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Overview

DC Motors (Brushed and Brushless)

Stepper Motors

Sean DeHart

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Electric Motor Basic Principles Interaction between magnetic field and

current carrying wire produces a force Opposite of a generator

Sean DeHart

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Conventional (Brushed) DC Motors Permanent magnets

for outer stator Rotating coils for

inner rotor  Commutation

performed with metal contact brushes and contacts designed to reverse the polarity of the rotor as it reaches horizontal

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2 pole brushed DC motor commutation

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Conventional (Brushed) DC Motors Common Applications:

Small/cheap devices such as toys, electric tooth brushes, small drills

Lab 3 Pros:

Cheap, simpleEasy to control - speed is governed by the

voltage and torque by the current through the armature

Cons:Mechanical brushes - electrical noise, arcing,

sparking, friction, wear, inefficient, shorting

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DC Motor considerations Back EMF - every motor is also a generator More current = more torque; more voltage = more speed Load, torque, speed characteristics

Shunt-wound, series-wound (aka universal motor), compound DC motors

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Brushless DC Motors Essential difference - commutation is

performed electronically with controller rather than mechanically with brushes

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Brushless DC Motor Commutation

Commutation is performed electronically using a controller (e.g. HCS12 or logic circuit)Similarity with stepper motor, but with less #

polesNeeds rotor positional closed loop feedback:

hall effect sensors, back EMF, photo transistors

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Delta               Wye

BLDC (3-Pole) Motor Connections

Has 3 leads instead of 2 like brushed DC Delta (greater speed) and Wye (greater

torque) stator windings 

Sean DeHart

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Brushless DC Motors Applications

CPU cooling fansCD/DVD PlayersElectric automobiles

Pros (compared to brushed DC)Higher efficiencyLonger lifespan, low maintenanceClean, fast, no sparking/issues with brushed

contacts Cons

Higher costMore complex circuitry and requires a controller

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Stepper Motor

A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.

Smriti Chopra

Main features

The sequence of the applied pulses is directly

related to the direction of motor shafts rotation.

The speed of the motor shafts rotation is directly related to the frequency of the input pulses.

The length of rotation is directly related to the number of input pulses applied.

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Stepper Motor Characteristics Open loop The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control. Brushless Very reliable since there are no contact brushes in the motor.

Therefore the life of the motor is simply dependant on the life of the bearing.

Incremental steps/changes The rotation angle of the motor is proportional to the input

pulse. Speed increases -> torque decreases

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Torque vs. Speed

Torque varies inversely with speed.

Current is proportional to torque.

Torque → ∞ means Current → ∞, which leads to motor damage.

Torque thus needs to be limited to rated value of motor.Smriti Chopra

Disadvantages of stepper motors

There are two main disadvantages of stepper motors:

Resonance can occur if not properly controlled. This can be seen as a sudden loss or drop in torque at certain

speeds which can result in missed steps or loss of synchronism. It occurs when the input step pulse rate coincides with the natural oscillation frequency of the rotor. Resonance can be minimised by using half stepping or microstepping.

Not easy to operate at extremely high speeds.

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Working principle

Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator.

When a phase winding of a stepper motor is energized with current, a magnetic flux is developed in the stator. The direction of this flux is determined by the “Right Hand Rule”.

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At position 1, the rotor is beginning at the upper electromagnet, which is currently active (has voltage applied to it).

To move the rotor clockwise (CW), the upper electromagnet is deactivated and the right electromagnet is activated, causing the rotor to move 90 degrees CW, aligning itself with the active magnet.

This process is repeated in the same manner at the south and west electromagnets until we once again reach the starting position. Smriti Chopra

Understanding resolution

Resolution is the number of degrees rotated per step.

Step angle = 360/(NPh * Ph) = 360/N

NPh = Number of equivalent poles per phase = number of rotor poles.

Ph = Number of phases. N = Total number of poles for all phases

together.

Example: for a three winding motor with a rotor having 4 teeth, the resolution is 30 degrees.

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Two phase stepper motors

There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.

unipolar bipolarSmriti Chopra

A unipolar stepper motor has two windings per phase, one for each direction of magnetic field. In this arrangement a magnetic pole can be reversed without switching the direction of current.

Bipolar motors have a single winding per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole.

Bipolar motors have higher torque but need more complex driver circuits.

Main difference

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Stepping modes

Wave Drive (1 phase on)A1 – B2 – A2 – B1(25% of unipolar windings , 50% of bipolar)

Full Step Drive (2 phases on)A1B2 – B2A2 – A2B1 – B1A1(50% of unipolar windings , full bipolar windings utilization)

Half Step Drive (1 & 2 phases on)A1B2 – B2 – B2A2 – A2 ----(increases resolution)

Microstepping (Continuouslyvarying motor currents)A microstep driver may split a full step into as many as 256 microsteps.

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Types of Stepper Motors

There are three main types of stepper motors:

Variable Reluctance stepper motor

Permanent Magnet stepper motor

Hybrid Synchronous stepper motor

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This type of motor consists of a soft iron multi-toothedrotor and a wound stator.

When the stator windings are energizedwith DC Current, the poles become magnetized.

Rotation occurs when the rotor teethare attracted to the energized statorpoles.

Variable Reluctance motor

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Permanent Magnet motor

The rotor no longer has teeth as withthe VR motor.

Instead the rotor ismagnetized with alternating northand south poles situated in a straightline parallel to the rotor shaft.

These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type.

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Hybrid Synchronous motor

The rotor is multi-toothed like the VR motor andcontains an axially magnetized concentricmagnet around its shaft.

The teeth on the rotor provide an evenbetter path which helps guide themagnetic flux to preferred locations inthe air gap.

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Applications

Stepper motors can be a good choice whenever controlled movement is required.

They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism.

These include printers plotters medical equipment fax machines automotive and scientific equipment etc.

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3phase induction motor-Not a variable speed motor 1 Phase induction motor-Not self starting, poor

power factor, efficiencyCommon single phase commutator motors are1.Series motors2.Universal motors3.Repulsion motors4.Repulsion –induction motors

If we connect normal dc series motor to ac what happens?1. Torque developed is not constant Magnitude2. Alternating flux induce eddy currents causing heat

and there by loss3. No inductive coupling between armature and field

since they are placed in quadrature4. Sparking in brushes is more due to transformer emf

induced5. Due to large voltage drop speed reduced6. Starting Torque is low, low pf

Modification needed for ac series motor.1. To reduce eddy current loss-Laminations used2. To reduce reactance-series field should contains less

number of turns3. To improve torque no. of armature conductors should

be large4. To reduce reactance compensating winding should be

used5. Operating voltage kept low to reduce inductance6. Reduce frequency to reduce inductance7. Interpoles to reduce armature resistance leads

ConstructionChanges should be employed to work in both ac and dc

OperationWhen motor is connected to an a.c. supply, the same alternating current flowsthrough the field and armature windings. The field winding produces an alternating flux that reacts with the current flowing in the armature to produce a torque. Since both armature current and flux reverse simultaneously, the torque always acts in the same direction. It may be noted that no rotating flux is produced in this type of machines; the principle of operation is the same as that of a d.c. series motor.

CharacteristicsThe operating characteristics of an a.c. series motor are similar to those of a d.c. series motor.(i) The speed increases to a high value with a decrease in load. In very small series motors, the losses are usually large enough at no load that limit the speed to a definite value (1500 - 15,000 r.p.m.).(ii) The motor torque is high for large armature currents, thus giving a highstarting torque.(iii) At full-load, the power factor is about 90%. However, at starting or whencarrying an overload, the power factor is lower

ApplicationsThe fractional horsepower a.c. series motors have high-speed (andcorresponding small size) and large starting torque. They can, therefore, be usedto drive:(a) high-speed vacuum cleaners (b) sewing machines(c) electric shavers (d) drills(e) machine tools etc.

A repulsion motor is similar to an a.c. series motor except that:

(i) brushes are not connected to supply but are short-circuited - currents are induced in the armature conductors by transformer action.

(ii)the field structure has non-salient pole construction.By adjusting the position of short-circuited brushes on the

commutator, the starting torque can be developed in the motor

ConstructionThe field of stator winding is wound like the main winding of a split-phasemotor and is connected directly to a single-phase source. The armature or rotor is similar to a d.c. motor armature with drum type winding connected to a commutator However, the brushes are not connected to supply but are connected to each other or short-circuited. Short-circuiting the brushes effectively makes the rotor into a type of squirrel cage. The major difficulty with an ordinary single-phase induction motor is the low starting torque. It has also better power factor than the conventional single-phase motor.

Construction

Principle of operation

The total armature torque in a repulsion motor can be shown to be

Ta = sin 2αwhere α = angle between brush axis and stator field axisFor maximum torque, 2α = 90° or α = 45°Thus adjusting α to 45° at starting, maximum torque can be obtainedduring the starting period. However, α has to be adjusted to give a suitable running speed.

Characteristics(i) The repulsion motor has characteristics very similar to those of an a.c. series motor i.e., it has a high starting torque and a high speed at no load.

(ii) The speed which the repulsion motor develops for any given load will depend upon the position of the brushes.

(iii) In comparison with other single-phase motors, the repulsion motor has a high starring torque and relatively low starting current.

Sometimes the action of a repulsion motor is combined with that of a single phase induction motor to produce repulsion-start induction-run motor (also called repulsion-start motor).

The machine is started as a repulsion motor with a corresponding high starting torque.

At some predetermined speed, a centrifugal device short-circuits the commutator so that the machine then operates as a single-phase induction motor.

This motor has the same general construction of a repulsion motor. The only difference is that it is equipped with a centrifugal device fitted on the armature shaft. When the motor reaches 75% of its full pinning speed, the centrifugal device forces a short-circuiting ring to come in contact with the inner surface of the commutator. This short-circuits all the commutator bars. The rotor then resembles squirrel-cage type and the motor runs as a single-phase induction motor. At the same time, the centrifugal device raises the brushes from thecommutator which reduces the wear of the brushes and commutator as well as makes the operation quiet.

Characteristics(i) The starting torque is 2.5 to 4.5 times the full-load torque and the startingcurrent is 3.75 times the full-load value.(ii) Due to their high starting torque, repulsion-motors were used to operatedevices such as refrigerators, pumps, compressors etc.However, they posed a serious problem of maintenance of brushes, commutatorarid the centrifugal device.

The repulsion-induction motor produces a high starting torque entirely due to repulsion motor action. When running, it functions through a combination of induction-motor and repulsion motor action.

ConstructionIt consists of a stator and a rotor (or armature).(i) The stator carries a single distributed winding fed from single-phase supply.(ii) The rotor is provided with two independent windings placed one inside theother. The inner winding is a squirrel-cage winding with rotor barspermanently short-circuited. Placed over the squirrel cage winding is arepulsion commutator armature winding. The repulsion winding is connected to a commutator on which ride short-circuited brushes. There is no centrifugal device and the repulsion winding functions at all times.

Operation(i) When single-phase supply is given to the stator winding, the repulsionwinding (i.e., outer winding) is active. Consequently, the motor starts as arepulsion motor with a corresponding high starting torque.(ii) As the motor speed increases, the current shifts from the outer to innerwinding due to the decreasing impedance of the inner winding withincreasing speed. Consequently, at running speed, the squirrel cagewinding carries the greater part of rotor current. This shifting of repulsion motor action to induction-motor action is thus achieved without any switching arrangement.

Characteristics(i) The no-load speed of a repulsion-induction motor is somewhat above thesynchronous speed because of the effect of repulsion winding. However,the speed at full-load is slightly less than the synchronous speed as in aninduction motor.(ii) The speed regulation of the motor is about 6%.(iii) The starting torque is 2.25 to 3 times the full-load torque; the lower valuebeing for large motors. The starting current is 3 to 4 times the full-loadcurrent.This type of motor is used for applications requiring a high starting torque withessentially a constant running speed. The common sizes are 0.25 to 5 H.P.

Very small single-phase motors have been developed which run at truesynchronous speed. They do not require d.c. excitation for the rotor. Because of these characteristics, they are called unexcited single-phase synchronous motors.The most commonly used types are:(i) Reluctance motors (ii) Hysteresis motorsThe efficiency and torque-developing ability of these motors is low; The output of most of the commercial motors is only a few watts

It is a single-phase synchronous motor which does not require d.c. excitation to the rotor. Its operation is based upon the following principle:Whenever a piece of ferromagnetic material is located in a magnetic field; a force is exerted on the material, tending to align the material so that reluctance of the magnetic path that passes through the material is minimum.

Construction(i) a stator carrying a single-phase winding along with an auxiliary winding to produce a synchronous-revolving magnetic field.(ii) a squirrel-cage rotor having unsymmetrical magnetic construction. This is achieved by symmetrically removing some of the teeth from the squirrel cage rotor to produce salient poles on the rotor.The salient poles created on the rotor must be equal to the poles on the stator.Note that rotor salient poles offer low reluctance to the stator flux and,therefore, become strongly magnetized.

Operation(i) When single-phase stator having an auxiliary winding is energized, asynchronously-revolving field is produced. The motor starts as a standardsquirrel-cage induction motor and will accelerate to near its synchronousspeed.(ii) As the rotor approaches synchronous speed, the rotating stator flux willexert reluctance torque on the rotor poles tending to align the salient-poleaxis with the axis of the rotating field. The rotor assumes a position whereits salient poles lock with the poles of the revolving field (ii)) Consequently, the motor will continue to run at the speed ofrevolving flux i.e., at the synchronous speed.

Operation(iii) When we apply a mechanical load, the rotor poles fall slightly behind thestator poles, while continuing to turn at synchronous speed. As the load on the motor is increased, the mechanical angle between the poles increases progressively. Nevertheless, magnetic attraction keeps the rotor locked to the rotating flux. If the load is increased beyond the amount under which the reluctance torque can maintain synchronous speed

Characteristics(i) These motors have poor torque, power factor and efficiency.(ii) These motors cannot accelerate high-inertia loads to synchronous speed.(iii)The pull-in and pull-out torques of such motors are weak.

Despite the above drawbacks, the reluctance motor is cheaper than any othertype of synchronous motor. They are widely used for constant-speedapplications such as timing devices, signaling devices etc

It is a single-phase motor whose operation depends upon the hysteresis effect i.e., magnetization produced in a ferromagnetic material lags behind the magnetizing force.

Construction (i) a stator designed to produce a synchronously-revolving field from asingle-phase supply. This is accomplished by using permanent-splitcapacitor type construction. Consequently, both the windings (i.e., startingas well as main winding) remain connected in the circuit during runningoperation as well as at starting. The value of capacitance is so adjusted as toresult in a flux revolving at synchronous speed.(ii) a rotor consisting of a smooth cylinder of magnetically hard steel, without winding or teeth.

Operation(i) When the stator is energized from a single-phase supply, a synchronously revolving field (assumed in anti-clockwise direction) is produced due to split-phase operation.(ii) The revolving stator flux magnetizes the rotor. Due to hysteresis effect, the axis of magnetization of rotor will lag behind the axis of stator field byhysteresis lag angle ,the rotor and stator poles are locked. If the rotor is stationary, the starting torque produced is given by:

OperationFrom now onwards, the rotor accelerates to synchronous speed with a uniform torque.(iii) After reaching synchronism, the motor continues to run at synchronous speed and adjusts its torque angle so as to develop the torque required by the load.

Characteristics(i) A hysteresis motor can synchronize any load which it can accelerate, nomatter how great the inertia. It is because the torque is uniform fromstandstill to synchronous speed.(ii) Since the rotor has no teeth or salient poles or winding, a hysteresis motoris inherently quiet and produces smooth rotation of the load.(iii) The rotor takes on the same number of poles as the stator field. Thus bychanging the number of stator poles through pole-changing connections,we can get a set of synchronous speeds for the motor.

ApplicationsDue to their quiet operation and ability to drive high-inertia toads, hysteresismotors are particularly well suited for driving (i)electric clocks(ii)Timing devices (iii) tape-decks (iv)from-tables and other precision audio-equipment.