Transformer is a device having two windings which are magnetically coupled & when one winding is excited by AC supply of certain frequency, the EMF will be induced in the second winding having same frequency as that of supply. The winding to which supply is given is called primary winding & across which EMF gets induced is known as secondary winding. The Induction motor can be regarded as Transformer.
- The difference between the transformer & IM is that the transformer works on alternating flux while IM works on RMF.
- The transformer has no air gap but the IM has distinct air gap between stator & rotor.
- In transformer, the frequency of the induced EMF and current in the primary & secondary is always same. However in IM, the frequency of EMF & current in stator side remains same but frequency of rotor EMF & current depends on the slip & slip depends on load on the motor. So we have a variable frequency on the rotor side.
At start of IM, N = 0 & slip S = 1.
Then frequency of supply to the stator & that of induced EMF in rotor is same.
- The last difference between transformer & IM is that the entire energy present in its secondary circuit of transformer is in electrical form while in IM part of the energy in the rotor is in electrical form & the remaining part is converted into mechanical form.
But in general an IM can be treated as a generalised transformer.
Let E1 be the stator EMF / Phase in volts
E2 be the rotor induced EMF / phase in volts at start when motor is at standstill
Then according to transformer there exists a fixed relation between E1& E2 called transformation ratio.
Therefore, at start when N = 0, S = 1 and we get
E2/E1=K=(Rotor Turns per Phase)/(Stator Turns per Phase)