7. Why rotor frequency of IM is not equal to the stator frequency?
An induction motor works by inducing voltages and currents in the rotor of the machine, and for
that reason, it is sometimes called a rotating transformer. Like a transformer, the primary (stator)
induces a voltage in the secondary (rotor) but unlike a transformer, in an induction motor the
secondary windings can move and due to this rotation of the rotor (the secondary winding of
induction motor), the induced voltage in it does not have the same frequency of the stator
(primary) voltage. The induction motor's rotor tries to catch up with the frequency of the stator's
rotating magnetic field.
If the induction motor’s rotor were turning at a frequency equal to the stator frequency, then the
rotor bars would be stationary relative to the magnetic field and there would be no induced
voltage. If eind was equal to 0, then there would be no rotor current and no rotor magnetic field.
With no rotor magnetic field, the induced torque would be zero, and the rotor would slow down
because of friction losses. An induction motor can thus speed up to near-synchronous speed, but
it can never exactly reach synchronous speed.
8. Suppose if the supply frequency increases then what happens to the speed of IM? And
why?
When a set of three-phase voltages are applied to the stator of the induction motor, a three-phase
set of stator currents starts flowing, which then produces a magnetic field rotating in a counterclockwise direction. The speed of this rotating magnetic field is given by:
nsync =
120fe
P
Where fe is the system frequency.
Here, the speed of the stator’s magnetic field is directly proportional to the supply frequency.
Hence, as the supply frequency increases, the speed of the rotating magnetic field increases
which results in an increase in the speed of the rotor. This relation can also be justified using the
following equation:
fr = sfe
Here, the speed of the rotor is directly proportional to the supply frequency for a constant slip. As
the supply frequency increases, the speed of the induction motor increases.
9. In IM, which winding is said to be armature winding? And why?
In induction motors, rotor winding is said to be armature winding.
In electrical machines, the term armature winding refers to the winding on which the voltage or
current is induced. In the case of an induction motor, the stator winding is supplied with a set of
three-phase voltage which produces a rotating magnetic field that induces a voltage in the rotor
windings.
10. In IM, which winding is said to be field winding? And why?
In induction motors, the stator winding is said to be field winding.
In electrical machines, the term field winding refers to the winding which produces a magnetic
field when a current flows through it. In the case of induction motors, the stator winding is
connected to a three-phase power supply and carries alternating current. This alternating current
produces a rotating magnetic field for driving the motor.
11. What are the types of rotors in an IM? What are the differences in their construction
and application?
There are two different types of induction motor rotors that can be placed inside the stator. One is
called a cage rotor, while the other is called a wound rotor.
A cage induction motor rotor consists of a series of conducting bars laid into the slots carved in
the face of the rotor and shorted at either end by a large shorting ring.
A wound rotor has a set of three-phase windings that are the same as the windings on the stator
and are usually Y-connected. The ends of the three rotor wires are tied to slip rings on the rotor’s
shaft. The rotor windings are shorted through the brush riding on the slip rings. Wound-rotor
induction motors have therefore their currents accessible at the stator brushes where extra
resistance can be inserted into the rotor circuit.
Wound-rotor induction motors are more expensive than cage induction motors and require much
more maintenance because of the wear associated with their brushes and slip rings. As a result,
wound-rotor induction motors are rarely used.
12.Why IM has a higher starting current?
The rotor is stationary at the start, which means that the motor is essentially operating as a
transformer, with the stator current being used to generate a rotating magnetic field. The slip
difference between the rotor speed and the synchronous speed is maximum i.e., s = 1, which
means that the relative velocity between the rotor conductor and rotating magnetic field is
maximum, which results in the maximum induced voltage. This makes the starting current
higher.
The other way to explain this higher current is that initially the rotor of the motor is stationary,
and it requires a significant amount of torque to overcome the inertia and to initiate the rotation.
This means that the torque required to start the motor is high and hence, the current required
from the power supply is high.
13. Draw a power flow diagram of IM. Discuss how Pout could be calculated?
From the above power flow diagram, we can see that the input power to the induction motor is in
the form of three-phase electric voltages and currents.
Pin = √3VT IL cos𝜃
The first losses encountered are I2 R losses in the stator windings PSCL . Then some of the power is
lost as hysteresis and eddy currents in the stator Pcore. The power remaining is transferred through
the air gap to the rotor called air-gap power PAG.
PAG = Pin − PSCL − Pcore
After the transfer, some of this power is lost as I2R losses in the rotor windings and the rest is
converted into mechanical form Pconv.
Pconv = PAG − PRCL
Finally, friction and windage losses PF&W and stray losses Pmisc is subtracted. The remaining
power is the output of the motor Pout.
Pout = Pconv − PF&W − Pmisc
If, instead of core losses rotational losses are given (Prot = Pcore + PF&W ), following changes
will happen:
PAG = Pin − PSCL
Pout = Pconv − Prot − Pmisc