One of the most common electric motors used in most applications is known as an induction motor. This motor is also called an asynchronous motor because it operates at a speed less than its synchronous speed. Here we have to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotating machine and depends on the frequency and number of poles of the machine.
An induction motor always operates at a speed lower than the synchronous speed because the rotating magnetic field that occurs in the stator will generate flux in the rotor that will cause the rotor to rotate, but due to the delay of the flow current in the rotor with Current flow in the stator, the rotor will never reach its rotating magnetic field speed, that is, the synchronous speed.
Basically, there are two types of induction motors that depend on the input power: single-phase induction motor and three-phase induction motor. The single-phase induction motor is not an automatic starter motor that we will discuss later and the three-phase induction motor is an automatic starter motor.
A typical motor consists of two parts namely stator and rotor like other type of motors.
1. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field,
2. An inside rotor attached to the output shaft that is given a torque by the rotating field.
The stator of an induction motor is laminated iron core with slots similar to a stator of a synchronous machine. Coils are placed in the slots to form a three or single phase winding
Type of rotors
Rotor is of two different types.
1. Squirrel cage rotor
2. Wound rotor
In the squirrel-cage rotor, the rotor winding consists of single copper or aluminium bars placed in the slots and short-circuited by end-rings on both sides of the rotor. Most of single phase induction motors have Squirrel-Cage rotor. One or 2 fans are attached to the shaft in the sides of rotor to cool the circuit.
In the wound rotor, an insulated 3-phase winding similar to the stator winding wound for the same number of poles as stator, is placed in the rotor slots. The ends of the star-connected rotor winding are brought to three slip rings on the shaft so that a connection can be made to it for starting or speed control.
It is usually for large 3 phase induction motors.
Rotor has a winding the same as stator and the end of each phase is connected to a slip ring.
Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, so it is not so common in industry applications.
Working Principle of Induction Motor
We need to give double excitement to make a machine turn. For example, if we consider a DC motor, we will provide a supply to the stator and another to the rotor through a brush arrangement. But in the induction motor we offer only one supply, so it is really interesting to know how it works. It is very simple, from the name itself we can understand that the process of induction is involved.
Actually, when we provide the supply to the stator winding, the flow will be generated in the coil due to the current flow in the coil. Now the winding of the rotor is arranged in such a way that it short-circuits in the rotor itself. The stator flux 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 to flow in the rotor coil. When the current will flow, another flow will be generated in the rotor. Now there will be two flows, one is the stator flux and another is the rotor flux and the rotor flux will lag back to the stator flux. Because of this, the rotor will feel a torque that will cause the rotor to rotate in the direction of the rotating magnetic flux. So the rotor speed will depend on the AC supply and the speed can be controlled by varying the input supply. This is the principle of operation of an induction motor of any type: simple 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
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? We will discuss it now.
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.