Construction and working principle of induction motor and it's application ~ Electrical & Electronics Knowledge

Construction of induction motors:-

Conversion of electrical power into mechanical power takes place in the rotating part of an electric motor. In A.C. motors, rotor receives electric power by induction in exactly the same way as the secondary of a two-winding transformer receives its power from the primary. Hence such motors are known as a rotating transformer i.e. one in which primary winding is stationary but the secondary is free to rotate.

An induction motor essentially consists of two main parts:

stator and

Rotor.

Stator:

The stator of an induction motor is in principle, the same as that of a synchronous motor (or) generator.

It is made up of a number of stampings, which are slotted to receive the windings.

The stator carries a 3-phase winding and is fed from a 3-phase supply.

It is wound for a definite number of poles, the exact number of poles being determined by the requirements of speed.

The number of poles are higher, lesser the speed and vice-versa.

The stator winding, when supplied with a 3-phase currents, produce a magnetic flux, which is of constant magnitude but which revolves at synchronous speed (Ns = 120 x f / p).

This revolving magnetic flux induces emf in rotor by mutual induction.

Rotor:

Squirrel cage Rotor:

Motors employing this type of rotor are known as squirrel cage induction motor.

(ii). Phase wound (or) slip-ring Rotor:

Motors employing this type of rotor are widely known as “phase-wound” motors or wound motor or “slip-ring” motors.

SQUIRREL CAGE ROTOR:

Almost 90 percentage of induction motors are squirrel-cage type, because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible.

The Rotor consists of cylindrical laminated core with parallel slots for carrying the rotor conductors which, it should be noted clearly, are not wires but consists of heavy bars of copper, aluminium or alloys.

One bar is placed in each slot; rather the bars are inserted from the end when semi-enclosed slots are used.

The rotor bars are brazed or electrically welded or bolted to two heavy and stout short circuiting end-rings, thus giving us, what is called a squirrel cage construction.

The Rotor bars are permanently short-circuited on themselves; hence it is not possible to add any external Resistance in series with the Rotor circuit for starting purposes.

The rotor slots are not quite parallel to the shaft but are purposely given a slight skew. This is useful in two ways.

It helps to make the motor run quietly by reducing the magnetic hum and

It helps in reducing the locking tendency of the rotor. i.e. the tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two.

PHASE-WOUND ROTOR:

This type of rotor is provided with 3-phase, double-layer, distributed winding consisting of coils are used in alternators.

The Rotor is wound for as many poles as the number of stator poles and is always wound 3-phase even when the stator is wound for two-phase.

The three phases are shorted internally.

The other three winding terminals are slip-rings mounted on the shaft with brushes resting on them.

These three brushes are further externally connected to a 3-phase star connected Rheostat.

This makes possible the introduction of additional resistance in the rotor circuit during the starting period for increasing the starting torque of the motor.

When running under normal conditions, slip-rings are automatically short circuited by means of a metal collar, which is pushed along the shaft and connects all the rings together.

Frame:

Made of close-grained alloy cast iron.

Stator and Rotor core:

Built from high quality low loss silicon steel laminations and flash enameled on both sides.

Stator and Rotor windings:

Have moisture proof tropical insulation and embodying mica and high quality varnishes.

Are carefully spaced for most effective air circulation and are rigidly braced to withstand centrifugal forces and any short circuit stresses.

Air gap:

The stator rabbets and bore are machined carefully to ensure uniformity of air gap.

Shaft and Bearings:

Ball and roller bearings are used to suit heavy duty, trouble free running and for enhanced service life.

Fans:

Light aluminium fans are used for adequate circulation of cooling air and are securely keyed onto the Rotor shaft.

Slip-Rings and Slip-Ring Enclosures:

Slip rings are made of high quality phosphor bronze and are of molded construction.

Working Principle Of An Induction Motor :-

In a DC motor, supply is needed to be given for the stator winding as well as the rotor winding. But in an induction motor only the stator winding is fed with an AC supply.

• Alternating flux is produced around the stator winding due to AC supply. This alternating magneting flux revolves with synchronous speed. The revolving flux is called as "Rotating Magnetic Field" (RMS).

The relative speed between stator RMF and rotor conductors causes an induced emf in the rotor conductors, according to the Faraday's law of electromagnetic induction. The rotor conductors are short circuited, and hence rotor current is produced due to induced emf. That is why such motors are called as induction motors.

(This action is same as that occurs in transformer, hence induction motors can be called as rotating transformers.)
Now, induced current in rotor will also produce alternating flux around it. This rotor flux lags behind the stator flux. The direction of induced rotor current, according to Lenz's law, is such that it will tend to oppose the cause of its production.
As the cause of production of rotor current is the relative velocity between rotating stator flux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same direction as that of stator flux to minimize the relative velocity. However, the rotor never succeeds in catching up the synchronous speed. This is the basic working principle of induction motor of either type, single phase of 3 phase.

Synchronous Speed:

The rotational speed of the rotating magnetic field is called as synchronous speed.

where, f = frequency of the spply

P = number of poles

Slip:

Rotor tries to catch up the synchronous speed of the stator field, and hence it rotates. But in practice, rotor never succeeds in catching up. If rotor catches up the stator speed, there wont be any relative speed between the stator flux and the rotor, hence no induced rotor current and no torque production to maintain the rotation. However, this won't stop the motor, the rotor will slow down due to lost of torque, the torque will again be exerted due to relative speed. That is why the rotor rotates at speed which is always less the synchronous speed.

The difference between the synchronous speed (N_s) and actual speed (N) of the rotor is called as slip.

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