EQUIVALENT CIRCUIT OF AN INDUCTION MOTOR BASIC INFORMATION AND TUTORIALS

The Equivalent Circuit of an Induction Motor

An induction motor relies for its operation on the induction of voltages and currents in its rotor circuit from the stator circuit (transformer action).  This induction is essentially a transformer operation, hence the equivalent circuit of an induction motor is similar to the equivalent circuit of a transformer. 

The Transformer Model of an Induction Motor

A transformer per-phase equivalent circuit, representing the operation of an induction motor is shown below:

	The transformer model or an induction motor, with rotor and stator connected by an ideal transformer of turns ratio aeff.

 

As in any transformer, there is certain resistance and self-inductance in the primary (stator) windings, which must be represented in the equivalent circuit of the machine.  They are - R₁ - stator resistance and

X₁ – stator leakage reactance

Also, like any transformer with an iron core, the flux in the machine is related to the integral of the applied voltage E₁.  The curve of mmf vs flux (magnetization curve) for this machine is compared to a similar curve for a transformer, as shown below:

The slope of the induction motor’s mmf-flux curve is much shallower than the curve of a good transformer.  This is because there must be an air gap in an induction motor, which greatly increases the reluctance of the flux path and thus reduces the coupling between primary and secondary windings.  The higher reluctance caused by the air gap means that a higher magnetizing current is required to obtain a given flux level.  Therefore, the magnetizing reactance X_m in the equivalent circuit will have a much smaller value than it would in a transformer.

The primary internal stator voltage is E₁ is coupled to the secondary E_R by an ideal transformer with an effective turns ratio a_eff.  The turns ratio for a wound rotor is basically the ratio of the conductors per phase on the stator to the conductors per phase on the rotor.  It is rather difficult to see a_eff clearly in the cage rotor because there are no distinct windings on the cage rotor. 

E_R in the rotor produces current flow in the shorted rotor (or secondary) circuit of the machine.

The primary impedances and the magnetization current of the induction motor are very similar to the corresponding components in a transformer equivalent circuit.