Unit 33 Three-Phase Motors

Transcription

1 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 threephase motor using a phase rotation meter. 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. 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. 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. 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.

2 Construction There are three basic types of three-phase motors: 1. squirrel-cage induction motor 2. wound-rotor induction motor 3. synchronous motor The magnetic field is concentrated between poles A1 and A2. 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. The magnetic field is concentrated between poles of phases A and B. Three-phase stator and three voltage sine waves. The magnetic field is concentrated between poles B1 and B2.

3 The magnetic field is concentrated between phases B and C. The magnetic field is concentrated between poles A1 and A2. The field has rotated 180. The magnetic field is concentrated between poles C1 and C2. The magnetic field is concentrated between phases B and C and has rotated 270. The magnetic field is concentrated between phases A and C. The magnetic field is concentrated between poles A1 and A2 and has rotated 360.

4 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. 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. RPM STATOR POLES Connecting the phase rotation meter to the motor. Synchronous Speed S = (120 x F) / P S = synchronous speed in RPM F = frequency in Hz P = number of stator poles Connecting the phase rotation meter to the line.

5 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 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. Dual-Voltage Motors The other 3 leads are connected internally. 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. Standard numbering for three-phase motors. Dual-Voltage Motors When motors are connected to their higherrated voltage on the name plate, a highvoltage connection pattern is required. When motors are connected to their lowerrated voltage on the name plate, a low-voltage connection pattern is required. 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.

6 Standard high-voltage wye connections. Standard low-voltage wye schematic. Standard high-voltage delta connections. Standard low-voltage wye chart and diagram. 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 delta schematic.

7 Standard low-voltage delta chart and diagram. 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. 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. Basic squirrel-cage rotor without laminations. Standard 12-lead motor schematic. Basic squirrel-cage rotor cutaway view.

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

9 Synchronous motor with DC excitation supplied through sliprings. The field-discharge resistor is connected in parallel with the rotor winding during starting. 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 motor schematic. 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. 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.

10 Selsyn motor schematic. Schematic of differential selsyn motor connections. 1. The basic types of three-phase motors are: squirrel cage induction motor wound rotor induction motor synchronous motor Selsyn motor schematic symbol. 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. Schematic of two selsyn motors connected.

11 5. Three factors that cause a magnetic field to rotate are: a. The fact that the voltages of a threephase 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. 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. 6. Two factors that determine the synchronous speed are: a. number of stator poles per phase. b. frequency of the applied voltage. 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 squirrelcage motor indicates the type of rotor bars used in the rotor construction. 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. 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.

12 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 squirrelcage motors of equal horsepower. 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. 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. 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. 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. 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.

13 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.