Introduction:
What
is the fundamental difference in working principle of induction motor and
transformer?That
is even though the equivalent circuit of motor and transformer is same rotor of
motor rotates whereas secondary of transformer do not.
Alternating flux machine and Rotating flux machine:
Induction
motor is a generalized transformer. Difference is that transformer is an
alternating flux machine while induction motor is rotating flux machine.
Rotating flux is only possible when 3 phase voltage (or poly phase) which is
120 degree apart in time is applied to a three phase winding (or poly phase
winding) 120 degree apart in space then a three phase rotating magnetic flux is
produced whose magnitude is constant but direction keeps changing. In
transformer the flux produced is time alternating and not rotating.
Air gap and Reluctance:
There
is no air gap between primary and secondary of transformer where as there is a
distinct air gap between stator and rotor of motor which gives mechanical
movability to motor. Because of higher reluctance ( or low permeability) of air
gap the magnetizing current required in motor is 25-40% of rated current of
motor where as in transformer it is only 2 -5 % of rated primary current.
Frequency:
In
an alternating flux machine frequency of induced EMF in primary and secondary
side is same where as frequency of rotor EMF depends on slip. During starting
when S = 1 the frequency of induced EMF in rotor and stator is same but after
loading it is not.
Other
difference is that the secondary winding and core is mounted on a shaft set in
bearings free to rotate and hence the name rotor. If at all secondary of a
transformer is mounted on shaft set at bearings the rate of cutting of mutual
magnetic flux with secondary circuit would be different from primary and their
frequency would be different. The induced EMF would not be in proportion to
number of turns ratio but product of turn ratio and frequency. The ratio of
primary frequency to the secondary frequency is called slip.
Any
current carrying conductor if placed in magnetic field experience a force so
rotor conductor experience a torque and as per Lenz’s Law the direction of
motion is such that it tries to oppose the change which has caused so it starts
chasing the field.
Power flow
diagram of induction motor:
Stator input electrical
power = A
Stator losses = B
Rotor losses = C
Mechanical output = P
A – ( B + C ) = P
Roughly B= 0.03A, C=
0.04A
A – 0.07A = P
0.93A = P, Hence
efficiency = (P/A) x 100 = 93%
