An induction motor is essentially a transformer with stator forming the primary and rotor forming (the short-circuited) rotating secondary (Fig. 20). This is so because the transfer of energy
Fig. 20. Induction motor as transformer.
from stator to the rotor of an induction motor takes place entirely inductively with the help of flux mutually linking the two.
The vector diagram (Fig. 21) is similar to that of a transformer.
V1 = E1 + I1 (R1 + jX1)
Er = 12 (R2 + jsX2)
However, the important differences between a transformer and an induction motor are:
- In induction motor the magnetic leakage and leakage reactance of rotor and stator are higher than in a transformer.
- The magnetic circuit of an induction motor has an air gap and this makes the per unit value of magnetising current much higher than that of a transformer.
- Because of the distributed windings in an induction motor the ratio of stator and rotor currents is not equal to the ratio of turns per phase in the rotor and stator windings.
- The losses in an induction motor are higher and, therefore, the efficiency is lower than in a transformer.
Fig. 21. Vector diagram of induction motor.