Working Principle of an Induction Motor – Explained

Three Phase Induction Motor

A 3-phase induction motor consists of two main parts, namely stator and rotor.

1. Stator: It is the stationary part of the motor. It has three main parts, namely.

(i) Outer frame,

(ii) Stator core and

(iii) Stator winding.

(i) Outer frame: It is the outer body of the motor. Its function is to support the stator core and also to protect the inner parts of the machine. For small machines the fame is casted but for large machines it is fabricated. To place the motor on the foundation, feet are provided in the outer frame. The frame of the motor is usually made of cast iron.

(ii) Stator core: When AC supply is given to the motor, an alternating flux is set -up in the stator core. This alternating field produces hysteresis and eddy current loss. To minimize these losses, the core is made of high grade silicon steel stampings. The stampings are assembled under hydraulic pressure and also are keyed to the frame. Each stamping is insulated from the other with a thin varnish layer. The thickness to the stamping usually varies from 0.3 to 0.5 mm. Slots are punched on the inner periphery of the stampings to accommodate stator winding.

(iii) Stator winding: The stator core carries a three phase winding which is usually supplied from a three phase supply system. The six terminals of the winding (two of each phase) are connected in the terminal box of the machine

Rotor: The rotating part of the motor is called rotor. Two types of rotors are used for 3-phase induction motors.

(i) Squirrel cage rotor

(ii) Phase wound or Slip Ring rotor

Working Principle of an Induction Motor– Three Phase

When 3-phase supply is fed to the stator winding of a 3-phase wound rotor induction motor, a resultant rotating magnetic field at constant angular velocity is produced in the stator core. The stator resultant MMF at four different instances along the airgap. Let this field is revolting in an anti-clockwise direction at synchronous speed 𝑛𝑠.

Where, 𝑛𝑠 =120𝑓/𝑃 (𝑟𝑝𝑚)

Where 𝑓 is the frequency of the input electrical power, and 𝑃 is the number of poles of the induction machine. The rotating magnetic field is cut by the stationary rotor conductors and also an emf is induced in the rotor conductors.

As the rotor conductors are short circuited, current flows through them, Furthermore, a resultant field is produced by the rotor current carrying conductors. This field tries to come in line with the stator rotating field, As a result, an electromagnetic torque is developed and rotor starts rotating in same direction as that of stator rotating field.

The rotor then run at a mechanical speed close to and less than the synchronous speed as it tries to attain synchronous speed but never reach. It is because if the rotor revolve at the synchronous speed then the relative speed between rotating stator field and rotor will be zero, therefore, neither emf will be induced in rotor conductors or current nor rotor field and hence no torque will be produced. Thus, an IM can never run at synchronous speed. This is the Working Principle of an Induction Motor -3 Phase.

3 Phase Induction Motor Types

Three phase induction motors are constructed into two major types:

1. Squirrel cage Induction Motors

2. Slip ring Induction Motors

Squirrel cage Induction Motors

It consists of Squirrel cage rotors. The motors in which these rotors are employed are called Squirrel cage induction motors. Because of simple and rugged construction, the most of the induction motors employed in the industry are of this type.

A squirrel cage rotor consists of a laminated cylindrical core having semi-closed circular slots at the outer periphery. Copper or aluminium bar conductors are placed in these slots and short circuited at each end by copper or aluminium rings, called short circuiting rings.

Thus, in these rotors, the rotor winding is permanently short-circuited and also no external resistance can be added in the rotor circuit. Figure 9.3 clearly shows that the slots are not parallel to the shaft but these are skewed.

The skewing provides the following advantages:

• Humming is reduced, that ensures quiet running.
• At different positions of the rotor, smooth and also sufficient torque is obtained.
• It reduces the magnetic locking of the stator and rotor,
• It increases the rotor resistance due to the increased length of the rotor bar conductors.

Slip ring Induction Motors

It consists of Phase wound rotor. The motors in which these rotors are employed are known as phase wound or slip ring induction motors. This rotor is also cylindrical in shape which consists of large number of stampings.

A number of semi-closed slots are punched at its outer periphery. A 3-phase insulated winding is placed in these slots. The rotor is wound for the same number of poles as that of stator. rotor winding is connected in star and also its remaining three terminals are connected to the slip rings.

The rotor core is keyed to the shaft. Similarly, slip-rings are also keyed to the shaft but these are insulated from the shaft. In this case, depending upon the requirement any external resistance can be added in the rotor circuit. In this case also the rotor is skewed.

A mild steel shaft is passed through the centre of the rotor and is fixed to it with key. The purpose of shaft is to transfer mechanical power.

WHAT IS SLIP OF INDUCTION MOTOR

• In practice, the rotor never succeeds in ‘catching up’ with the stator field. If it really did so, then there would be no relative speed between the two, hence no rotor e.m.f., no rotor current and so no torque to maintain rotation. That is why the rotor runs at a speed which is always less than the speed of the stator field. The difference in speeds depends upon the load on the motor.
• The difference between the synchronous speed Ns and the actual speed N of the rotor is known as Slip of Induction Motor.
• Though it may be expressed in so many revolutions/second, yet it is usual to express it as a percentage of the synchronous speed. Actually, the term ‘slip’ is descriptive of the way in which the rotor ‘slips back’ from synchronism.
• Sometimes, Ns − N is called the Slip Speed.

Induction Motor Vs Synchronous Motor

Advantages of Induction Motor:

• It has very simple & extremely rugged, almost unbreakable construction (especially squirrel cage type).
• Its cost is low. It is very reliable.
• It has sufficiently high efficiency. In normal running condition, no brushes are needed, hence frictional losses are reduced. It has a reasonably good power factor.
• It requires minimum of maintenance.
• It starts up from rest and needs no extra starting motor & has not to be synchronized.

Disadvantages of Induction Motor:

• Its speed cannot be varied without sacrificing some of its efficiency.
• Just like a d.c. shunt motor, its speed decreases with increase in load.
• Its starting torque is somewhat inferior to that of a d.c. shunt motor

Induction Motor Applications

The applications of squirrel cage induction motors and slip-ring (phase wound) induction motors are given below:

1. Squirrel cage induction motor application:

These motors are mechanically robust and are operated almost at constant speed. These motors operate at high power factor and also have high over load capacity. However, these motors have low starting torque. (i.e., these motors cannot pick-up heavy loads) and draw heavy current at start. On the bases of these characteristics, these motors are best suited for:

• Printing machinery
• Flour mills
• Saw mills
• Shaft drives of small industries
• Pumps
• Prime-movers with small generators etc.

2. Slip-ring induction motor application:

These motors have all the important characteristics (advantage) of squirrel cage induction motors and at the same time have the ability to pick-up heavy loads at start drawing smaller current from the mains. Accordingly these motors are best suited for;

• Rolling mills
• Lifts & hoists
• Big flour mills
• Large pumps
• Line shafts of heavy industries
• Prime-moves with medium & also large generators.

INDUCTION MOTOR FAQ

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