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.

Unit 33 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.

Unit 33 Three-Phase 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.

Unit 33 Three-Phase Motors

• 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.

Unit 33 Three-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.

Unit 33 Three-Phase Motors

Construction

There are three basic types of three-phase motors:

1. squirrel-cage induction motor 2. wound-rotor induction motor 3. synchronous motor

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Three-phase stator and three voltage sine waves .

Unit 33 Three-Phase Motors

The magnetic field is concentrated between poles A1 and A2.

Unit 33 Three-Phase Motors

The magnetic field is concentrated between poles of phases A and B.

Unit 33 Three-Phase Motors

The magnetic field

is concentrated

between poles B1

and B2.

Unit 33 Three-Phase Motors

The magnetic field is concentrated between phases B and C.

Unit 33 Three-Phase Motors

The magnetic field is concentrated

between poles C1 and C2.

Unit 33 Three-Phase Motors

The magnetic field is concentrated

between phases A and C.

Unit 33 Three-Phase Motors

The magnetic field is concentrated between poles A1 and A2. The field has rotated 180°.

Unit 33 Three-Phase Motors The magnetic

field is concentrated between phases B and C and has rotated 270°.

Unit 33 Three-Phase Motors

The magnetic

field is

concentrated

between poles

A1 and A2 and

has rotated 360°.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

RPM STATOR POLES

3600 2

1800 4

1200 6

900 8

Unit 33 Three-Phase Motors

Synchronous Speed

• S = (120 x F) / P

• S = synchronous speed in RPM

• F = frequency in Hz

• P = number of stator poles

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Connecting the phase rotation meter to the motor .

Unit 33 Three-Phase Motors

Connecting the phase rotation meter to the line .

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Standard numbering for three-phase motors.

Unit 33 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.

Unit 33 Three-Phase Motors

Standard high-voltage wye connections.

Unit 33 Three-Phase Motors

Standard high-voltage delta connections.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Standard low-voltage wye schematic.

Unit 33 Three-Phase Motors

Standard low-voltage wye chart and diagram.

Unit 33 Three-Phase Motors

Standard low-voltage delta schematic.

Unit 33 Three-Phase Motors

Standard low-voltage delta chart and diagram.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Standard 12-lead motor schematic.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Basic squirrel-cage rotor without laminations.

Unit 33 Three-Phase Motors

Basic squirrel-cage rotor cutaway view.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

External resistors are connected to the rotor of a wound-rotor motor.

Unit 33 Three-Phase Motors

Wound-rotor motor schematic symbol.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Synchronous motor with DC excitation supplied through sliprings .

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

The field-discharge resistor is connected in parallel with the rotor winding during starting.

Unit 33 Three-Phase Motors

Synchronous motor schematic.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Selsyn motor schematic.

Unit 33 Three-Phase Motors

Selsyn motor schematic symbol.

Unit 33 Three-Phase Motors

Schematic of two selsyn motors connected.

Unit 33 Three-Phase Motors

Schematic of differential selsyn motor connections.

Unit 33 Three-Phase Motors

Review:

1. The basic types of three-phase motors are:

– squirrel cage induction motor – wound rotor induction motor – synchronous motor

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

Review:

6. Two factors that determine the synchronous speed are:

a. number of stator poles per phase.

b. frequency of the applied voltage.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.

Unit 33 Three-Phase Motors

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.