What is Three Phase Induction Motor ?

The induction motor is an electric motor that runs on AC (alternating current) and it is of two type single phase motor that work on single-phase power supply and three-phase motor that work on three-phase power supply.

The three-phase motor is an alternating current machine (AC) that is used to convert the three phase alternating power into mechanical power. The three-phase power so obtained is used to drive the load for various application. The motor as like other electric motor like AC motor or DC motor have a stator winding to obtain the three phase supply and a rotor to produce the mechanical power. The rotor and the stator are separated by the air gap inside the yoke of the machine.

Three Phase Induction Motor Working Principle

The three-phase motor principle is based on the electromagnetic induction. That is when the three phase supply is given to the stator winding a rotating magnetic field is produced. This rotating magnetic field when links with the rotor's short circuited winding, an electromotive force (emf) is induced in the rotor winding. The emf induced in the rotor winding is due to the Lenz's law, the emf produces a current. Due to current flowing in the conductor a force is experienced by all the conductors of the rotor. The force aids in moving the rotor in the direction of the rotating magnetic field.

Construction of Three Phase Induction Motor

The three-phase motor is constructed with two main parts that is rotor and the stator, where both the stator and the rotor are the part of the yoke of induction motor. The stator and the rotor have a gap between them that ranges from 0.5 mm to 4 mm and the depends on the rating of the motor.

The diagram shows the basic structure of the three-phase motor that as a three-phase power supply, stator winding, rotor bars and the body frame
Three Phase Induction Motor

Stator of Three Phase Induction motor

The stator of the three-phase motor is made of the thin silicon steel lamination, the lamination are used to reduce the eddy current losses and the hysteresis losses of the core. The stator has a number of slots on which the three-phase motor winding is placed such that a three phase star of delta connected winding is formed.

The winding of the stator is connected to the three-phase power supply, the supply given produces magnetic field that has a synchronous speed. The speed of the machine is given by the expression:


Here, P is the number of Poles formed by the stator and the f is the frequency of the supply.

Rotor of Three phase Induction Motor

The rotor of the induction motor is one that rotates and does the major work of conversion of electrical energy into mechanical energy. The rotor has a number of slots on which the conductor or rotor bars are placed. For three-phase motor there are two types of rotor:

  1. Squirrel cage rotor
  2. Wound type or slip ring rotor

Squirrel Cage Rotor

The squirrel cage rotor has laminated core. The slots are there on the outer surface of the rotor and all the slots are parallel to each other. The conductor bars made of either the aluminium or the copper are placed in these slots. It is to be noted that all the conductor bars are short-circuited at the end with the help of the copper ring or the aluminium ring depending on the type of the conductor used. The three phase induction motor that use the squirrel cages are named as squirrel cage induction motor.

The rotor bars on the surface of the cage are not straight to the axis of the rotor, the rotor are somewhats skewed. The skewing is done to reduce the noise during the operation of the rotor, the rotor being simple in design produces the uniform torque. Also, the locking of the rotor with the stator teeth is somewhat reduced in case of the cage type rotor.

Shows the cross section of the squirrel cage rotor that has a short ring at the end to get the rotor bars short circuited
Squirrel Cage Rotor

Wound rotor motor

The wound rotor three phase induction motor has the armature on which the slots are made in the form of three phase distributed winding, the winding is somewhat similar to the three phase winding on the stator of the motor with the change that the winding of the rotor are connected in star. The winding of the induction motor are connected at the end with the slip ring. The purpose of the sling ring is to connect the rotor circuit with the starting resistance and the purpose of the starting resistance is to reduce the high amount of current at the starting and the control the starting torque.

The wound rotors have advantage over the slip ring rotors that high starting torque can be achieved with the wound rotor. The wound rotor can be used for speed changes and torque changes by connecting the external resistance to the circuit via the slip ring.

Shows the diagram of the wound rotor circuit in which there is a slip ring at the end to accommodate the connection of external resistance in the circuit.
Wound Rotor Circuit

Common Mistakes

  • Make sure that three-phase power supply given to the three-phase motor is as per the rating of the motor.
  • It is to be noted that speed change is not possible via the rotor circuit in case of squirrel cage motor.
  • The load on the three-phase motor must not exceed the rate load.
  • The electric motor are of two types AC motor and DC motor, the mode of supply to both must be AC and DC respectively.
  • The three phase motor is self starting so does not add any external prime mover to start the motor.

Context and Application

The electric motors have a variety of use in the field of engineering both and consumer and producer level. The three-phase motors like three phase induction motor is used in application that demands the high torque at starting and low value of current at the starting. The two type of rotors squirrel cage rotor and and wound rotor motor, both of these rotor have different applications.

  • Wound rotor motor have their application where the speed control is required like in compressor, cranes, hoists, elevators and many more.
  • Squirrel cage rotor motor has its application that requires constant speed like in fans, toys, air compressors and many more.

Some application areas of three phase induction motor are:

  • Bachelor of technology in electrical engineering
  • Masters of engineering in power and machine
  • In research work for efficient machine design

In three phase induction motor the torque and the speed shares an inverse relationship, the increase in the torque decreases the speed and the increase in speed decreases the torque.

Speed Control of three-phase motors

Consider the formula for the torque produced by the induction motor.


Here, E is the voltage induced in the rotor, s is the slip of the motor, R2 is the rotor resistance and X2 is the rotor reactance.

Consider that emf induced in the rotor is directly proportional to the voltage supplied to the stator V, if the supplied voltage is change the induced voltage is also changed and hence the torque.

The formula shows that the change in resistance that is connected to the rotor can change the torque as they both shares the inverse relationship.

Practice Problems

Q1. Which type of rotor is best suitable for resistance control.

(a). Squirrel cage rotor

(b). Wound rotor motor

(c). Deep cage rotor

(d). None of these

Correct option (b)

Explanation: Wound rotor motor have its rotor bars end connected with the slip ring, the slip ring allows addition of the resistance for speed control.

Q2. Use of lamination in in the stator and rotor is:

(a). To reduce the current

(b). Reduce the flux

(c). Reduces the core losses

(d). Reduce the copper losses

Correct option (c)

Explanation: The purpose of the lamination is to reduce the core losses that are the hysteresis and the eddy current losses.

Q3. Stator can have star or delta connected winding.

(a) True

(b) False

Correct option (a)

Explanation: The stator can be connected in both the star as to regulate the voltage required for the supply.

Q4. Squirrel cage rotor have its rotor bars open circuited for connection of external resistance.

(a) True

(b) False

Correct option (b)

Explanation: Squirrel cage rotor have its ends short circuited with the material same as the rotor bars.

Q5. If the number of pole is 2 and the frequency is 60 Hz. Find the speed of the field.

(a) 3600 rpm

(b) 1200 rpm

(c) 2400 rpm

(d) 3000 rpm

Correct option (a)

Explanation: Consider the calculation shown.

Ns=120fP=120602=3600 rpm

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