Induction Motor:

One of the most common electrical motor used in most applications which is known as induction motor. This motor is also called as asynchronous motor because it runs at a speed less than its 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 which is produced in the stator will generate flux in the rotor which will make the rotor to rotate, but due to the lagging of flux current in the rotor with flux current in the stator, the rotor will never reach to its rotating magnetic field speed i.e. the synchronous speed. There are basically two types of induction motor that depend upon the input supply - single phase induction motor and three phase induction motor. Single phase induction motor is not a self starting motor which we will discuss later and three phase induction motor is a self-starting motor.

Working Principle of Induction Motor:

We need to give double excitation to make a machine to rotate. For example if we consider a DC motor, we will 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 that how it works. It is very simple, from the name itself we can understand that induction process is involved. Actually when we are giving the supply to the stator winding, flux will generate in the coil due to flow of  current in the coil. Now the rotor winding is arranged in such a way that it becomes short circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited, according to Faraday's law of electromagnetic induction, current will start flowing in the coil of the rotor. When the current will flow, another flux will get generated in the rotor. Now there will be two flux, one is stator flux and another is rotor flux and the rotor flux will be lagging w.r.t to the stator flux. Due to this, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply. This is the working principle of an induction motor of either type – single and three phase. .

Types Induction Motor

1.   Single Phase Induction Motor

1.   Split phase induction motor

2.   Capacitor start induction motor

3.   Capacitor start capacitor run induction motor

4.   Shaded pole induction motor

2.   Three Phase Induction Motor

1.   Squirrel cage induction motor

2.    Slip ring induction motor

Key Differences between Single Phase & Three Phase Induction Motor:

Single Phase Induction Motor	Three Phase Induction Motor
The AC asynchronous motor that runs on single phase AC power supply.	AC asynchronous motor that runs on three phase AC power supply.
It has 2 terminals thus it requires only two wires to power it up.	It has 3 terminals and requires three or four (including neutral) wires to operate.
It is not a self-start motor.	It is a self-start motor.
It requires extra starting mechanism.	It does not require external starting mechanism.
The direction control is a little difficult and it is changed by swapping the polarity of starter winding.	The direction control is simple and it is done by swapping any two input phases.
Their types include; Spilt phase, shaded pole, capacitor start, capacitor start capacitor run induction motor etc.	Their types are; squirrel cage induction motor and wound type induction motor.
There are two slips i.e. forward and backward slip.	It has only forward slip.
It generates mechanical noise and vibration.	It operates smoothly with less noise.
The copper losses are high due to single winding carrying all the current.	The copper loses are low because windings share the current.
Its efficiency is lower.	It has high efficiency.
They are made for low power rating usually below 5kW.	They are made with power rating above 5kW.
It offers very limited starting torque.	It offers very high starting torque.
Its size is large considering same ratings.	Its size is smaller.
Its design is simple and easier to construct	It design is complex.
Its maintenance is very easy.	Its maintenance is relatively difficult.
It is cheaper.	It is expensive.
It is more reliable (less cost and easier to repair).	It is less reliable (high cost and expensive maintenance).
It is used in household and offices and other driving other small loads in industries such as fans, water pumps, drills etc.	It is used for driving heavy mechanical loads in industries.

We had 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? Question arises How it works?

 Why is Three Phase Induction Motor Self Starting?

In three phase system, there are three single phase line with 120° phase difference. So the rotating magnetic field is having the same phase difference which will make the rotor to move. If we consider three phases a, b and c, when phase a is 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.