Working of Induction Motor as a Transformer

Induction motors, commonly known as “asynchronous motors”, are the AC fed electric motors. They never run at synchronous speed because torque development is not possible at synchronous speed. An induction motor works on the principle of Faraday's law of electromagnetic induction which is similar to the transformer, therefore, an induction motor is often referred to as a rotating transformer.

In this article, we will discuss the working of an induction motor as a transformer in detail. But, before proceeding to the main concept, let’s first revise some basics of induction motors and transformers to ensure we can easily grasp the concept.

Introduction to Induction Motor & Transformer

Introduction to Induction Motor

We already published a dedicated article about the basics of induction motors. In that article, we discussed that an induction motor consists of two major parts: stator and rotor. The stator is a hollow cylindrical stationary structure. The rotor is also a cylindrical structure that is placed in the cavity of the stator with an optimum air gap. Slots are provided on the inner periphery of the stator and the outer periphery of the rotor for the placement of windings.

When the AC voltage is applied to the stator winding, it sets up a rotating magnetic field in the air gap. This rotating magnetic field induces emf and current in the rotor as the rotor is short circuited at the both ends. This induced current in the rotor produces its own rotating magnetic field in the area gap, and due to the interaction of these two fields, torque is developed in the induction motor.

Introduction to Transformer

A transformer is a static device that transfers power from one voltage level to another voltage level. It is basically a magnetically coupled circuit which comprises two coils or windings (primary and secondary winding) tightly coupled through the magnetic core. A transformer works on the principle of Faraday's law of electromagnetic induction, same as the induction motor, as we mentioned earlier in this article.

In a transformer voltage is applied to the primary winding that produces magnetic flux in the magnetic core. This magnetic flux links with the secondary winding and primary winding as well and produces emf in both windings.

Let us first consider a transformer with no load. In this condition, the current flowing through the secondary winding is zero, consequently, the current flowing through the primary winding is also zero in case of an ideal transformer, or it is a no-load current in the case of a practical transformer.

But in the case of a transformer with load, the current flowing through the secondary winding is not zero, it is equal to the current required by the load. The current flowing through the secondary winding produces its own magnetic flux that opposes the main flux produced by the primary winding in the core due to its demagnetizing nature. This will cause the reduction in induced voltage in primary winding, consequently the rise in primary current. This increased amount of primary current produces its magnetic flux that is exactly equal and opposite to the magnetic flux produced by the secondary current, in order to fulfil the load current requirement.

Working of Induction Motor as a Transformer

The working of an induction motor is based on the principle of Faraday's law of electromagnetic induction. According to this principle, when a conductor is placed in a time varying magnetic field, an emf is induced in the conductor.

Let us understand this in detail.

When the AC voltage is applied to the stator winding, it produces a time varying magnetic field that rotates in the air gap at synchronous speed, known as rotating magnetic field. Similar to the transformer, where AC voltage is given to the primary winding produces a time varying magnetic field in the magnetic core, although it is stationary unlike an induction motor.

In the case of a transformer, due to the magnetic core, the magnetizing current (current required to set up the magnetic field) requirement is less as compared to an induction motor. Thus, an induction motor works at a low lagging power factor.

The rotating magnetic flux produced by the stator links the both stator winding and rotor winding, and induces emf in both the windings. Similar to the transformer, where the flux produced by the primary winding links with both primary as well as second winding and induces emf in both primary and secondary.

Let us denote the induced emf in the primary winding of the transformer is E₁ and in the secondary winding is E₂, as shown in figure of the transformer with load above.

E1 = √2 π. F. N₁. ɸ

E2 = √2 π. F. N₂. ɸ

In the above equation, F is the frequency of supply AC supply, N₁ and N₂ is the number of turns in the primary and secondary winding and ɸ is the flux setup in the magnetic core.

Similarly, for the induction motor, let us say induced emf in the stator winding is E_s and in rotor winding is E_r.

E_s = kw_s.√2 π. F_s. N_s. ɸ

E_r = kw_r .√2 π. F_r. N_r. ɸ

Where, F_r = s.F_s,

s is the slip in the induction motor, F_s is the frequency of the applied voltage to the induction motor, F_r is the frequency of induced voltage in the induction motor, kw_s and kw_r are the winding factor of stator and rotor respectively and Ns and Nr are the number of turns of the stator and rotor winding respectively

As the rotor is short-circuited at the both ends, a current is also induced in the rotor winding similar to the transformer on load. The direction of this induced current in the rotor winding in such a way that it satisfies Lenz's law, according to which the direction of induced emf and current in the conductor through electromagnetic induction in such a way that it opposes the cause of it. Here in this case the cause of the production of emf and current is the relative speed between the stator magnetic field and the stationary rotor conductor.

This induced current in the rotor winding produces its own magnetics flux that also rotates in the air gap in the opposite direction of the stator field, at the same speed as the stator magnetic field i.e. synchronous speed. These two fields are stationary relative to each other and oppose to each other. This will cause the reduction in induced emf in the stator winding, similarly in the case of transformer on load discussed previously in the transformer.

The reduction of induced emf in the stator results in increases in stator current. The increased amount of stator current produces an additional magnetic field that is exactly equal and opposite to the rotor magnetic field in the air gap.

Now, the magnetic field rotates in the air gap is the resultant of the magnetic field produced by the stator current (Fs) and the magnetic field produced by the additional stator current). And Due the interaction of this resultant field and the rotor magnetic field, torque is developed in the induction motor that derives the rotor in the same direction of the resultant magnetic field.

Similarities between Induction Motor and Transformer

The working principle of both the transformer and the induction motor is the same i.e. based on Faraday’s Law of electromagnetic induction.

Stator of the induction motor acts as a primary of the transformer, whereas the rotor of the induction motor acts as the secondary of the transformer.

They both require magnetizing current to operate, although it is high in the case of an induction motor and low in case of a transformer.

Due to magnetizing current requirements, they both operate at low lagging power factor. However, because an induction motor requires high magnetizing current, it operates at a much lower lagging power factor.

They both have the same no load and rated load behaviour. They both maintain constant flux in core or in the air gap respectively.

Induction Motor Vs Transformer

A transformer converts electrical energy at one voltage level to another voltage level through the high permeability magnetic core. Whereas, an induction motor converts electrical energy to mechanical through air gap.

Due to the high permeability magnetic core, a transformer requires less magnetizing current, approximately 2-6% of rated current. Whereas due to air gap, the magnetizing current required by an induction motor is high as compared to a transformer, around 30-50% of the rated current.

In transformer windings are concentrated, whereas in induction motors these are distributed.

Induced emf in both windings in the transformer is based on the turns ratio, whereas induced emf in the induction motor depends on turns ratio only at stand-still condition and it changes with speed under running condition.