INDUCTION MOTOR
An induction motor (also known as an asynchronous motor) is a commonly used AC electric
motor. In an induction motor, the electric current in the rotor needed to produce torque is
obtained via electromagnetic induction from the rotating magnetic field of the stator
winding. The rotor of an induction motor can be a squirrel cage rotor or wound type rotor.
Induction motors are referred to as ‘asynchronous motors’ because they operate at a speed
less than their synchronous speed.
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. The induction motor always
runs at speed less than its synchronous speed.
The rotating magnetic field produced in the stator will create flux in the rotor, hence causing
the rotor to rotate. Due to the lag between the flux current in the rotor and the flux current
in the stator, the rotor will never reach its rotating magnetic field speed (i.e. the synchronous
speed).
Induction motors are classified into two types based on the input supply: single phase and
three phase. Single phase induction motors are not self-starting, whereas three phase
induction motors are self-starting.
Working Principle of Induction Motor
We need to give double excitation to make a DC motor to rotate. In the DC motor, we 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 interesting to know how an induction
motor works.
It is simple, from the name itself we can understand that here, the induction process is
involved. When we give the supply to the stator winding, a magnetic flux gets produced in
the stator due to the flow of current in the coil. The rotor winding is so arranged that each
coil becomes short-circuited.
The flux from the stator cuts the short-circuited coil in the rotor. As the rotor coils are short-
circuited, according to Faraday’s law of electromagnetic induction, the current will start
flowing through the coil of the rotor. When the current through the rotor coils flows,
another flux gets generated in the rotor.
Now there are two fluxes, one is stator flux, and another is rotor flux. The rotor flux will be
lagging with respect to the stator flux. Because of that, the rotor will feel a torque which will
make the rotor to rotate in the direction of the rotating magnetic field. This is the working
principle of both single and three-phase induction motors.
Types of Induction Motors
The types of induction motors can be classified depending on whether they are a single
phase or three phase induction motor.
Single Phase Induction Motor
The types of single phase induction motors include:
1. Split Phase Induction Motor
2. Capacitor Start Induction Motor
3. Capacitor Start and Capacitor Run Induction Motor
4. Shaded Pole Induction Motor
Three Phase Induction Motor
The types of three phase induction motors include:
1. Squirrel Cage Induction Motor
2. Slip Ring Induction Motor
Type of starter
1. DOL starter
Direct On-Line (D.O.L.) Starting Method
Direct On-Line (D.O.L.) starting is the simplest and most economical method to start a 3-
phase induction motor. In this method, the motor is connected directly to the power supply,
causing it to start immediately. Small three-phase induction motors can be started directly
online (DOL), meaning the rated supply voltage is directly applied to the motor. However,
this results in a very high starting current, typically 5 to 7 times the rated current, and the
starting torque is usually 1.5 to 2.5 times the full load torque.
Advantages
Simple and cost-effective
Provides full starting torque
Minimal components required
Disadvantages
High inrush current (6-8 times the rated current)
Mechanical stress on the motor components
Voltage dips in the electrical network
2. Star Delta Starter
Stator Resistance Starting Method
In this method, external resistors are connected in series with each phase of the stator
winding during startup. These resistors cause a voltage drop across them, resulting in a
reduced voltage being applied to the motor terminals. Consequently, the starting current is
lowered. As the motor accelerates, these external resistors are gradually removed from the
stator circuit in steps. Once the motor reaches its rated speed, the resistors are eliminated,
and the full line voltage is applied directly across the motor terminals.
Advantages
Reduced inrush current
Lower starting torque, reducing mechanical stress
Disadvantages
Power loss in resistors
Reduced efficiency during start-up
Requires additional components
3. Auto transformer Starter
Autotransformer Starting Method
An autotransformer is used to step down the supply voltage during startup, via tapping
arrangements. This limits the starting current. Once the motor picks up sufficient speed, it is
reconnected directly to the supply voltage. This method provides a controlled and gradual
increase in voltage, thereby limiting the starting current and torque.
Advantages
Reduced inrush current
Smooth acceleration
Higher efficiency compared to stator resistance starting
Disadvantages
More complex and expensive than D.O.L. starting
Requires additional space for the autotransformer
Efficiency & Losses of induction motor
There are two types of losses occur in three phase induction motor. These losses are
1. Constant or fixed losses
2. Variable losses
Constant or Fixed Losses
Constant losses are those losses which are considered to remain constant over normal
working range of induction motor. The fixed losses can be easily obtained by performing no-
load test on the three phase induction motor. These losses are further classified as-
1. Iron or core losses,
2. Mechanical losses,
3. Brush friction losses.
Variable Losses
Variable losses, also known as copper losses, occur due to the current in
the stator and rotor windings. As the load changes, the current and thus
these losses change. These losses are determined by performing a
blocked rotor test on a three-phase induction motor. The main function of
an induction motor is to convert electrical power into mechanical power,
which involves different stages of power flow.
Efficiency of Three Phase Induction Motor
Efficiency is defined as the ratio of the output to that of input,
Rotor efficiency of the three phase induction motor ,
Gross mechanical power developed / rotor input
Three phase induction motor efficiency,
Three phase induction motor efficiency
Significance of torque slip
Definition: In Induction Motor, a slip is a speed among the rotary magnetic flux as well as
rotor expressed in terms of for every unit synchronous speed. It can be measured in
dimensionless & the value of this motor cannot be zero.
importance of Slip in an Induction Motor
The importance of slip in the induction motor can be discussed below based on the values of
a slip because the motor behaviour depends mainly on the slip’s value.
When the Value of Slip is ‘0’
If the slip value is ‘0’ then the speed of the rotor is equivalent to revolving magnetic flux. So
there is no motion among the coils of the rotor as well as revolving magnetic flux. So, there
is no flux cutting act in the rotor coils. Therefore, emf will not be generated within rotor
coils for generating rotor current. So this motor will not work. So, it essential to have a
positive slip value in this motor and due to this reason, the slip will never become ‘0’ in an
induction motor.
When the Value of Slip is ‘1’
If the slip value is ‘1’ then the rotor in the motor will be stationary
When the Value of Slip is ‘-1’
If the slip value is ‘-1’ then the speed of the rotor in the motor is more comparable with the
synchronously revolving magnetic flux. So, this is possible only when the rotor within the
motor is turned in the revolving magnetic flux direction using the prime mover
This is only possible when the rotor is turned in the direction of revolving magnetic flux by
some prime mover. In this condition, the motor operates as an induction generator.
When the Value of Slip is >1
If the slip value of the motor is greater than one then the rotor will turn in the opposite
direction to the revolution of magnetic flux. So if the magnetic flux is revolving in the
direction of clockwise, then the rotor will turn rotating in the anti-clockwise direction. So,
the speed among them will be like (Ns + Nr). In braking or Plugging of this motor, the slip is
greater than ‘1’ is attained to rapidly bring the rotor of the motor at rest.
Formula
The formula of the slip in the induction motor is given below.
Slip = (Ns-Nr/Ns)*100
In the above equation, ‘Ns’ is the synchronous speed in rpm whereas the ‘Nr’ is the
rotational speed in the rpm (revolution for each second)
