# Comparison between Induction Motor and Transformer

An induction motor may be considered to be a transformer with a rotating short-circuited secondary. The stator winding corresponds to the transformer primary and the rotor winding to the transformer secondary.

However, the following differences between the two are worth noting: ### 1. Air Gap

Unlike a transformer, the magnetic circuit of a 3-phase induction motor has an air gap. Therefore, the magnetizing current in a 3-phase induction motor is much larger than that of the transformer.

For example, in an induction motor, it may be as high as 30-50 % of rated current whereas it is only 1-5% of rated current in a transformer.

### 2. Leakage Reactance

In an induction motor, there is an air gap and the stator and rotor windings are distributed along the periphery of the air gap rather than concentrated on a core as in a transformer.

Therefore, the leakage reactances of stator and rotor windings are quite large compared to that of a transformer.

### 3. Mechanical and Electrical Outputs

In an induction motor, the inputs to the stator and rotor are electrical but the output from the rotor is mechanical. However, in a transformer, input, as well as output, is electrical.

### 4. Slip

The main difference between the induction motor and transformer lies in the fact that the rotor voltage and its frequency are both proportional to slip s.

If f is the stator frequency, E₂ is the per phase rotor e.m.f. at standstill and X₂ is the standstill rotor reactance/phase, then at any slip s, these values are:

Rotor e.m.f./phase, E₂’  = s E₂

Rotor reactance/phase, X₂’  = sX₂
Rotor frequency, f’ = sf