Induction Motor Basics

Introduction to Induction Motor Basics:

As talking about induction motor basics, we know that an electrical motor is a device converts electrical energy into mechanical energy. Then this type of energy is supplied to the various loads. And induction motors are one of the types of motors. These machines are called induction machines because the rotor voltage which produces the rotor current and rotor magnetic field is induced in the rotor windings rather than the physical connection by wires. The distinguishing feature of induction motors is that the requirement of the dc field current is not necessary to run the machine. An induction machine can also be used to operate as a motor or generator but there are the distinguished disadvantages as the induction machine to operate as the generator. That’s why they are operated as motors and are known as Induction motors.

Construction of Induction Motor:

An induction motor has got the same stator as one having the synchronous machine. But the rotor construction is different than one with the synchronous machine. There are two types of rotors which can be placed inside the stator are

  1. Cage Rotor: A cage induction motor rotor consists of a series of conducting bars laid into slots carved in the face of the rotor and shorted at either end large shorting rings. The design is said to be the cage rotor because the rotor conductors when examined would like one of the exercise wheels on which hamsters or squirrels run. It is also known by the name of the squirrel cage rotor.


  1. Wound Rotor: A wound rotor has a complete set of three-phase windings that are mirror images of windings on the stator. The three phases of the rotor windings are usually connected in star-connection fashion, and the ends of the three rotor wires are tied to slip rings on the rotor’s shaft. The rotor windings are shorted through brushes riding on the slip rings. Wound-rotor induction motors, therefore, have their rotor currents accessible at the stator brushes, where they can be examined and where extra resistance can be inserted into the rotor circuit. It is possible to take advantage of this feature to modify the torque-speed characteristic of the motor.


Wound rotor induction motors are much more expensive than the squirrel cage induction motors because of the require much more maintenance due to wear associated with their brushes and slip rings. Due to which they are rarely used than the squirrel cage induction motors.

Basic Operation:

When input voltages are applied to the stator, the stator currents start flowing which produces the magnetic field Bswhich is rotating in the counter-clockwise direction. The speed at which the rotation of the magnetic field occurs is given by,

Ns = 120f/P


f = AC power supply frequency

P = Number of poles

The magnetic-filed Bs passes over the rotor bars induces the voltage in them. The voltage induced in the given rotor bar is given by

eind = (v x B).l


v = velocity of the bar relative to the magnetic field

B = magnetic flux density

l = length of conductor in a magnetic field

It is the relative motion of the rotor compared to the stator magnetic field that produces induced voltage the bar of the rotor. The velocity of the upper bars of the rotor relative to the magnetic field is to the right due to which direction of induced voltage is out of the page whereas the direction of the induced voltage in the lower bars of the rotor is into the page. Due to which current flows in and out of the lower and upper bars of the rotor, respectively. As we know that the rotor assembly is inductive that’s why the rotor current lags behind the rotor voltages. Thus, the flow of the rotor current produces rotor magnetic-field is BR. Therefore, the induced torque in the machine is given by

Tind = kBRxBS

Hence, the resulting rotor induced torque is counterclockwise, the rotor will accelerate in that direction.

As we know that the voltage induced in the rotor bar of the induction motor depends on the speed of the relative motion of the rotor magnetic fields. Since the behavior of an induction motor depends on the voltage and current of the rotor, the main thing to talk about the relative motion or speed of the magnetic fields of the rotor. Two terms are commonly used to describe this relative speed. One is the slip speed which is defined as the difference between the synchronous speed and rotor speed of the motor.

Nslip = Ns – Nm


Nslip = slip speed of the machine

Ns = speed of the magnetic fields

Nm = mechanical shaft speed of the motor         

The other term used to describe the relative motion is slip, which is the relative speed expressed on the percentage basis. Therefore, it can be written as

%s = (Nslip/Ns)*100

%s = (Ns – Nm/Ns)*100 

The slip can also be defined in terms of slip and it can be written as

Nm = (1-s)*Ns

As we know that the induction motor works by inducing voltages and currents in the rotor of the machine and for that reason, it is also known as the rotating transformer. Like a transformer, stator winding acts as the primary winding of the transformer while the rotor winding of the induction motor acts as the secondary winding of the transformer and we know that the primary induces voltages in the secondary but in induction motor, the frequency does not hold the same principle as of the transformer. That’s why the rotor frequency is directly proportional to the difference between the speed of the magnetic field Ns and the speed of the rotor Nm. Therefore, we can write that

fr = sf 


fr = rotor rotation frequency

f = AC power supply frequency

We can write

fr =(Ns – Nm/Ns)*f


Ns = 120f/P


fr = (Ns – Nm)*(P/120f)*f

fr = (P/120)*(Ns – Nm)

Hence, the rotor frequency can be determined by the above relation.

induction motor basics

Related Topics;

  1. Three Phase Induction Motor Basics
  2. Speed Control of Induction Motors
  3. Starting Methods of Induction Motors
  4. Why induction motor is used in electric vehicle?
  5. Why induction motor is called asynchronous motor?