2. 2
Induction Machines
• 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. for the slip.
s
s
n
nn
s
−
=
3. 3
Figure
17.37
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)
8. 8
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).
9. 9
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 two-
phase 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.
10. 10
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).
11. 11
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.
Rotor
R
Main Winding, L and R
Starting Winding
Main Supply
12. 12
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.
13. 13
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
Starting Winding
Power Supply
Rotor
CapacitorSwitch
