Motors & Motor StartersMotors & Motor Starters

Prepared By:

Erik Redd

&

Jeremy Roberts

MotorsMotors

AC-MotorsParts of an Electric Motor

A. Stator : Stationary FrameB. Rotor : Revolving Part

The rotary motion in an ac-motor is caused by the fundamental law of magnetism.

This law states that like poles repel and unlike poles attract.

Diagram of an ac-motor Diagram of an ac-motor

This shows a three phase, two pole stator.

Where A, B, and C are the three phases

Diagram of the Three PhasesDiagram of the Three Phases

Fig. 13-2 Pg. 244 Poles 1 and 4 are at their greatest magnetic field at

time equal to one, because phase A (red line) is connected to those poles, and the same for the other poles when their corresponding phases are at maximum current magnitude.

Synchronous SpeedSynchronous Speed

Speed at which it takes the motor to go one cycle and one revolution.

S=[120*frequency}] (# poles)

Example:For a three-phase, 60 Hertz, 2 pole motor:S=[120*60]/2=3600 revolutions per minute

Polyphase Squirrel-Cage Polyphase Squirrel-Cage Induction MotorsInduction Motors

• The most common three-phase motor

• Does not have solid poles• Instead, it has laminations:

numerous flat sheets held together in a package. They are insulated from each other (this reduces Eddy currents) making up the stator

• The difference between induction and synchronous motors is that the rotor for an induction motor can travel at a different speed than the stator. This is called Slip.

• slip= Syn. rpm – Motor rpm *100 Syn. rpm

Example.

A 2 pole, 60 Hz motor runs at a full-load speed of 1760 rpm.

What is the slip?

Ans. %slip= 3600-1760*100

3600

=51.1%

Single-Phase MotorsSingle-Phase Motors

Supplied by single source of ac voltage Rotor must be spun by hand in either direction,

does not have a starting mechanism Has no starting torque Three different types of single-phase motors: split-

phase, capacitor start, permanent split-capacitor, and shaded-pole motors

Resistance Split-Phase MotorsResistance Split-Phase Motors

Has a start winding and a main winding

Winding currents are out of phase by 30 degrees, this produces a flux field that starts the motor

• Main winding current (IM) and start winding current (IS) lags supply voltage (VL)

Start (inrush) current is high Needs centrifugal starting

switch or relay to disconnect the start winding (protects it from over heating)

Efficiency is between 50-60%

Capacitor-Start MotorsCapacitor-Start Motors

Has the same winding and switch mechanism arrangement as split-phase but adds a short time-rated capacitor in series with the start winding

The time shift phase between the main and start winding is close to 90 degrees

IS leads VL Efficiency is between 50-65% Capacitor controls the inrush

current

Permanent Split-Capacitor Permanent Split-Capacitor MotorsMotors

Winding arrangement is the same as the capacitor and split-phase motors

Capacitor can run continuously, rated in microfarads for high-voltage ratings

No centrifugal switch is needed IM lags VL, while IS leads VL

• Efficiency is between 50-70%

Shaded Pole MotorsShaded Pole Motors

Simple construction, least expensive

Has a run winding only, shading coils are used instead of the start winding

Stator is made up of a salient pole, one large coil per pole, wound directly in a single large slot

A small shift in the rotor causes torque and starts the motor

Efficiency is between 20-40%

DC MotorsDC Motors

• Consists of an armature winding and a stator winding

• Armature windings act as the rotor• Has three different classifications: constant torque,

constant horsepower, or a combination of the two• Standard industrial dc motors are shunt wounded• Modifications of the dc motor are: shunt wound,

stabilized shunt exciting fields, compound wound motors, and series wound motors

Armature Voltage ControlArmature Voltage Control

Is used for motor speeds below base speedOutput torque= T=k*ø*IA

k is machine constant

ø is the main pole flux

IA is the armature current

Shunt Field ControlShunt Field Control

Is used for motor speeds above base speedHorsepower, (HP)= Torque*rpm

5252

Where torque is in lb-ft

Speed RegulationSpeed Regulation

Speed Regulation (IR)= no load rpm- full load rpm

full load rpm

Brushless DC MotorsBrushless DC Motors

Three phase ac power is converted into dc by the input side of the motor to charge up a bank of storage capacitors

These capacitors are called the Buss The purpose of the buss is to store energy

and supply dc power to transistors in the output side as the motor requires the power to start up

Brushless DC MotorsBrushless DC Motors

Figure 13-21, page 264 shows the input power section

It consists of three fuses, six diodes, a choke, and two capacitors

The fuses protect the diodes The choke protects against line transients The motor control may run at very low speeds at

very high torques while drawing little current from the ac line

Brushless DC MotorsBrushless DC Motors

This picture is a representation of the encoders (rotor part of the motor) telling the corresponding transistors (stator) to turn on in order to get maximum torque from the motor

Picture of a Brushless MotorPicture of a Brushless Motor

Motor Control StartersMotor Control Starters

Motor will draw high inrush current while the starter will slow current down

Starter reduces the amount of torque needed to start the motor

Magnetic Motor StarterMagnetic Motor Starter

Normally open contactsNot always possible to control amount of

work applied to the motorHas overloads

– Motor may be overloaded resulting in damage to the motor

– Open due to excessive motor current, high temperature, or a combination of both

Full-Voltage StarterFull-Voltage Starter

Contains one set of contacts

Motor is directly connected to the line voltage

Reversing Motor StarterReversing Motor Starter

Contains two starters of equal size Two starters connect to the motor Interlocks are used to prevent both starters from

closing their line contacts at the same time Figure 14-4A

Reduced-voltage Motor StarterReduced-voltage Motor Starter

Applies a percentage of the total voltage to start (50% - 80%)

After motor rotates, switching is provided to apply full voltage

Torque will be reduced when starting Four types:

1) Autotransformer2) Primary Resistance3) Wye – Delta4) Part Winding

Autotransformer StarterAutotransformer Starter

Two contactors are used:1) Start contactor

- Closes first and connects motor to the line through an autotransformer

- Deenergizes2) Run contactor

- Motor switches to this contacter which has full voltage

Primary Resistor StarterPrimary Resistor Starter

Two contactor 1) Line contactor - First to energize connecting motor to the line voltage through a resistor

- After preset time, contactor opens2) Accelerating contactor - Energizes

- Causes smooth acceleration to full voltage

Wye – Delta StarterWye – Delta Starter

Three contactors are used 1) Line contactor and start contactor

- Energizes first and connects motor in wye

putting about 58% of line voltage across

each motor phase

- Contacts open after preset time

2) Run contactor

- Energizes connecting motor in delta and

putting full voltage on the motor

Part Winding StarterPart Winding Starter

Starter supplies about 48% of normal starting torque Not truly a reduced-voltage means Two Types

1) Two-Step - one winding connected to full voltage line and, after a preset time, the other connects 2) Three-Step – one winding is connected in series with a resistor to the voltage line; after interval, resistor is shorted out and then second line is connected to full voltage line

Solid-State Motor StarterSolid-State Motor Starter

For lower starting torque and smooth acceleration

Used on conveyors, pumps, compressors, etc.

Standard Modes of OperationStandard Modes of Operation

Motor voltage gradually increases during acceleration

Creates a kick start pulse of 500% of full load amperage for high friction

Used when necessary to limit currentUsed when motor requires a full voltage

start