inw unit 33 33 three-phase motors construction there are three basic types of three-phase motors: 1....
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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 MotorsPhase 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.