedance of the windings unequal. The main winding has a high inductance and a low resistance. The current, therefore, lags the voltage by a large angle. The starting winding is designed to have a fairly low


of 127

inductance and a high resistance. Here the current lags the voltage by a smaller angle. For maximum starting torque, the 30‐degree phase difference still generates a rotating field. This supplies enough torque to start the motor. When the motor comes up to speed, a speed‐controlled switch disconnects the starting winding from the line, and the motor continues to run as an induction motor. The starting torque is not as great as it is in the capacitor‐start. For example, suppose the current in the main winding lags the voltage by 70º. The current in the auxiliary winding lags the voltage by 40º. The currents are, therefore, out of phase by 30º. The magnetic fields are out of phase by the same amount. Although the ideal angular phase difference is 90º

Applications, Advantages and disadvantages: The resistance start motor is used in applications where the starting torque requirement is less than that provided by the capacitor start motor. Apart from the cost, this motor does not offer any major advantage over the capacitor start motor. A comparison for the popular types of a split phase motors is shown in the below image.


of 127

C‐ Universal motor:

Universal motor

Universal motors are mostly operated on AC power, but they can operate on either AC or DC. Tools and appliances are among the most frequent applications. Please review the previous topic “Classification of Electric Motors – Part One” for more information about Universal motor.


of 127

2‐ Three Phase, Squirrel Cage, Induction Motor: Almost 90% of the three‐phase AC Induction motors are of Squirrel Cage type. Here, the rotor is of the squirrel cage type and it works as explained earlier. The power ratings range from one‐third to several hundred horsepower in the three‐phase motors. Motors of this type rated one horsepower or larger, cost less and can start heavier loads than their single‐phase counterparts. Three phase Squirrel cage Induction motors are classified by application with a design letter which gives an indication of key performance characteristics of the motor, these classification are made by NEMA and IEC. The main Classifications of Three phase Squirrel cage Induction motors are shown in the below image.

Three Phase, Squirrel Cage, Induction Motor


of 127

3‐ Single Phase, Wound Rotor, Induction Motor This category have many types as shown in the below image.

A‐ Repulsion motor Construction:

Repulsion motor


of 127

The motor has a stator and a rotor but there is no electrical connection between the two and the rotor current is generated by induction. The rotor winding is connected to a commutator which is in contact with a pair of short‐circuited brushes which can be moved to change their angular position relative to an imaginary line drawn through the axis of the stator. The motor can be started, stopped and reversed, and the speed can be varied, simply by changing the angular position of the brushes.

The principle difference between an AC series motor and repulsion motors is the way in which power is supplied to armature. In Ac series motor the armature receives voltage by conduction through the power supply. But In repulsion motors the armature is supplied by induction from the stator windings.


of 127

Disadvantages of Repulsion Motor:

1. Occurrence of sparks at brushes. 2. Commutator and brushes wear out quickly. This is primarily due to arcing and

heat generated at brush assembly. 3. The power factor is poor at low speeds. 4. No load speed is very high and dangerous.

Application of Repulsion motors: Because of excellent starting and accelerating characteristics, repulsion‐induction motors are ideal for:

1. Value Operators. 2. Farm Motor Applications. 3. Hoists. 4. Floor Maintenance Machines. 5. Air Compressors. 6. Laundry Equipment. 7. Mining Equipment.

Types: The various types of motors which works under the repulsion principle are:

1. Repulsion‐start Induction‐run motor. 2. Repulsion Induction motor.


of 127

A‐ Repulsion‐start induction‐run

A repulsion‐start induction motor is a single phase motor having the same windings as a repulsion motor , When an induction motor drives a hard starting load like a compressor, the high starting torque of the repulsion motor may be put to use. The induction motor rotor windings are brought out to commutator segments for starting by a pair of shorted brushes. At near running speed, a centrifugal switch shorts out all commutator segments, giving the effect of a squirrel cage rotor, the brushes may also be lifted to prolong bush life. This means that they started as repulsion motors but running as induction motor Starting torque is 300% to 600% of the full speed value as compared to under 200% for a pure induction motor.

B‐ Repulsion‐Induction Motor A repulsion‐induction motor is a form of repulsion motor which has a squirrel‐cage winding in the rotor in addition to the repulsion motor winding. A motor of this type may have either a constant speed or varying‐speed characteristic.


of 127

4‐ Three Phase, Wound Rotor, Induction Motor

Three Phase, Wound Rotor, Induction Motor

• This type of 3‐phase induction motor has high starting torque, which makes it

ideal for applications where standard NEMA design motors fall short. The wound‐rotor motor is particularly effective in applications where using a squirrel‐cage motor may result in a starting current that's too high for the capacity of the power system.

• In addition, the wound‐rotor motor is appropriate for high‐inertia loads having a long acceleration time.

• The slip‐ring motor or wound‐rotor motor is a variation of the squirrel cage induction motor. While the stator is the same as that of the squirrel cage motor, it has a set of windings on the rotor which are not short‐circuited, but are terminated to a set of slip rings. These are helpful in adding external resistors and contactors.


of 127

Wound Rotor

• The slip necessary to generate the maximum torque (pull‐out torque) is directly proportional to the rotor resistance. In the slip‐ring motor, the effective rotor resistance is increased by adding external resistance through the slip rings. Thus, it is possible to get higher slip and hence, the pull‐out torque at a lower speed.

• A particularly high resistance can result in the pull‐out torque occurring at almost zero speed, providing a very high pull‐out torque at a low starting current. As the motor accelerates, the value of the resistance can be reduced, altering the motor characteristic to suit the load requirement. Once the motor reaches the base speed, external resistors are removed from the rotor. This means that now the motor is working as the standard induction motor.

• This motor type is ideal for very high inertia loads, where it is required to generate the pull‐out torque at almost zero speed and accelerate to full speed in the minimum time with minimum current draw.

Applications: They are generally used to drive high‐inertia loads (e.g., large pumps, cranes, grinders).


of 127

Second: Synchronous motor Synchronous Motor: So called because rotor tries to line up with the rotating magnetic field in the stator. It has the stator of an induction motor, and the rotor of a dc motor. A synchronous motor is an AC motor, which runs at constant speed fixed by frequency of the system. It requires direct current (DC) for excitation and has low starting torque, and therefore suited for applications that start with a low load, such as air compressors, frequency changes and motor generators. Synchronous motors are able to improve the power factor of a system, which is why they are often used in systems that use a lot of electricity. Differences between Synchronous and Induction motors:

1. Synchronous motors are not as widely used as induction machines because their rotors are more complex and they require exciters.

2. Synchronous motors are used in large industrial applications in situations where their ability to provide leading power factor helps to support or stabilize voltage and to improve overall power factor.

3. In ratings higher than several hundred horsepower, synchronous machines are often more efficient than induction machines and so very large synchronous machines are sometimes chosen over induction motors.

4. Unlike an induction motor, the synchronous motor is excited by an external DC source and, therefore, requires slip rings and brushes to provide current to the rotor.

5. In the synchronous motor, the rotor locks into step with the rotating magnetic field and rotates at synchronous speed. If the synchronous motor is loaded to the point where the rotor is pulled out of step with the rotating magnetic field, no torque is developed, and the motor will stop.

6. A synchronous motor is not a self‐starting motor because torque is only developed when running at synchronous speed; therefore, the motor needs some type of device to bring the rotor to Synchronous speed.

Construction: Like the asynchronous (Induction) motor, the synchronous motor consists of a stator and a rotor separated by the air gap. It differs from the asynchronous motor in that the flux in the air gap is not due to a component of the stator current: it is created by magnets or by the field coil current provided by an external DC source energizing a winding placed in the rotor.


of 127

The main components of a synchronous motor are as follows: 1‐ Stator:

Stator

The stator consists of a housing and a magnetic circuit generally comprising silicon steel laminations and a 3‐phase coil similar to that of an asynchronous motor supplied with 3‐phase AC to produce a rotating field. The stator produces a rotating magnetic field that is proportional to the frequency supplied. This motor rotates at a synchronous speed, which is given by the following equation: Ns = 120 f / P Where: f = frequency of the supply frequency P= number of poles


of 127

2‐ Rotor

Rotor

Synchronous rotors are designed primarily for applications requiring highly efficient motors. Each pole assembly is made from high strength steel laminations with a DC field winding encircling the pole body. The field winding consists of a rectangular section of insulated copper wire wound directly on an insulated pole body and bonded by a high temperature, high strength insulating epoxy resin which, when cured, results in a coil impervious to dirt, moisture and other contaminants. The rotor carries field magnets or coils through which a direct current flows and which create interposed North and South poles. Unlike asynchronous (Induction) machines, the rotor rotates with no slip at the speed of the rotating field. There are two types of rotor structures as follows:

1. Salient pole rotor. 2. Round or cylindrical rotor (Non‐salient‐pole rotor).


of 127

a‐ Salient Pole Rotor

Salient Pole Rotor

• Salient pole structure is used for low speed applications, such as hydroelectric generators.

• Salient‐pole rotor: four and more poles.

b‐ Round or Cylindrical Rotor (Non‐salient‐Pole Rotor)

Round or Cylindrical Rotor (Non‐salient‐Pole Rotor)


of 127

• Round rotor structure is used for high speed synchronous machines, such as

steam turbine generators. • Non‐salient‐pole rotor: usually two‐ and four‐pole rotors.

3‐ Amortisseur (starting winding) Synchronous motors are provided with an Amortisseur, or starting winding, consisting of copper alloy bars located in the pole face, parallel to the shaft, and brazed at the ends to copper alloy rings. The Amortisseur winding is tailored for the application to provide the required starting performance. 4‐ Stator Frame The stator frame contains and supports the other parts and may include bearing housings. 5‐ Other Parts Large machines include additional parts for cooling the machine, supporting the rotor, lubricating and cooling the bearings, and various protection and measurement devices. Operation:

Operation of a Synchronous Motor


of 127

The operation of a synchronous motor is simple to imagine. The 'Stator' winding, when excited by a poly‐phase (usually 3‐phase) supply, creates a rotating magnetic field inside the motor. The rotor winding, which acts as a permanent magnet, supplied with a DC current and creating a field which simply locks in with the rotating magnetic field and rotates along with it? During operation, as the rotor field locks in with the rotating magnetic field, the motor is said to be in synchronization and a torque is developed. Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the breakdown load, the motor falls out of synchronization i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization. Applications:

1. Synchronous motors find applications in all industrial applications where constant speed is necessary.

2. Improving the power factor as synchronous condensers. 3. Low power applications include positioning machines, where high precision is

required, and robot actuators. 4. Mains synchronous motors are used for electric clocks. 5. Record player turntables. 6. Large plant compressors. 7. Fans, pumps, and large industrial grinders. 8. Mills in the steel industry. 9. Larger high‐speed motors are popular in the natural‐gas pipeline system.

Advantages: Synchronous motors have the following advantages over non‐synchronous motors:

1. Speed is independent of the load, provided an adequate field current is applied.

2. Accurate control in speed and position using open loop controls, e.g. stepper motors.

3. They will hold their position when a DC current is applied to both the stator and the rotor windings.

4. Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.


of 127

5. Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).

6. They run either at the synchronous speed or they do not run at all.

Types: There are two major types of synchronous motors as follows:

1. Non‐excited motors. 2. DC‐excited motors.

1‐ Non‐excited motors These motors employ a self‐starting circuit and require no external excitation supply. In non‐excited motors, the rotor is made of solid steel. At synchronous speed it rotates in step with the rotating magnetic field of the stator, so it has an almost‐constant magnetic field through it. The external stator field magnetizes the rotor, inducing the magnetic poles needed to turn it. The rotor is made of a high‐retentively steel such as cobalt steel. These are manufactured in three types as follows:

• Reluctance motors. • Hysteresis motors. • Permanent magnet motors.

A‐ Reluctance motors Reluctance motor is A synchronous‐induction motor. The rotor has salient poles and a cage so that it starts like an induction motor, and runs like a synchronous motor. Principle of operation:

Reluctance Rotor


of 127

• A classic squirrel cage rotor with notches (or flats) in the rotor periphery. The number of notches will correspond to the number of poles in the stator winding. The sections of the rotor periphery between the high reluctance areas are known as salient poles. Since these poles create a low reluctance path for the stator flux, they are attracted to the poles of the stator field.

• The reluctance synchronous rotor starts and accelerates like a regular squirrel cage rotor, but as it approaches the rotational speed of the field, a critical point is reached where there is an increased acceleration and the rotor “snaps” into synchronism with the stator field.

• If the load (particularly inertial) is too great, the motor will not attain synchronous speed. Motor “pull‐in” torque is defined as the maximum load that the motor can accelerate and pull into synchronism at rated voltage and frequency.

• An applied load greater than the rated “pull‐in” torque will prevent the motor from pulling the load into synchronism and will result in rough, non‐uniform operation.

Reluctance synchronous motors may be designed for poly‐phase operation, as well as single‐phase versions in split‐phase, CS and PSC configurations. Reluctance synchronous motors ratings range from sub‐fractional to about 30 hp. Sub‐fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral horsepower motors use squirrel cage construction with toothed rotors.

Switched Reluctance Motors


of 127

Switched Reluctance Motors

• The switched reluctance motor (SRM) is an electric motor in which torque is

produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is maximized.

• SRM is a type of synchronous machine. It has wound field coils of a DC motor for its stator windings and has no coils or magnets on its rotor.

• It can be seen that both the stator and rotor have salient poles; hence, the machine is a doubly salient, singly excited machine.

• Stator windings on diametrically opposite poles are connected in series or parallel to form one phase of the motor.

• Several combinations of stator and rotor poles are possible, such as 6/4 (6 stator poles and 4 rotor poles), 8/4, 10/6 etc.

• The configurations with higher number of stator/rotor pole combinations have less torque ripple.

Applications:

1. Flameproof drive systems for potentially explosive atmospheres. 2. Washing machine. 3. Environmentally friendly air conditioning system for passenger trains. 4. Servo systems for advanced technology weaving machine.


of 127

B‐ Hysteresis motors:

Hysteresis motors

Although the stator in a hysteresis synchronous design is wound much like that of the conventional squirrel cage motor, its rotor is made of a heat‐treated cast permanent magnet alloy cylinder (with a nonmagnetic support) securely mounted to the shaft like "hard" cobalt steel. This material has a wide hysteresis loop (high retentively), meaning once it is magnetized in a given direction, it requires a large reverse magnetic field to reverse the magnetization.

Stator of a Hysteresis Motor


of 127

The motor’s special performance characteristics are associated with its rotor design. The rotor starts on the hysteresis principle and accelerates at a fairly constant rate until it reaches the synchronous speed of the rotating field.

Cobalt Hysteresis Ring Rotor

• Instead of the permanently fixed poles found in the rotor of the reluctance synchronous design, hysteresis rotor poles are “induced” by the rotating magnetic field. During the acceleration period, the stator field will rotate at a speed faster than the rotor, and the poles which it induces in the rotor will shift around its periphery. When the rotor speed reaches that of the rotating stator field, the rotor poles will take up a fixed position.

• If the load is increased beyond the capacity of the motor, the poles on the periphery of the rotor core will shift.

• If the load is then reduced to the “pullin” capacity of the motor, the poles will take up fixed positions until the motor is again overloaded or stopped and restarted.

• The hysteresis rotor will “lock‐in” at any position, in contrast to the reluctance rotor which has only the “lock‐in” points corresponding to the salient poles on the rotor.

Applications: Hysteresis motors are manufactured in sub‐fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.


of 127

C‐ Permanent‐Magnet Synchronous Motors

Permanent‐Magnet Synchronous Motors

The stator portion of Permanent‐Magnet Synchronous Motors has an uneven distribution of magnetic Poles and the solid steel rotor has permanent magnets embedded in it, the purpose of this is to give the rotor a preferred starting point while providing an apparent shift in field during starting due to the uneven reluctance of the stator.

Permanent‐Magnet clock Motor and Rotor

They are not self‐starting. Because of the constant magnetic field in the rotor these cannot use induction windings for starting, and must have electronically controlled variable frequency stator drive.

Some of these motors have a spring return mechanism to reverse the rotation just in case it starts turning the wrong way.


of 127

Applications:

Industrial drives, e.g., pumps, fans, blowers, mills, hoists, handling systems, elevators and escalators, people movers, light railways and streetcars (trams), electric road vehicles, aircraft flight control surface actuation. Advantages: The use of permanent magnets (PMs) in construction of electrical machines brings the following benefits:

1. No electrical energy is absorbed by the field excitation system and thus there are no excitation losses which mean substantial increase in the efficiency.

2. Higher torque and/or output power per volume than when using electromagnetic excitation.

3. Better dynamic performance than motors with electromagnetic excitation (higher magnetic flux density in the air gap).

4. Simplification of construction and maintenance. 5. Reduction of prices for some types of machines.

Disadvantages:

1. High cost of permanent magnets. 2. Magnet corrosion and possible demagnetization. 3. Large air gap in surface mount PM machines.

2‐ DC‐excited motors They are made in sizes larger than 1 hp, these motors require direct current for excitation which can be supplied from a separate source or from a dc generator directly connected to the motor shaft. These motors are commonly used in analog electric clocks, timers and other devices where correct time is required. Two common approaches are used to supply a DC current to the field circuits on the rotating rotor:

1. Supply the DC power from an external DC source to the rotor by means of slip rings and brushes “Brush type Synchronous motors”.

2. Supply the DC power from a special DC power source mounted directly on the shaft of the machine “brushless type Synchronous motors”.


of 127

A‐ Brush type Synchronous motors:

Brush type Synchronous motors

The field exciter for a brush‐type motor is typically a DC generator with its rotor mounted on the motor shaft. The output of the DC generator is fed via brushes and slip rings to the motor field windings. A brush‐style exciter is typically not used in a high speed application due to ignition problems caused by the brushes’ physical contact with the slip ring. Proper and regular maintenance, though difficult to perform, can reduce the occurrence of ignition problems in brush‐type exciters B‐ Brushless type Synchronous motors:

A rotor of large synchronous machine with a brushless exciter mounted on the same shaft.


of 127

The field exciter for a brushless synchronous motor typically consists of an AC generator with the field windings on its stator, armature windings on its rotor, and with its rotor mounted on the motor shaft. The output of the generator is rectified by solid‐state rectifier elements also mounted on the rotor shaft and fed directly to the motor field windings without the need for brushes or slip rings. Because of the proliferation of solid‐state power electronic technology, and because the brushless‐type motors require less maintenance almost all new synchronous motors are brushless‐type.

Solid‐State Rectifier for Brushless Motor

It is possible to adjust the field current on the main machine by controlling the small DC field current of the exciter generator (located on the stator). Note: In either design; brush and brushless, the field excitation to the exciter may be varied to vary the power‐factor operation of the motor, and in fact power factor correction is one common use of synchronous motors since they can be made to operate at leading power factors.


of 127

3‐ Stepper motor: Stepper motor is a special type of synchronous motor which is designed to rotate a specific number of degrees for every electric pulse received by its control unit. Typical steps are 7.5 or 15 degree per pulse. It is a motor that can rotate in both directions, move in precise angular increments, sustain a holding torque at zero speed, and be controlled with digital circuits. It moves in accurate angular increments known as steps, in response to the application of digital pulses to the electric drive circuit. Generally, such motors are manufactured with steps per revolution. Depending on its electrical power supply, it may be: A‐ Unipolar: if its coils are always supplied in the same direction by a single voltage, it requiring only one power source, hence the name unipolar. B‐ Bipolar: when its coils are supplied sometimes in one direction and sometimes in the other, it requiring two power sources. They sometimes create a North Pole, and sometimes a South pole, hence the name bipolar. Stepper motors, unlike ordinary DC motors, are brushless and can divide a full 360° into a large number of steps, for example 200. Operating principles:


of 127

Stepper motors operate differently from normal DC motors, which rotate when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple "toothed" electromagnets arranged around a central gear‐shaped piece of iron. The electromagnets are energized by an external control circuit, such as a micro controller. To make the motor shaft turn, first one electromagnet is given power, which makes the gear's teeth magnetically attracted to the electromagnet's teeth. When the gear's teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a "step," with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle. Advantages:

1. Low cost. 2. Can work in an open loop (no feedback required). 3. Excellent holding torque (eliminated brakes/clutches). 4. Excellent torque at low speeds. 5. Low maintenance (brushless). 6. Very rugged ‐ any environment. 7. Excellent for precise positioning control. 8. No tuning required.

Disadvantages: Some of the disadvantages of stepper motors in comparison with servo motors are as follows:

1. Rough performance at low speeds unless you use micro‐stepping. 2. Consume current regardless of load. 3. Limited sizes available. 4. Noisy. 5. Torque decreases with speed (you need an oversized motor for higher torque

at higher speeds). 6. Stepper motors can stall or lose position running without a control loop.

Applications of Stepper motor:

1. Cruise control. 2. Auto air vents. 3. Light leveling.


of 127

4. Printers. 5. Industrial machines. 6. Automotive gauges. 7. Office equipment. 8. Computer drives. 9. Medical scanners. 10. Scientific Instrumentation.

Types of Stepper Motors: 1‐ Variable‐Reluctance Step Motors

Variable‐Reluctance Step Motors

The construction of variable‐reluctance (VR) motors is generally as shown in above image, there is a stator assembly consisting of an insulated lamination stack with copper coils wound around the teeth. The stator assembly is positioned within a housing or main frame such that its location is secured. The rotor assembly consists of a steel magnetic core, a steel output shaft, and bearings. The rotor assembly is centrally located inside the stator assembly by end frames or bearing supports.


of 127

2‐ Permanent‐Magnet‐Rotor Step Motors

Permanent‐Magnet‐Rotor Step Motors

The PM step motor is illustrated in above image. It consists of two sets of stamped steel cups with diagonal teeth facing the rotor. Each set of cups circumscribes a coil of wire. The two sets are positioned with respect to each other such that they circumscribe the rotor but they are offset from each other by one‐half of a tooth pitch. The permanent‐magnet‐rotor step motor is commonly referred to as the stamped‐construction or sheet‐metal step motor. It is sometimes called simply a PM step motor but should not be confused with the hybrid permanent‐magnet step motor. The rotor in a stamped‐construction motor is a smooth cylindrical permanent magnet radially magnetized with alternating N and S poles. The stator has two cup‐shaped halves with formed stator teeth. Each half contains a circular, bobbin‐wound coil. Because of this simple design, the price is low, but step accuracy and speed may not equal the performance of other step‐motor types.


of 127

3‐ Hybrid Permanent‐Magnet Step Motors

Hybrid Permanent‐Magnet Step Motors

The hybrid step motor is generally constructed as shown in above image. It has a stator assembly similar to that of the VR motor, but the rotor consists of three sections. Two pieces are similar to the VR step‐motor rotor, but a magnet is placed between them, and they are offset circumferentially from each other by one‐half tooth pitch. This motor is termed a hybrid because it uses elements of both variable reluctance and permanent‐magnet‐rotor step motors. The commonly known version is the 1.8 step‐angle motor. It was originally designed as an ac two‐phase synchronous inductor motor for low‐speed applications. Its stator construction is similar to that of a variable‐reluctance step motor with salient poles (multiple teeth per pole).The phase windings may be either monofilar or bifilar coils, as discussed for the stamped‐construction motor. The rotor contains a cylindrical permanent magnet axially magnetized and enclosed on each end by a soft‐iron cup with uniformly spaced teeth. As for the variable‐reluctance motor, the number of stator phases and differing number of stator and rotor teeth determine the step angle.


of 127

Third: Linear motors

Linear motors should be thought of as rotary electric motors that have been cut along a radial plane and unrolled. The resultant motor is a linear electric motor that can produce linear motion without the need of pneumatic or hydraulic cylinders or translation of rotary motion with the use of belts, pulleys, or screws. This is desirable because the extra machine parts make the machine more complicated, and there are more parts that will wear out, and need replacement.

However, because linear motors do not have the luxury of 360 degree contained rotation, they must either increase the length of the primary, coil assembly, and keep a short moving secondary, magnet assembly, or increase the length of the secondary, and keep a short moving primary. There is a diagram that can be found below illustrating the differences between these two options. So, a linear motor is an electric motor that has had its stator and rotor "unrolled" so that instead of producing a torque (rotation) it produces a linear force along its length. Linear electric motors can drive a linear motion load without intermediate gears, screws, or crank shafts. Applications:

1. Sliding doors and various similar actuators. 2. Accelerating cars for crash tests. 3. Transportation (Trains).


of 127

4. Robotics & Material Handling. 5. Elevators. 6. Compressors & Pumps. 7. Catapults and Launchers. 8. Curtain pullers.

Types: There are two main types of Linear Motors as follows:

1. Linear induction motor (LIM). 2. Linear synchronous motor (LSM).

1‐ Linear induction motor (LIM)

Linear induction motor (LIM)

A linear induction motor (LIM) is an AC asynchronous linear motor that works by the same general principles as other induction motors but is very typically designed to directly produce motion in a straight line. Characteristically, linear induction motors have a finite length primary, which generates end‐effects, whereas with a conventional induction motor the primary is arranged in an endless loop. Linear motors frequently run on a 3 phase power supply. Despite their name, not all linear induction motors produce linear motion, some


of 127

linear induction motors are employed for generating rotations of large diameters where the use of a continuous primary would be very expensive. Construction:

Traditional Linear Motors

A linear electric motor's primary typically consists of a flat magnetic core (generally laminated) with transverse slots which are often straight cut with coils laid into the slots. The secondary is frequently a sheet of aluminum, often with an iron backing plate. Some LIMs are double sided, with one primary either side of the secondary, and in this case no iron backing is needed. Two sorts of linear motor exist, short primary, where the coils are truncated shorter than the secondary, and a short secondary where the conductive plate is smaller. Short secondary LIMs are often wound as parallel connections between coils of the same phase, whereas short primaries are usually wound in series. The primaries of transverse flux LIMs have a series of twin poles lying transversely side‐by‐side, with opposite winding directions. Principles of operation


of 127

a‐ Moving magnetic field In this design of electric motor, the force is produced by a moving linear magnetic field acting on conductors in the field. Any conductor, be it a loop, a coil or simply a piece of plate metal, that is placed in this field will have eddy currents induced in it thus creating an opposing magnetic field, in accordance with Lenz's law. The two opposing fields will repel each other, thus creating motion as the magnetic field sweeps through the metal. b‐ End effect Unlike a circular induction motor, a linear induction motor shows end effects. With a short secondary, the behavior is almost identical to a rotary machine, provided it is at least two poles long, but with a short primary reduction in thrust occurs at low slip (below about 0.3) until it is eight poles or longer. However, because of end effect, linear motors cannot 'run light'‐ normal induction motors are able to run the motor with a near synchronous field under low load conditions. Due to end effect this creates much more significant losses with linear motors. c‐ Levitation In addition, unlike a rotary motor, an electrodynamics levitation force is shown, this is zero at zero slip, and tends to a constant positive lift force as slip increases in either direction.


of 127

2‐ Linear synchronous motor (LSM)

Linear synchronous motor (LSM)

A linear synchronous motor (LSM) is a linear motor in which the mechanical motion is in synchronism with the magnetic field, i.e., the mechanical speed is the same as the speed of the traveling magnetic field. The thrust (propulsion force) can be generated as an action of the following two fields:

1. traveling magnetic field produced by a polyphase winding and an array of magnetic poles N, S,...,N, S or a variable reluctance ferromagnetic rail (LSMs with a.c. armature windings);

2. Magnetic field produced by electronically switched d.c. windings and an array of magnetic poles N, S,...,N, S or variable reluctance ferromagnetic rail (linear stepping or switched reluctance motors).

The part producing the traveling magnetic field is called the armature or forcer. The part that provides the d.c. magnetic flux or variable reluctance is called the field excitation system (if the excitation system exists) or salientpole rail, reaction rail, or variable reluctance platen.


of 127

Motor Selection Procedures

First: AC Motors Selection Procedures The type of motor chosen for an application depends on the characteristics needed in that application which include:

1. The power supply, 2. System requirements, 3. Motor class, 4. Motor insulation type, 5. Motor Duty Cycle, 6. Bearing type, 7. Method of mounting the motor, 8. The cost and size of the motor, 9. Method of speed control, 10. Environmental conditions.

1‐ The Power Supply The power supply is distinguished by its number of phases, rated voltage and frequency as follows: 1.1 Number of phases

1. A power system can be either single‐phase or poly‐phase. 2. Single‐phase power is most commonly found in homes, rural areas and in

small commercial establishments. 3. A poly‐phase power system consists of 2 or more alternating currents of

equal frequency and amplitude but offset from each other by a phase angle. 4. For motors, an advantage of three‐phase power is simpler construction which

requires less maintenance. Also, a more powerful machine can be built into a smaller frame and will generally operate at a higher efficiency than single‐phase motors of the same rating.

1.2 Voltage: 1.2. A‐ Motor Nameplate Voltage The motor nameplate voltage is determined by the available power supply which


of 127

must be known in order to properly select a motor for a given application. The nameplate voltage will normally be less than the nominal distribution system voltage to allow for a voltage drop in the system between the power source and the motor leads. The bellow image lists motor nameplate voltages and provides the best match to distribution system voltages and meets current motor design practices.


of 127

1.2. B‐ Dual Voltage Motors Poly‐phase and single‐phase motors may be furnished as dual voltage ratings under the following conditions:

• Both voltages are standard for the particular rating as listed in the above image.

• The two voltages are in a ratio of either 1:2 or 1:Ã3 (e.g. 230/460, 60 Hz; 2300/4000, 60 Hz; or 220/380, 50 Hz).

• Single‐phase voltage ratios are 1:2 only.

1.2. C‐ Voltage Unbalance Unbalanced line voltages applied to a poly‐phase motor result in unbalanced currents in the stator windings. Even a small percentage of voltage unbalance will result in a larger percentage of current unbalance, thus increasing temperature rise and possibly result in nuisance tripping. Percent voltage unbalance is calculated as follows: Percent Unbalance = (100 x Maximum Voltage Deviation from Average Voltage) / Average Voltage Note: Motor operation above 5% voltage unbalance is not recommended. Unbalanced voltages will produce the following effects on performance characteristics:

• Torques: Unbalanced voltage results in reduced locked‐rotor and breakdown torques for the application.

• Full‐Load Speed: Unbalanced voltage results in a slight reduction of full‐load speed.

• Current: Locked‐rotor current will be unbalanced to the same degree that voltages are unbalanced but locked‐rotor KVA will increase only slightly. Full‐load current at unbalanced voltage will be unbalanced in the order of six to ten times the voltage unbalance.

• Temperature Rise: A 3.5% voltage unbalance will cause an approximate 25% increase in temperature rise.

1.3 Frequency 1.3. A‐ Standard Frequency The predominant frequency in the United States is 60 hertz. However, 50 hertz systems are common in other countries. Other systems, such as 40 and 25 hertz are isolated and relatively few in number.


of 127

1.3. B‐ 50 Hz Operation of 60 Hz Motors General Electric standard motors rated at 60 Hz may be successfully operated at 50 Hz at reduced voltage and horsepower as shown in the following table:

• Rated Hp at 50 Hz = Nameplate Hp x Derate Factor. • Allowable voltage variation at derated Hp = ±5%. • Select motor overload protection for 60 Hz Amps and 1.0 Service Factor. • Motor speed = 5/6 nameplate rated speed. • Service Factor = 1.0 • Sixty hertz motors intended for use as shown above should be ordered as 60

Hz motors with no reference to 50 Hz operation.

1.3. C‐ Dual Frequency Motors that require 50 and 60 Hz operation of the same motor are non‐NEMA defined motors and will be nameplated as such. When this is a motor requirement, it must be specified with the order. 1.4 Voltage and Frequency Variation All motors are designed to operate successfully with limited voltage and frequency variations. However, voltage variation with rated frequency must be limited to ±10% and frequency variations with rated voltage must be limited to ±5%. The combined variation of voltage and frequency must be limited to the arithmetic sum of 10%. Variations are expressed as deviation from motor nameplate values, not necessarily system nominal values. The allowable ±10% voltage variation is based upon the assumption that horsepower will not exceed nameplate rating and that motor temperature may increase.


of 127

The following conditions are likely to occur with variations in voltage:

• An increase or decrease in voltage may result in increased heating at rated horsepower load. Under extended operation this may accelerate insulation deterioration and shorten motor insulation life.

• An increase in voltage will usually result in a noticeable decrease in power factor. Conversely, a decrease in voltage will result in an increase in power factor.

• Locked‐rotor and breakdown torque will be proportional to the square of the voltage. Therefore, a decrease in voltage will result in a decrease in available torque.

• An increase of 10% in voltage will result in a reduction of slip of approximately 17%. A voltage reduction of 10% would increase slip by about 21%.

The following conditions are likely to occur with variations in frequency:

• Frequency greater than rated frequency normally improves power factor but decreases locked rotor and maximum torque. This condition also increases speed, and therefore, friction and winding losses.

• Conversely, a decrease in frequency will usually lower power factor and speed while increasing locked‐rotor maximum torque and locked‐rotor current.

1.5 Variable Frequency Operation Motors are available for use on variable frequency inverters. Generally speaking, there are three different types of inverters:

• VVI is a square wave inverter in which voltage and frequency vary in proportion (constant volts per hertz).

• PWI is a pulse width modulated inverter and the same as the VVI type except pulses are varied in time to simulate a sine wave.

• CCI is a constant current inverter, which utilizes a square wave current supply as opposed to voltage.


of 127

2‐ System requirements: This will include:

• Rated Speed (Speed measured in shaft revolutions per minute (RPM)). • Torque. • Horsepower. • Torque‐Speed performance of a motor. • Torque, Speed and Current Relation of a motor.

2.1 Rated speed The speed at which an induction motor operates is dependent upon the input power frequency and the number of electrical magnetic poles for which the motor is wound. The higher the frequency, the faster the motor runs. The more poles the motor has, the slower it runs. The speed of the rotating magnetic field in the stator is called synchronous speed. To determine the synchronous speed of an induction motor, the following equation is used: Synchronous Speed (rpm) = (60 x 2 x Frequency) / Number of poles Actual full‐load speed (the speed at which an induction motor will operate at nameplate rated load) will be less than synchronous speed. The difference between synchronous speed and full‐load speed is called slip. Percent slip is defined as follows: Percent Slip = (Synchronous Speed ‐ Full Load Speed) X 100 / Synchronous Speed Induction motors are built having rated slip ranging from less than 5% to as much as 20%. A motor with a slip of less than 5% is called a normal slip motor. Motors with a slip of 5% or more are used for applications requiring high starting torque (conveyor) and/or higher than normal slip (punch press) where, as the motor slows down, increased torque allows for flywheel energy release. 2.2 Torque Torque is one key motor characteristic (in addition to horsepower) that determine the size of motor for an application. Torque is merely a turning effort or force acting


of 127

through a radius. 2.3 Horsepower Horsepower take into account how fast the motor shaft is turned. Turning the shaft rapidly requires more horsepower than turning it slowly. Thus, horsepower is a measure of the rate at which work is done. By definition, the relationship between torque and horsepower is as follows: Full‐load torque in lb‐ft = (Hp x 5252) / Full‐Load rpm 2.4 Torque‐Speed performance of a motor The following graph illustrates a typical speed torque curve for a NEMA design B induction motor. An understanding of several points on this curve will aid in properly applying motors.


of 127

2.4. A‐ Locked‐Rotor Torque Locked‐rotor torque is the torque which the motor will develop at rest (for all angular positions of the rotor) with rated voltage at rated frequency applied. It is also sometimes known as “starting torque” and is usually expressed as a percentage of full‐load torque. 2.4. B‐ Pull‐Up Torque Pull‐up torque is the minimum torque developed during the period of acceleration from locked rotor to the speed at which breakdown torque occurs. For motors which do not have a definite breakdown torque (such as NEMA design D) pull‐up torque is the minimum torque developed up to rated full‐load speed. It is usually expressed as a percentage of full‐load torque. 2.4. C‐ Breakdown Torque Breakdown torque is the maximum torque the motor will develop with rated voltage applied at rated frequency without an abrupt drop in speed. Breakdown torque is usually expressed as a percentage of full‐load torque. 2.4. D‐ Full‐Load Torque Full‐load torque is the torque necessary to produce rated horsepower at full‐load speed. In pound‐feet, it is equal to the rated horsepower times 5252 divided by the full‐load speed in rpm. Full‐load torque in lb‐ft = (Hp x 5252) / Full‐Load rpm 2.5 Torque, Speed and Current Relation of a motor In addition to the relationship between speed and torque, the relationship of motor current to these two values is an important application consideration. The speed/torque curve is repeated below with the current curve added to demonstrate a typical relationship.


of 127

Two important points on this current curve need to be examined: 2.5. A‐ Full‐Load Current The full‐load current of an induction motor is the steady‐state current taken from the power line when the motor is operating at full‐load torque with rated voltage and rated frequency applied. 2.5. B‐ Locked‐Rotor Current Locked‐rotor current is the steady‐state current of a motor with the rotor locked and with rated voltage applied at rated frequency. NEMA has designated a set of code letters to define locked‐rotor KVA/HP. This code letter appears on the nameplate of all AC squirrel‐cage induction motors. KVA per horsepower is calculated as follows: For three‐phase motors: KVA/HP = √3 x current (in amperes) x volts / (1000 x Hp) For single‐phase motors: KVA/Hp = current (in amperes) x volts / (1000 x Hp)


of 127

The locked‐rotor kilovolt‐ampere‐per‐horsepower range includes the lower figure up to, but not including, the higher figure. For example, 3.14 is letter “A” and 3.15 is letter “B”. By manipulating the preceding equation for KVA/Hp for three‐phase motors the following equation can be derived for calculating locked‐rotor current: LRA = (1000 x Hp x Locked‐Rotor KVA/Hp) / (√3 x Volts) This equation can then be used to determine approximate starting current of any particular motor. For instance, the approximate starting current for a 7 1/2 Hp, 230 volt motor with a locked‐rotor KVA code letter G would be: LRA = (1000 x 7.5 x 6.0) / (√3 x230) = 113 Amps


of 127

3‐ Motor class 3.1 Poly‐phase Motors, 1‐200 HP NEMA has designated several specific types of motors (see Fig.1), each type having unique speed/torque relationships (see Fig.2) and these designs are described below along with some typical applications for each.

Fig (1): NEMA Motor Class

Fig (2): NEMA Motor Speed/torque Curves


of 127

Table (1): Locked‐Rotor Torque

The pull‐up torque for NEMA design A and B motors listed in the above Locked‐Rotor Torque Table with rated voltage and frequency applied will not be less than the following:

Table (2): Pull‐up Torque

Note: The pull‐up torque of Design C motors, with rated frequency and voltage applied will not be less than 70 percent of the locked‐rotor torque in the above Locked‐Rotor Torque Table.


of 127

Table (3): Breakdown Torque

3.2 Poly‐phase Motors Larger Than 500 HP Ratings larger than those listed in Tables 1, 2 and 3 are not covered by NEMA design letters, but have minimum torques established by NEMA MG1 as follows:

Locked‐rotor current of these designs will normally not exceed 650% of full‐load current, and will normally be within NEMA locked‐rotor KVA limits for a code G or H motor.


of 127

3.3 Single‐Phase Motors NEMA Design L NEMA design L motor is a single‐phase integral horsepower motor designed to withstand full‐voltage starting. Starting performance of these motors is dependent upon a “start” winding controlled by a centrifugal mechanism and switch.

Upon energization of the motor, both the “start” winding and the “run” winding of the motor are connected to the line. As the motor comes up to speed, the centrifugal mechanism will actuate and open the switch, removing the start provisions from the line. The rpm at which this occurs is known as “switch speed”. The motor will then operate at full‐load with only the run windings connected. Maximum locked‐rotor currents for Design L, 60 Hz motors are shown in the following table:

3.4 Service Factor Service factor is defined as the permissible amount of overload a motor will handle within defined temperature limits.


of 127

When voltage and frequency are maintained at nameplate rated values, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.However, locked‐rotor torque, locked‐rotor current and breakdown torque are unchanged. NEMA has defined service factor values for standard poly‐phase drip‐proof, 60 Hz motors as shown in the following table:

4‐ Motor Insulation Type NEMA has classified insulation systems by their ability to provide suitable thermal endurance. The total temperature is the sum of ambient temperature plus the motor’s temperature rise. The following image illustrate the temperature rise limits established for various insulation classes per NEMA MG1, Part 12.


of 127

Note: An additional 10 degrees C measured temperature rise is permitted when temperatures are measured by detectors embedded in the winding.

4.1 Motor Temperature A major consideration in both motor design and application is heat. Excessive heat will cause the following effects on motors:

1. Accelerate motor insulation deterioration and cause premature insulation failure.

2. Cause a breakdown of bearing grease, thus damaging the bearing system of a motor.

and, The total temperature a motor must withstand is the result of two factors:

1. External or ambient temperature. 2. Internal or motor temperature rise.

Most motors are designed to operate in a maximum ambient temperature of 40°C. If the ambient temperature exceeds 40°C, the motor may need to be modified to compensate for the increase in total temperature. The temperature rise is the result of heat generated by motor losses during operation as follows:

• At no‐load, friction in the bearings, core losses (eddy current and hysteresis losses), and stator I2R losses contribute to temperature rise.

• At full‐load, additional losses which cause heating are rotor I2R losses and stray load losses. (NOTE: I = current in amps and R = resistance of the stator or rotor in ohms).

Since current increases with an increase in motor load and under locked‐rotor, temperature rise will be significantly higher under these conditions. Therefore, applications requiring frequent starting and/or frequent overloads may require special motors to compensate for the increase in total temperature. 4.2 Motor Cooling Since the total temperature of a motor is greater than the surrounding environment, heat generated during motor operation will be transferred to the ambient air. The rate of heat transfer affects the maximum load and/or the duty cycle of a specific motor design. Factors affecting this rate of transfer are: a. Motor enclosure Different enclosures result in different airflow patterns which alter the amount of ambient air in contact with the motor.


of 127

b. Frame surface area Increasing the area of a motor enclosure in contact with the ambient air will increase the rate of heat transfer. General Electric motor enclosures often are cast with many ribs to increase their surface area for cooler operation. c. Airflow over motor The velocity of air moving over the enclosure affects the rate of heat transfer. Fans are provided on most totally‐enclosed and some open motors to increase the velocity of air over the external parts. d. Ambient air density A reduction in the ambient air density will result in a reduction of the rate of heat transfer from the motor. Therefore, total operating temperature increases with altitude. Standard motors are suitable for operation up to 3300 feet; motors with service factor may be used at altitudes up to 9900 feet at 1.0 service factor. 5‐ Motor Duty Cycle Applications The duty cycle of a motor describes the energization / de‐energization, and load variations, with respect to time for any given application. Duty cycle applications may be divided into three basic classifications: 1. Continuous duty It is a requirement of service that demands operation at an essentially constant load for an indefinitely long time.This is the most common duty classification and accounts for approximately 90% of motor applications. To size a motor for a specific application with this duty cycle classification, select proper horsepower based upon continuous load. 2. Intermittent duty It is a requirement of service that demands operation for alternate intervals of load and no‐load; or load and rest; or load, no‐load and rest; each interval of which is definitely specified.


of 127

Intermittent duty

Select a motor for these applications to match the horsepower requirements under the loaded condition. In some instances, such as a hoist or elevator application, savings in the purchase price of a motor may be possible by designing for the intermittent duty cycle. 30 or 60 minute motors are normally specified. Frequent starts, however, can increase motor heating.

3. Varying duty It is a requirement of service that demands operation at loads and for intervals of time, which may be subject to wide variation. For this classification of duty cycle, a horsepower versus time curve will permit determination of the peak horsepower required and a calculation of the root‐mean‐square (RMS) horsepower will indicate the proper motor rating from a heating standpoint. The following example demonstrates the method for selecting a motor for a varying duty cycle based upon peak horsepower and RMS horsepower requirements assuming constant frequency.

Example for a Varying duty Motor


of 127

6‐ Bearing Types

Bearings, mounted on the shaft, support the rotor and allow it to turn. Not all bearings are suitable for every application; a universal, all‐purpose bearing does not exist. The choice of bearing arrangement is based on the following qualities:

• Load carrying capacity in the axial and radial direction. • Overspeed and duration. • Rotating speed. • Bearing life.

The size of the bearing to be used is initially selected on the basis of its load carrying capacity, in relation to the load to be carried, and the requirements regarding its life and reliability. Other factors must also be taken into consideration, such as operating temperature, dirty and dusty environmental conditions, and vibration and shocks affecting bearings in running and resting conditions.


of 127

There are many types of bearings on the market, each with different characteristics and different uses, these types were listed before in topic “Electrical Motors Basic Components “and you can review them there. 7‐ Method of mounting the motor

7.1 Motor Mounting Configurations Motors may be furnished for any of twelve mounting configurations classified under 3 main mounting positions:

• Floor Mounting Positions, • Ceiling Mounting Positions, • Wall Mounting Positions.

A‐ Floor Mounting Positions The typical floor mounting positions are illustrated in fig(1) , and are referred to as F‐1 and F‐2 mountings.


of 127

Fig.1

The conduit box can be located on either side of the frame to match the mounting arrangement and position. The standard location of the conduit box is on the left‐hand side of the motor when viewed from the shaft end. This is referred to as the F‐1 mounting. The conduit opening can be placed on any of the four sides of the box by rotating the box in 90° steps. B‐ Ceiling Mounting Positions With modification, a foot‐mounted motor can be mounted on a ceiling. Typical ceiling mounts are shown in fig (2) . Ceiling mounted positions have the prefix C.

Fig.2

C‐ Wall Mounting Positions, With modification, a foot‐mounted motor can be mounted on a wall. Wall mounting positions have the prefix W. Wall mounting positions can be divided to two types according to the shaft direction as follows:


of 127

C.1 Shaft Horizontal Typical wall mounts, Shaft Horizontal are shown in the fig (3).

Fig.3

C.2 Shaft Vertical Typical wall mounts, Shaft Vertical are shown in the fig (4).

fig.4

Note: Motors which are to be mounted vertically must be specified shaft up or down to obtain a suitable bearing and lubrication system.


of 127

7.2 Motor Mounting Faces

Fig.5

It is sometimes necessary to connect the motor directly to the equipment it drives as in fig (5) where a motor is connected directly to a gear box. The motor be connected to the equipment by one of the following two methods: (1) C‐face The face, or the end, of a C‐face motor has threaded bolt holes. Bolts to mount the motor pass through mating holes in the equipment and into the face of the motor.

(2) D‐flange The bolts go through the holes in the flange of a D‐flange motor and into threaded mating holes of the equipment.


of 127

8‐ The cost and size of the motor This point will be explained later in our “Motor advanced course” that will be issued shortly. 9‐ Method of Motor speed control There are four major motor control topics or categories to consider. Each of these has several subcategories and sometimes the subcategories overlap to some extent. These four categories are:

• Motor Starting, • Motor Protection, • Motor Stopping, • Motor Operational Control.

The motor speed control is classified under the forth category “Motor Operational Control” which – and other categories‐ will be explained in detail in our “Motor advanced course” that will be issued shortly. However, Motor speed controls can be divided into two basic categories:

• Passive device speed controls, • Solid state controls.

A‐ Passive device speed controls: Passive device controls consist of fixed or variable resistors, or variable transformers that are used to adjust the magnetic field strength, voltage levels or other motor characteristics (depending on the motor type), in order to control motor speed.


of 127

Passive device speed controls

Passive devices such as resistors increase the motor circuit resistance, causing increased power dissipation in the form of heat. This additional heat produces no useful work and decreases the overall efficiency of the system. B‐ Solid state speed controls: Solid state controls utilize more complex circuits consisting of active devices like diodes, thyristors, transistors, integrated circuits and in some cases, microprocessors to control motor voltage, power supply frequency, or to provide electronic commutation and thereby control motor speed.

Solid state speed controls


of 127

With the development of semiconductors, it became possible to vary motor speed through voltage switching rather than by adding resistance to the drive circuit. Instead of varying the level of resistance, switching amplifiers vary the time during which full line voltage is applied to the armature. The net effect is an average voltage which is roughly equivalent to a voltage level obtained by the variable resistance‐ type control. Examples for Solid state controls are:

• Half‐Wave SCR Controls. • Full‐Wave SCR Controls. • Pulse Width Modulation Control.

10‐ Environmental conditions

10.1 Motor enclosures


of 127

The type of enclosure required is dependent upon the surrounding atmosphere in which the motor is installed and the amount of mechanical protection and corrosion resistance required. The two general classes of motor enclosure are: 1‐ Open enclosure An open machine is one having ventilating openings which permit passage of external air over and around the winding of the motor. 2‐ totally‐enclosed enclosure A totally‐enclosed machine is constructed to prevent the free exchange of air between the inside and outside of the motor, but not sufficiently enclosed to be termed air‐tight. I explained the types of motor enclosures in topic “Electrical Motors Basic Components “and you can review them there. 10.2 Altitude The rating of standard motors assumes operation at sea level in a 40°C ambient. For purposes of standardization it is considered that there is no difference in motor operating temperature between sea level and 3300 feet altitude. The cooling effect of ventilating air is a function of its density. The atmospheric pressure and density at higher altitudes is reduced and the air cannot remove as much motor heat, causing the motor to run hotter. As a general guide, motor temperature rise increases 1% for every 330 feet above 3300 feet. To keep motor heating within safe limits at altitudes above 3300 feet, there are the following alternatives: A. Supply a motor designed for standard sea level operation which can either be: (1) Operated at less load (a motor with service factor rating of 1.15 or higher can be operated at altitudes up to 9000 feet with a 1.0 service factor), or (2) Operated in a lower ambient temperature per the following table:


of 127

Notes:

• It should be remembered that, although the outdoor ambient temperature at higher altitudes is low, motors probably will be installed indoors in higher ambient temperatures.

• Motors applied per A(1) or A(2) above, will have no special altitude or temperature data on the nameplate.

10.3 Ambient Temperatures Standard motors are designed so that the temperature rise produced within the motor, added to the standard 40°C ambient temperature, will not exceed the winding‐insulation temperature limit. The motor may work in two up‐normal ambient temperature as follows: 1‐ High Ambient (If the ambient temperature exceeds 40°C) A. in this case, The temperature rise produced in the motor must be offset by:

• Reducing the load and consequent motor losses. A motor rated for a 40°C ambient temperature and operating in a 50°C ambient, will, if rated 1.15 service factor, carry rated Hp with no overload (1.0 SF) and, if rated 1.0 service factor, carry 90% of rated Hp.

• Applying a special motor design.

B. The temperature limit may be raised by the substitution of a higher temperature insulation system, special grease and bearings. Notes:

• Motors applied per A(1) above will not have special ambient temperature or service factor data on the nameplate.

• The choice between A(2) and B rests with the motor designer who also may have to use a frame size larger than is standard for the rating. Explosion‐


of 127

proof motors may require frame sizes different from the corresponding totally‐enclosed motors.

Warning:

• The maximum allowable ambient temperature for explosion‐proof machines is 60°c.

2‐ Low Ambient (If the ambient temperature is less than minus 40°C) In this case, a Special low‐temperature grease and special steel shafts may be required.


of 127

DC Motors Selection Procedures

Choosing a dc motor type and associated equipment for a given application requires consideration of several factors which are: 1‐ Speed range The minimum and maximum speeds for an application determine the motor base speed. 2‐ Allowable speed variation Applications requiring constant speed at all torque values should use a shunt‐wound motor. If speed changes with load and speed variation must be minimized to less than 2%, a regulator employing tachometer feedback must be used. 3‐ Torque requirements • The torque requirements at various operating speeds should be determined.

• Many applications are essentially constant torque, such as conveyors.

• Others, such as centrifugal blowers, require torque to vary as the square of the speed. In contrast, machine tools and center winders are constant horsepower, with torque decreasing as speed increases. Thus, the speed‐torque relationship determines the most economical motor.


of 127

4‐ Reversing

• This operation affects the power supply and control. • When the motor cannot be stopped for switching series fields before reverse

operation, compound and stabilizing windings should not be used if full load torque is needed in both directions.

• Bi‐directional operation may also affect brush adjustments.

5‐ Duty rating DC motors carry one of three ratings:

• Continuous duty is applied to motors that will continuously dissipate all the heat generated by internal motor losses without exceeding rated temperature rise.

• Definite time, intermittent duty motors will carry rated load for specified time without exceeding rated temperature rise. These motors must be allowed to cool to ambient before load is repeated.

• Indefinite time, intermittent duty is usually associated with some RMS load of a duty‐cycle operation.

6‐ Peak torque

• The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Brush and commutator damage depends on sparking severity and duration. Therefore, peak torque depends on the duration and frequency of occurrence of the overload.

• Peak torque is often limited by the maximum current that the power supply can deliver.

• Motors can commutate greater loads at low speed without damage. • NEMA standards specify that dc machines must deliver at least 150% rated

current for one minute at any speed within rated range, but most motors exceed this requirement.

7‐ Heating

• The temperature of a dc motor is a function of ventilation and losses in the machine. Some losses in core, shunt‐field, and brush friction are independent of load, and vary with speed and excitation.

• Several methods can predict operating temperature. The best method is to use thermal capability curves available from the manufacturer.


of 127

This is the end of our course "Introduction to Motors Basics”. So, please keep following our new course for motors”Motors Advanced Course".

Course motor 1-an introduction to electrical motors basics

Design

Transcript of Course motor 1-an introduction to electrical motors basics