Three Phase Induction Motor

What is a 3-Phase Induction Motor?

The three phase induction motor is a rotating electronic machine that is operated for a three-phase supply. This 3-phase induction motor is also called an asynchronous motor. These AC motors are of two types such as squirrel cage and slip-ring type 3-phase induction motor. A 3-phase induction motor (A.C.) consists of two major parts such as

  • Stator 
  • Rotor  

What is Stator in 3-Phase Induction Motor?

The stator of a three-phase induction motor is made up of several slots to form a 3-phase winding circuit which we connect to a 3-phase AC source of the induction motor. We arrange the three-phase air in the slots in such a way that they can create a rotating magnetic field. When we switch the source of the three-phase AC supply, a rotating magnetic field is created.

What is Rotor in 3-Phase Induction Motor?

Conductors are bars made of copper or aluminum attached to each slot and briefly propagated by end rings. The slots are not made parallel to the axis of the shaft but are slightly slaked as this arrangement reduces. The magnetic humming noise can avoid motor stalling.

What is the Working Principle of 3-Phase Induction Motor?

3-phase induction motor operates according to the principle of electromagnetic induction which was invented back in 1830 by the English physicist Michael Faraday. Faraday observed that when a conductor, such as a coil or wire loop, is placed in a modified magnetic position, there is an EMF present flowing electrical energy or EMF that is generated in the conductor. Since both the width and the direction will change. It is expressed mathematically by relating the electric field curl to the rate of change of magnetic flux.

When the stator of the three-phase induction motor is driven from a three-phase electrical power source, each coil creates a magnetic field whose poles (north or south) change position throughout the cycle to the same position as the AC current. Since the AC current is shifted by 120 in each of the three phases of the current, the magnetic polarization of the three coils does not occur simultaneously. This condition produces a stator known as RMF or a rotating magnetic field.


As the rotor sits in the center of the store coils, the changing magnetic field from the stator flows into the rotor coil, resulting in an inverse magnetic field generated by the rotor. The rotor field tries to align its polarity with the stator field, the result being net torque is applied to the motor shaft. And it starts rotating as it tries to align its field. Note that in a 3-phase induction motor, there is no direct electrical connection to the rotor; Magnetic induction causes motor rotation

What is Slip in Induction Motor?

Slip is the difference between synchronous speed and asynchronous speed. The difference between the synchronous speeds of the electric motor magnetic field. The rotation speed of the shaft is measured in rpm or frequency. The slip increases with increasing load providing a greater torque.

What is 3-Phase Motor Controller?

Since the source of RMF in the stator coils is a speed that is generated by a three-phase AC motor, it is a function of the frequency of the AC supply. Therefore, some AC motor controllers operate the motor using the AC current. That is input to create a modulated or controlled frequency input thereby controlling the speed of the motor. Another method that can be used to control motor speed is by changing the slip (described earlier). As the slip increases the speed of the motor (e.g. the speed of the rotor) decreases.

How to Produce Magnetic Flux?

A rotating magnetic field in the stator is the first part of the operation. In order to generate torque and thus rotate, the rotors must carry some current. In induction motors, this current comes from the rotor conductor. The rotating magnetic field produced in the stator cuts the conductive bars of the rotor and induces an e.m.f The rotor windings of induction motors are either closed by external resistance or shorted directly.

Therefore, the e.m.f transmitted to the rotor flows a current against the rotating magnetic field in the stator and carries a twisting speed or torque to the rotor. As a result, the rotor speed will not reach the synchronous speed of e.m.f in the stator. If the speed is combined, then no e.m.f. Induced in the rotor, no current will flow and therefore no torque will be generated. The difference between stator (synchronous speed) and rotor speed is called slip.

What is the Difference Between Induction Motor and Synchronous Motor?

The induction motor and synchronous motor work by obeying the different principles. There are lots of differences between induction motor and synchronous motor. These are given below.

Three Phase Induction MotorSynchronous Motor
Three Phases or single phase Induction motor converts electrical energy into mechanical energy at a variable speed.The synchronous motor converts electrical energy into mechanical energy at a constant speed.
Induction motor speed can be changed by changing the load.If the load is increased on the synchronous motor after the synchronous motor run at a constant speed
The induction motor works and follows the faradays Law of the electromagnetic induction process.Constant speed motor always follows and works by the magnetic locking.
A three-phase induction motor is a self-starting motor.A synchronous motor cannot start by itself.
The induction motor consists of two main parts stator and rotor.The synchronous motor consists of two main parts stator and rotor.
Induction runs at 1500 rpm.Synchronous motor runs from 150 to 1800rpm. (speed never decreases)
The slip in induction motor is different.Synchronous motor slip is zero (0)
Maintenance cost is low           Maintenance cost is high    

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