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. The induction motor always runs at speed less than synchronous speed. The rotating magnetic field, which is 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 it’s rotating magnetic field speed, i.e., the synchronous speed.
There are basically two types of induction motor. The types of induction motor 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 the 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 interesting to know how an induction motor works. It is simple, from the name itself we can understand that here, the induction process is involved. When we give the supply to the stator winding, a magnetic flux gets produced in the stator due to the flow of current in the coil. The rotor winding is so arranged that each coil becomes short-circuited.
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 in respect of the stator flux. Because of that, the rotor will feel a torque which will make the rotor to rotate in the direction of the rotating magnetic field. Aforementioned is the working principle of an induction motor of either type – single and three phase.
Types Induction Motor
Single Phase Induction Motor
- Split Phase Induction Motor
- Capacitor Start Induction Motor
- Capacitor Start and Capacitor Run Induction Motor
- Shaded Pole Induction Motor
Three Phase Induction Motor
- Squirrel Cage Induction Motor
- 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 gets magnetized, the rotor will move towards the phase a winding a, in the next moment phase b will get magnetized 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 a 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 the 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 the 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 a 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 applications. It has 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.