We know that, transformer is a device
in which two windings are magnetically coupled and when one winding is
excited by a.c. supply of certain frequency, the e.m.f. gets induced in
the second winding having same frequency as that of supply given to the
first winding. The winding to which supply is given is called primary
winding while winding in which e.m.f. gets induced is called secondary
winding. The induction motor can be regarded as the transformer.
The difference is that the normal transformer is an alternating flux
transformer while induction motor is rotating flux transformer. The
normal transformer has no air gap as against this an induction motor has
distinct air gap between its stator and rotor.
In an alternating flux transformer the frequency of induced e.m.f. and
current in primary and secondary is always same. However in the
induction motor frequency of e.m.f. and current on the stator side
remains same but frequency of rotor e.m.f. and current depends on the
slip and slip depends on load on the motor. So we have a variable
frequency on the rotor side. But it is important to remember that at
start when N = 0 the value of slip is unity (s = 1), then frequency of
supply to the stator and of induced e.m.f. in the rotor is same. The
effect of slip on the rotor parameters is already discussed in the
last difference is that in case of the alternating flux transformer the
entire energy present in the secondary circuit, is in the electrical
form. As against this, in an induction motor part of its energy in the
rotor circuit is in electrical form and the remaining part is converted
into mechanical form.
|Fig. 1 Induction motor as a transformer|
motor can be treated as a generalised transformer as shown in the Fig.
1. In this, the slip ring induction motor with star connected stator and
rotor is shown.
Key Point : So if stator
supply voltage is known and ratio of stator to rotor turns per phase is
known then the rotor induced e.m.f. on standstill can be obtained.