Induction Motor as a Transformer

We know that, transformer is a device in which two windings are magnetically coupled and when one winding is excited by a.c. supply of certain frequency, the e.m.f. gets induced in the second winding having same frequency as that of supply given to the first winding. The winding to which supply is given is called primary winding while winding in which e.m.f. gets induced is called secondary winding. The induction motor can be regarded as the transformer.

The difference is that the normal transformer is an alternating flux transformer while induction motor is rotating flux transformer. The normal transformer has no air gap as against this an induction motor has distinct air gap between its stator and rotor.

In an alternating flux transformer the frequency of induced e.m.f. and current in primary and secondary is always same. However in the induction motor frequency of e.m.f. and current on the stator side remains same but frequency of rotor e.m.f. and current depends on the slip and slip depends on load on the motor. So we have a variable frequency on the rotor side. But it is important to remember that at start when N = 0 the value of slip is unity (s = 1), then frequency of supply to the stator and of induced e.m.f. in the rotor is same. The effect of slip on the rotor parameters is already discussed in the previous section.

And last difference is that in case of the alternating flux transformer the entire energy present in the secondary circuit, is in the electrical form. As against this, in an induction motor part of its energy in the rotor circuit is in electrical form and the remaining part is converted into mechanical form.

Fig. 1 Induction motor as a transformer

In general, an induction motor can be treated as a generalised transformer as shown in the Fig. 1. In this, the slip ring induction motor with star connected stator and rotor is shown.

So if E₁ = Stator e.m.f. per phase in volts.

E₂= Rotor induced e.m.f. per phase in volts at start when motor is at standstill.

Then according to general transformer there exists a fixed relation between E₁and E₂called transformer ratio.

.^.. At start when N =0 , s =1

and we get,

Key Point : So if stator supply voltage is known and ratio of stator to rotor turns per phase is known then the rotor induced e.m.f. on standstill can be obtained.

Practical Commutation

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation. Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as...

Demagnetising and Cross Magnetizing Conductors

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the Fig.1. Fig. 1 The brushes are lying along the new position of MNA which is at angle θ from GNA. The conductors in the region AOC = BOD = 2θ at the top and bottom of the armature are carrying current in such a direction as to send the flux in armature from right to left. Thus these conductors are in direct opposition to main field and called demagnetizing armature conductors. The remaining armature conductors which are lying in the region AOD and BOC carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux. These conductors are shown in the Fig. 2 Fig. 2 ...

Winding Pitches

Now we will consider the definitions of different types of winding pitches, required in the design of the armature winding. 1.1 Back Pitch The distance which is measured interns of armature conductors i.e. between top and bottom coil sides of a coil measured around the back of the armature i.e. away from the commutator is called back pitch and is denoted by . It is measured interms of coil sides. Note : Since it is difference between odd and even number, it is always odd number. The size of the coil is decided by the back pitch. The back pitch is normally equal to coilsides per pole. It is shown for lap and wave type of winding in the Fig. 1. Fig. 1 The back pitch for the winding shown in the Fig. 1 is y b = 14 - 1 = 13 as coilsides 1and 14 are of coil number 1. 1.2 Front Pitch It is defined as the distance measured between two c...

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