Unit 33 Three-Phase Motors -...

Unit 33

Three-Phase Motors

Unit 33 Three-Phase Motors

Objectives:

• Discuss the basic operating principles of

three-phase motors.

• List factors that produce a rotating

magnetic field.

• List different types of three-phase motors.


Objectives:

• Discuss the operation of squirrel-cage motors.

• Show connection of dual-voltage motors for proper operation on the desired voltage.

• Discuss the operation of consequent pole motors.


Objectives:

• Discuss the operation of wound rotor motors.

• Discuss the operation of selsyn motors.

• Discuss the operation of synchronous

motors.

• Determine the direction of rotation of a three-

phase motor using a phase rotation meter.


• Three-phase motors are used throughout the U.S. and Canada as the prime mover for industry.

• These motors convert three-phase AC into mechanical energy to operate all types of machinery.

• They are smaller, lighter, and have higher efficiencies per horsepower than single-phase motors.


• Three-phase motors are extremely rugged

and require minimal maintenance.

• These motors can be operated 24/7 for

years without problems.

• Nikola Tesla patented the first induction

motors as rotating transformers.


Construction

There are three basic types of three-

phase motors:

1. squirrel-cage induction motor

2. wound-rotor induction motor

3. synchronous motor


Rotating Magnetic Field

• The principle of operation for all three-phase motors is the rotating magnetic field.

• The magnetic field rotation is caused by:

– voltages are 120° out of phase.

– voltages periodically change polarity.

– the arrangement of the stator windings.


Three-phase stator

and three voltage

sine waves.


The magnetic field

is concentrated

between poles A1

and A2.


The magnetic

field is

concentrated

between poles

of phases A and

B.


The magnetic field

is concentrated

between poles B1

and B2.


The magnetic field

is concentrated

between phases B

and C.


The magnetic field is

concentrated

between poles C1

and C2.


The magnetic field is

concentrated

between phases A

and C.


The magnetic field

is concentrated

between poles A1

and A2. The field

has rotated 180°.


The magnetic

field is

concentrated

between

phases B and

C and has

rotated 270°.


The magnetic

field is

concentrated

between poles A1

and A2 and has

rotated 360°.


Synchronous Speed

• Synchronous speed is the rotational

speed of the magnetic field.

• Synchronous speed is determined by:

– the number of stator poles per phase.

– the frequency of the applied voltage.


RPM STATOR POLES

3600 2

1800 4

1200 6

900 8


Synchronous Speed

• S = (120 x F) / P

• S = synchronous speed in RPM

• F = frequency in Hz

• P = number of stator poles


Phase Rotation

• The direction of rotation is either clockwise or counterclockwise.

• Reversing any two of the stator leads will reverse the direction of rotation.

• A phase rotation meter can determine the direction of rotation.

• Motor stator leads are often called T leads.


Connecting the phase rotation meter to the motor.


Connecting the phase rotation meter to the line.


Dual-Voltage Motors

• Many motors are designed to operate on two different voltages, such as 240 V and 480 V.

• This type of motor has two windings for each phase.

• Most dual-voltage motors bring out 9 leads to the terminal box.


Dual-Voltage Motors

• The other 3 leads are connected internally.

• Review: There are two connection leads per winding; there are two windings per phase; there are three phases. This makes 12 connection leads. Of these 12 leads 9 are usually brought out to the terminal box, 3 are connected internally.


Dual-Voltage Motors

• When motors are connected to their

higher-rated voltage on the name plate, a

high-voltage connection pattern is

required.

• When motors are connected to their

lower-rated voltage on the name plate, a

low-voltage connection pattern is required.


Dual-Voltage Motors

• The identification of connection leads is

standardized to T1 through T12.

• The correct connection patterns are

usually shown on the motor name plate.

• The NEC® states the required name plate

data.


Standard numbering for three-phase motors.


High-Voltage Connections

• High-voltage connections require the windings to be series configured.

• The high-voltage connections can be either wye or delta, depending on how the motor was constructed and designed.

• A terminal chart is another way to identify proper T lead connections.


Standard high-voltage wye connections.


Standard high-voltage delta connections.


Low-Voltage Connections

• Low-voltage connections require the windings to be parallel configured.

• The low-voltage connections can be either wye or delta, depending on how the motor was constructed and designed.

• A terminal chart is another way to identify proper T lead connections.


Standard low-voltage wye schematic.


Standard low-voltage wye chart and diagram.


Standard low-voltage delta schematic.


Standard low-voltage delta chart and diagram.


12-Lead Dual-Voltage Motors

• Some motors will have 12 T leads brought to the terminal box instead of the usual 9 leads.

• These motors are intended for wye-delta starting.

• Wye-delta starting helps limit inrush starting current.


Standard 12-lead motor schematic.


Squirrel-Cage Induction Motors

• The rotor on this type of motor resembles a squirrel cage.

• The rotor contains bars connected to the end rings.

• The current flow in the rotor is produced by induced voltage from the rotating magnetic field of the stator.


Basic squirrel-cage rotor without laminations.


Basic squirrel-cage rotor cutaway view.


Torque

• Three factors determine the amount of motor torque:

– the strength of the stator magnetic fields.

– the strength of the rotor magnetic fields.

– the phase angle difference between the rotor and stator fields.


Slip

• An induction motor never reaches

synchronous speed.

• Slip is the difference between

synchronous speed and rotor speed.

• Percent slip is the ratio of slip to

synchronous speed times 100.


Wound-Rotor Induction Motor

• This motor is very popular in industry because of its high starting torque and low starting current.

• A squirrel-cage motor and a wound-rotor motor have similar stator windings.

• The rotor has wire windings instead of bars.


External resistors are connected to the rotor of a

wound-rotor motor.


Wound-rotor motor schematic symbol.


Synchronous Motors

• This motor is not an induction motor. It does not depend on induced current in the rotor to produce a torque.

• It operates at constant speed from no load to full load.

• This motor must have DC excitation to operate.


Synchronous motor with DC excitation supplied

through sliprings.


Synchronous Motors

• The operating speed and the speed of the rotating magnetic field (synchronous speed) are the same.

• It operates at constant speed from no load to full load.

• This motor can be used for power factor correction.


Synchronous Motors

• A set of squirrel-cage bars known as the amortisseur winding are used to start the synchronous motor.

• A synchronous motor must never be started with DC current connected to the rotor.

• A field-discharge resistor is used to safely control excessive current and voltage.


The field-discharge resistor is connected in parallel

with the rotor winding during starting.


Synchronous motor schematic.


Selsyn Motors

• Selsyn motors are used for position control and angular feedback information.

• Selsyn motors contain three-phase windings, although they operate on single-phase AC.

• A differential selsyn unit can be used to determine the algebraic rotation sum of two other selsyn units.


Selsyn motor schematic.


Selsyn motor schematic symbol.


Schematic of two selsyn motors connected.


Schematic of differential selsyn motor connections.


Review:

1. The basic types of three-phase motors

are:

– squirrel cage induction motor

– wound rotor induction motor

– synchronous motor


Review:

2. All three-phase motors operate on the principle of a rotating magnetic field.

3. The speed of the rotating magnetic field is called the synchronous speed.

4. The direction of rotation of any three-phase motor can be changed by reversing the connection of any two stator leads.


Review:

5. Three factors that cause a magnetic field to rotate are:

a. The fact that the voltages of a three-phase system are 120°out of phase with each other.

b. The fact that voltages change polarity at regular intervals.

c. The arrangement of the stator windings.


Review:

6. Two factors that determine the

synchronous speed are:

a. number of stator poles per phase.

b. frequency of the applied voltage.


Review:

7. The direction of rotation of a three-phase motor can be determined with a phase rotation meter before power is applied to the motor.

8. Dual-voltage motors will have 9 or 12 leads brought out at the terminal connection box.


Review:

9. Dual-voltage motors intended for high-

voltage connection have their phase

windings connected in series.

10.Dual-voltage motors intended for low-

voltage connection have their phase

windings connected in parallel.


Review:

11.Motors that bring out 12 leads are generally intended for wye-delta starting.

12.Maximum torque is developed when stator and rotor flux are in phase with each other.

13.The code letter on the nameplate of a squirrel-cage motor indicates the type of rotor bars used in the rotor construction.


Review:

14.The torque of an induction motor is

determined by:

a. the magnetic field strength of the stator.

b. the magnetic field strength of the rotor.

c. the phase angle difference between

rotor and stator flux.


Review:

15.Wound-rotor motors have three sliprings

on the rotor shaft to provide external

connection to the rotor.

16.Wound-rotor motors have higher starting

torque and lower starting current than

squirrel-cage motors of equal horsepower.


Review:

17.The speed of a wound-rotor motor can be controlled by permitting resistance to remain in the rotor circuit during operation.

18.Synchronous motors operate at a synchronous speed.

19.Synchronous motors operate at a constant speed from no load to full load.


Review:

20.When load is connected to a synchronous

motor, stress develops between the

magnetic fields of the rotor and stator.

21.Synchronous motors must have DC

excitation from an external source.


Review:

22.DC excitation is provided to some synchronous motors through two sliprings located on the rotor shaft, and other motors use a brushless exciter.

23.Synchronous motors have the ability to produce a leading power factor by overexcitation of the DC current supplied to the rotor.


Review:

24. Synchronous motors have a set of type A

squirrel-cage bars used for starting. This

squirrel-cage winding is called the amortisseur

winding.

25. A field-discharge resistor is connected across

the rotor winding during starting to prevent high

voltage in the rotor due to induction.


Review:

26.Changing the DC excitation current does

not affect the speed of the motor.

27.Selsyn motors are used to provide

position control and angular feedback

information.


Review:

28.Although selsyn motors contain three-

phase windings, they operate on single-

phase AC.

29.A differential selsyn unit can be used to

determine the algebraic sum of the

rotation of two other selsyn units.

Unit 33 Three-Phase Motors -...

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