3 phase motors
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Unit 32 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 32 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 32 Three-Phase Motors
Objectives:• Discuss the operation of wound rotor
motors.• Discuss the operation of selsyn motors.
Unit 32 Three-Phase Motors
Objectives:• Discuss the operation of synchronous
motors.• Determine the direction of rotation of a
three-phase motor using a phase rotation meter.
Unit 32 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 32 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 32 Three-Phase Motors
Construction• There are three basic types of three-
phase motors:1. squirrel-cage induction motor2. wound-rotor induction motor3. synchronous motor
Unit 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 Three-Phase Motors
The field-discharge resistor is connected in parallel with the rotor winding during starting.
Unit 32 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 32 Three-Phase Motors
Review:1. The basic types of three-phase motors
are:– squirrel cage induction motor– wound rotor induction motor– synchronous motor
Unit 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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 32 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.
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