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Induction Machines
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Simple!
Stator winding is similar to that of synchronous machine!
Mostly used as motors.
No separate excitation is required for the rotor.
The rotor typically consists of one of two arrangements:
– Squirrel cage.
– Wound rotor.
Induction motor operates by virtue of currents induced from the stator field in
the rotor.
Speed of rotation n is below the synchronous speed ns. See eq. 17.73 for the
slip.
Solve Ex 17.12; 17.13; 17.14
ns  n
s
ns
0
Squirrel cage induction motor; (b) conductors in rotor; (c) photograph of squirrel
cage induction motor; (d) views of Smokin’ Buckey motor: rotor, stator, and cross
section of stator (Courtesy: David H. Koether Photography)
Figure
17.37
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Circuit model for induction machine
Figure
17.38
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Equivalent circuit of an induction machine
Figure 17.40
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Performance curve for induction motor
Figure
17.42
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General configuration of adjustable-frequency drive
Figure
17.45
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Single-Phase Motors
• Single-phase motors are used mostly to operate home
appliances such as air conditioners, refrigerators, pumps,
and fans.
• They are designed to operate on 120 V or 240 V.
• They range in capacity from fractional horsepower to
several horsepower depending on the application.
• Voltage is induced in the rotor as a result of magnetic
induction, and a magnetic field is produced around the
rotor. This field will always be in opposition to the stator
field (according to Lenz’s law).
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Split-Phase Motors
Split-phase motors use inductance, capacitance, or resistance to
develop a starting torque. These motors split the current flow
through two separate windings to simulate a two-phase power
system. A rotating magnetic field can be produced with a twophase system.
• There are three types of split-phase motors
depending on the means of starting:
– The resistance-start induction-run motor.
– The capacitor-start induction-run motor.
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The Resistance-Start Induction-Run Motor
• The out-of-phase condition between start and run winding
current is caused by the start winding having more
resistance than the run winding.
• The amount of starting torque produced is determined by:
– The strength of the magnetic field of the stator.
– The strength of the magnetic field of the rotor.
– The phase angle difference between current in the start
winding and current in the run winding (maximum
torque is achieved when these two currents are 90o out
of phase with each other).
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The main winding has a high inductance and a low resistance. The current
lags the voltage by a large angle. The starting winding have a low
inductance and a high resistance. The current lags the voltage by a smaller
angle. Suppose the current in the main winding lags the voltage by 80°. The
current in the auxiliary winding lags the voltage by 40°. The currents are,
hence, out of phase by 40°, which is enough to generate rotating field.
Main Winding, L and R
Starting Winding
Rotor
Main Supply
R
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The Capacitor-Start Induction-Run Motor
• The stator consists of the main winding and a starting
winding. The starting winding is connected in parallel with
the main winding and is placed at right angles to it.
• A 90-degree electrical phase difference between the two
windings is obtained by connecting the auxiliary winding in
series with a capacitor and a starting switch. When the
motor is energized, the starting switch is closed. This places
the capacitor in series with the auxiliary winding.
• Now we will have an RC circuit (Starting winding and the
capacitor) and an RL circuit (main winding). The currents in
each winding are therefore 90° out of phase, so are the
magnetic fields that are generated.
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When nearly full speed is obtained, a centrifugal
device (the switch) cuts out the starting winding. The
motor then runs as a single-phase induction motor.
Main Winding
Rotor
Power Supply
Switch
Starting Winding
Capacitor
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