Working Principle of an Induction Motor

The motor which works on the principle of electromagnetic induction is known as the induction motor. The electromagnetic induction is the phenomenon in which the electromotive force induces across the electrical conductor when it is placed in a rotating magnetic field.

The stator and rotor are two essential parts of the motor. The stator is the stationary part, and it carries the overlapping windings while the rotor carries the main or field winding. The windings of the stator are equally displaced from each other by an angle of 120°.

The induction motor is the single excited motor, i.e., the supply is applied only to the one part, i.e., stator. The term excitation means the process of inducing the magnetic field on the parts of the motor.

When the three phase supply is given to the stator, the rotating magnetic field produced on it. The figure below shows the rotating magnetic field set up in the stator.

Consider that the rotating magnetic field induces in the anticlockwise direction. The rotating magnetic field has the moving polarities. The polarities of the magnetic field vary by concerning the positive and negative half cycle of the supply. The change in polarities makes the magnetic field rotates.

The conductors of the rotor are stationary. This stationary conductor cut the rotating magnetic field of the stator, and because of the electromagnetic induction, the EMF induces in the rotor. This EMF is known as the rotor induced EMF, and it is because of the electromagnetic induction phenomenon.

The conductors of the rotor are short-circuited either by the end rings or by the help of the external resistance. The relative motion between the rotating magnetic field and the rotor conductor induces the current in the rotor conductors. As the current flows through the conductor, the flux induces on it. The direction of rotor flux is same as that of the rotor current.

Now we have two fluxes one because of the rotor and another because of the stator. These fluxes interact each other. On one end of the conductor the fluxes cancel each other, and on the other end, the density of the flux is very high. Thus, the high-density flux tries to push the conductor of rotor towards the low-density flux region. This phenomenon induces the torque on the conductor, and this torque is known as the electromagnetic torque.

The direction of electromagnetic torque and rotating magnetic field is same. Thus, the rotor starts rotating in the same direction as that of the rotating magnetic field.

The speed of the rotor is always less than the rotating magnetic field or synchronous speed. The rotor tries to the run at the speed of the rotor, but it always slips away. Thus, the motor never runs at the speed of the rotating magnetic field, and this is the reason because of which the induction motor is also known as the asynchronous motor

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what is synchronous generator or alternator ?

 An alternator consists of two parts, the stator, and the rotor. The stator is the stationary part of the machine, and the rotor is the rotating part of the machine. The stator carries the armature winding in which the voltage is generated, and the output is taken from it. The rotor of the machine produces the main flux.

Stator Construction

The parts of the stator are the frame, stator core, stator windings, and cooling arrangement. The frame of the stator is made up of cast iron for a small-size machine, and welded steel for large size machines. To reduce the hysteresis and eddy current losses, the stator core is assembled with high-grade silicon content steel laminations. A 3-phase winding is put in the slots cut on the inner periphery of the stator. The winding is star connected and is distributed over several slots. When current flows in a distributed winding it produces an essentially sinusoidal space distribution of e.m.f.

Rotor Construction

The rotor construction is of two types

Salient-pole type.

Cylindrical rotor type.

Construction of three phase synchronous machines

Salient-pole rotor

The term salient means 'projecting'. A salient-pole consists of poles that are projected out from the surface of the rotor core. These are used for the rotors for four or more poles.

The rotor is subjected to changing magnetic fields that is why it is made of steel laminations to reduce the eddy current losses. Identical dimensions poles are assembled by stacking laminations to the required length and then riveted together. After the field coil is placed around each pole body, these poles are fitted by a dove-tail joint to a steel spider keyed to the shaft. Salient-pole rotors have faces to damp out the rotor oscillations during a sudden change in load conditions. A non-uniform air gap accompanies a salient-pole synchronous machine.

The air gap is minimum under the pole centers, and it is maximum in between the poles. The pole faces are so shaped that the radical air gap length increases from the pole center to the pole tips so that the flux distribution in the air gap is sinusoidal. This will help the machine to generate sinusoidal e.m.f.

To give the alternate north and south polarities, the individual field-pole windings are connected in series. The end of the field windings is connected to a d.c winding by the brushes on the slip-rings.

The salient-pole generators have a large number of poles and lower operating speed. Salient-pole alternators that are driven by water turbines are called hydro-alternators or hydro generators.

Cylindrical Rotor

Cylindrical-rotor machines are also known as non-salient pole rotor machine. The construction of the rotor is such that it forms a smooth cylinder. It has no physical poles as in the salient-pole construction. These rotors are made up of solid forgings of high-grade nickel-chrome-molybdenum steel.

In about two-thirds of the rotor periphery, slots are cut at regular intervals and parallel to the shaft.

The d.c field windings are connected in these slots. The winding is of distributed type. The unslotted portion of the rotor forms two pole faces. These machines have a small diameter and long axial length.

Such construction limits the centrifugal forces. Thus, the cylindrical rotors are useful in high-speed machines.

Steam or gas turbines drive Cylindrical-rotor machines. Cylindrical-rotor synchronous generators are called turbo-alternators or turbo-generators.