Unit 32 Three-Phase Motors

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
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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 singlephase motors.
Unit 32
Three-Phase Motors
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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 threephase motors:
1. squirrel-cage induction motor
2. wound-rotor induction motor
3. synchronous motor
Unit 32
Three-Phase Motors
Rotating Magnetic Field
• The principle of operation for all threephase 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
Three-phase stator and three voltage sine
waves.
Unit 32
Three-Phase Motors
The magnetic field is concentrated between
poles 1A and 1B.
Unit 32
Three-Phase Motors
The magnetic field is concentrated between
poles of phases 1 and 2.
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Three-Phase Motors
The magnetic field is concentrated between
poles 2A and 2B.
Unit 32
Three-Phase Motors
The magnetic field is concentrated between
phases 2 and 3.
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Three-Phase Motors
The magnetic field is concentrated between
poles 3A and 3B.
Unit 32
Three-Phase Motors
The magnetic field is concentrated between
phases 1 and 3.
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
RPM
STATOR POLES
3600
2
1800
4
1200
6
900
8
Unit 32
Three-Phase Motors
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Synchronous Speed
S = (120 x F) / P
S = synchronous speed in RPM
F = frequency in Hz
P = number of stator poles
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Three-Phase Motors
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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
Connecting the phase rotation meter to the
motor.
Unit 32
Three-Phase Motors
Connecting the phase rotation meter to the
line.
Unit 32
Three-Phase Motors
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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
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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
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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
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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.
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Three-Phase Motors
Standard numbering for three-phase motors.
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Three-Phase Motors
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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
Standard high-voltage wye connections.
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Three-Phase Motors
Standard high-voltage delta connections.
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Three-Phase Motors
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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
Standard low-voltage wye schematic.
Unit 32
Three-Phase Motors
Standard low-voltage wye chart and diagram.
Unit 32
Three-Phase Motors
Standard low-voltage delta schematic.
Unit 32
Three-Phase Motors
Standard low-voltage delta chart and diagram.
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Three-Phase Motors
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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
Standard 12-lead motor schematic.
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Three-Phase Motors
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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.
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Three-Phase Motors
Basic squirrel-cage rotor without laminations.
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Three-Phase Motors
Basic squirrel-cage rotor cutaway view.
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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.
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Three-Phase Motors
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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.
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Three-Phase Motors
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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
External resistors are connected to the rotor of
a wound-rotor motor.
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Three-Phase Motors
Wound-rotor motor schematic symbol.
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Three-Phase Motors
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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.
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Three-Phase Motors
Synchronous motor with DC excitation
supplied through sliprings.
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Three-Phase Motors
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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
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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.
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Three-Phase Motors
Synchronous motor schematic.
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Three-Phase Motors
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Selsyn Motors
Selsyn motors are used for position
control and angular feedback information.
Selsyn motors contain three-phase
windings, although they operate on singlephase AC.
A differential selsyn unit can be used to
determine the algebraic rotation sum of
two other selsyn units.
Unit 32
Three-Phase Motors
Selsyn motor schematic.
Unit 32
Three-Phase Motors
Selsyn motor schematic symbol.
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Three-Phase Motors
Schematic of two selsyn motors connected.
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Three-Phase Motors
Schematic of differential selsyn motor
connections.
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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 threephase 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 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.
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 highvoltage connection have their phase
windings connected in series.
10. Dual-voltage motors intended for lowvoltage 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.
Unit 32
Three-Phase Motors
Review:
28. Although selsyn motors contain threephase windings, they operate on singlephase AC.
29. A differential selsyn unit can be used to
determine the algebraic sum of the
rotation of two other selsyn units.