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Induction Motor | Working Principle | Types of Induction Motor

One of the most commonly used electrical motors is induction motor. We also call this motor as asynchronous motor because it runs at a speed less than its synchronous speed.

Synchronous Speed

Here we need to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine, and it depends upon the frequency and number poles of the machine.

An induction motor always runs at a speed less than synchronous speed. Because the rotating magnetic field produced in the stator will create flux in the rotor and hence will make the rotor to rotate. Due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach its rotating magnetic field speed i.e. the synchronous speed. Induction Motor There are basically two types of induction motors. The types of induction motors depend upon the input supply. The single phase induction motor and three phase induction motor. Single phase induction motor is not a self-starting motor and three phase induction motor is a self-starting motor.

Working Principle of Induction Motor

We need to give double excitation to make a DC motor to rotate. In a DC motor, we give one supply to the stator and another to the rotor through brush arrangement. But in induction motor, we give only one supply, so it is really interesting to know how an induction motor works. It is very simple, from the name itself we can understand that here, induction process is involved. Actually, when we give the supply to the stator winding, a magnetic flux is produced in the stator due to the flow of current in the coil. The rotor winding is arranged in such a way that each coil becomes short-circuited in the rotor itself.

The flux from the stator cuts the short-circuited coil in the rotor. As the rotor coils are short-circuited, according to Faraday's law of electromagnetic induction, current will start flowing through the coil of the rotor. When the current through the rotor coils flows, another flux gets generated in the rotor. Now there are two fluxes, one is stator flux and another is rotor flux. The rotor flux will be lagging with respect to the stator flux. Because of that, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic field. This is the working principle of an induction motor of either type – single and three phase.

Types Induction Motor

Single Phase Induction Motor

  1. Split Phase Induction Motor
  2. Capacitor Start Induction Motor
  3. Capacitor Start and Capacitor Run Induction Motor
  4. Shaded Pole Induction Motor

Three Phase Induction Motor

  1. Squirrel Cage Induction Motor
  2. Slip Ring Induction Motor
We have already mentioned above that single phase induction motor is not a self-starting and three phase induction motor is self-starting. So what is self-starting? When the machine starts running automatically without any external force to the machine, then it is called as self-starting. For example, we see that when we put on the switch the fan starts to rotate automatically, so it is self-starting. Point to be noted that fan used in home appliances is single phase induction motor which is inherently not self-starting. How? A question arises how it works? We will discuss it now.

Why is Three Phase Induction Motor Self Starting?

In three phase system, there are three single phase lines with 120° phase difference. So the rotating magnetic field has the same phase difference which will make the rotor to move. If we consider three phases a, b, and c when phase a is magnetised, the rotor will move towards the phase a winding a, in the next moment phase b will get magnetised and it will attract the rotor and then phase c. So the rotor will continue to rotate.

Working Principle of Three Phase Induction Motor - Video

Why Single Phase Induction Motor is not Self Starting?

It has only one phase still it makes the rotor to rotate, so it is quite interesting. Before that, we need to know why single phase induction motor is not a self-starting motor and how we overcome the problem. We know that the AC supply is a sinusoidal wave and it produces a pulsating magnetic field in uniformly distributed stator winding. Since we can assume the pulsating magnetic field as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting, and hence the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by an external force, then the motor will start to run. We can solve this problem by making the stator winding into two winding, one is main winding, and another is auxiliary winding. We connect one capacitor in series with the auxiliary winding. The capacitor will make a phase difference when current flows through both coils. When there is phase difference, the rotor will generate a starting torque, and it will start to rotate. Practically we can see that the fan does not rotate when the capacitor gets disconnected from the motor, but if we rotate with the hand, it will start rotating. That is why we use a capacitor in the single phase induction motor.
There are several advantages of induction motor which make this motor to have wider application. It is having good efficiency up to 97%. But the speed of the motor varies with the load given to the motor which is a disadvantage of this motor. The direction of rotation of induction motor can easily be changed by changing the phase sequence of three-phase supply, i.e. if RYB is in a forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor but in single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.



MMANSOOR ALI commented on 10/02/2018
This site is very useful for me and for others EE students
pPRIYA commented on 22/02/2018
clear explanation
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