2 Principle of operation and comparison to
5 Speed control
6 Starting of induction motor
7 Types of starters
An induction motor
(IM) is a type of
motor where power
is supplied to the
rotating device by
Three-phase induction motors
The induction motor with a wrapped rotor was
invented by Nikola Tesla in 1888 in the United
States. In his scientific work, Tesla laid the
foundations for understanding the way the motor
The induction motor with a cage was invented by
Mikhail Dolivo-Dobrovolsky about a year later in
Europe. Technological development in the field
has improved to where a 100 hp (73.6 kW) engine
from 1976 takes the same volume as a 7.5 hp (5.5
kW) engine did in 1897. Currently, the most
common induction motor, is the cage rotor motor.
Principle of operation
The basic difference between an induction motor and
a synchronous AC motor is that in the latter a current
is supplied onto the rotor. This then creates a
magnetic field which, through magnetic interaction,
links to the rotating magnetic field in the stator
which in turn causes the rotor to turn. It is called
synchronous because at steady state the speed of the
rotor is the same as the speed of the rotating
magnetic field in the stator.
The induction motor does not have any direct
supply onto the rotor; instead, a secondary
current is induced in the rotor. To achieve
this, stator windings are arranged around the
rotor so that when energised with a polyphase
supply they create a rotating magnetic field
pattern which sweeps past the rotor
At the moment illustrated, the current in the stator
coil is in the direction shown and increasing. The
induced voltage in the coil shown drives current and
results in a clockwise torque.
Note that this simplified motor will turn once it is
started in motion, but has no starting torque.
Various techniques are used to produce some
asymmetry in the fields to give the motor a starting
In a three phase induction motor, the induced emf in the rotor
circuit depends on the slip of the induction motor and the
magnitude of the rotor current depends upon this induced emf
(electromotive force). When the motor is started, the slip is equal
to 1 as the rotor speed is zero, so the induced emf in the rotor is
large. As a result, a very high current flows through the rotor. This
is similar to a transformer with the secondary coil short circuited,
which causes the primary coil to draw a high current from the
mains. Similarly, when an induction motor starts, a very high
current is drawn by the stator, on the order of 5 to 9 times the full
load current. This high current can damage the motor windings
and because it causes heavy line voltage drop, other appliances
connected to the same line may be affected by the voltage
fluctuation. To avoid such effects, the starting current should be
The rotational speed of the rotor is controlled by the number of
pole pairs (number of windings in the stator) and by the frequency
of the supply voltage. Before the development of cheap power
electronics, it was difficult to vary the frequency to the motor and
therefore the uses for the induction motor were limited. There are
various techniques to produce a desired speed. The most
commonly used technique is PWM(Pulse Width Modulation), in
which a DC signal is switched on and off very rapidly, producing a
sequence of electrical pulses to the inductor windings. The
dutycycle of the pulses, also known as the mark-space ratio,
determines the average power input to the motor.
TYPES OF MOTORS
Based on type of phase supply
○ Three phase induction motor (self starting
single phase induction motor (not self
Squirrel cage induction motor
Slip ring induction motor
The rotor bars in squirrel-cage induction motors
are not straight, but have some skew to reduce
noise and harmonics.
The motor's phase type is one of two types
1.Single-phase induction motor
2.3-phase induction motor
In a DC machine, the stator winding is excited by DC
current and hence the field produced by this
winding is time invariant in nature. In this machine the
conversion of energy from electrical to mechanical form or
vice versa is possible by one of the following ways:
1.rotating the rotor in the field produced by the stator
2.feeding external dc current through carbon brushes to
Electric motors convert electrical power to mechanical
power in its rotor (rotating part).
There are several ways to supply power to the rotor.
AC motor this power is induced in the rotating device.
An induction motor can be called a rotating
because the stator(stationary part) is essentially
the primary side of the transformer and the rotor
(rotating part) is the secondary side.
Induction motors are widely used, especially polyphase
induction motors, which are frequently used in industrial
The stator consists of wound 'poles' that carry the
supply current that induces a magnetic field in the
conductor. The number of 'poles' can vary between
motor types but the poles are always in (i.e. 2,4,6 etc)
pairs. There are two types of rotor
2.Slip ring rotor
The most common rotor is a squirrel-cage rotor. It is
made up of bars of either solid copper (most common)
or aluminum that span the length of the rotor, and are
connected through a ring at each end
Slip Ring Rotor
Resistance-start ac induction motor.
The relationship between the supply frequency, f, the
number of pole pairs, p, and the synchronous speed, n,
is given by f = p*n.
From this relationship:
Speed of rotating field (n) = f/P (revs.s-1)
Speed of rotor = n(1-S) (rev.s-1)
where S is the slip.
Slip is calculated using:
% slip = (n - r) / n * 100
where r is the rotor speed
In contrast, a synchronous motor always runs at either a
constant speed N=(120f)/P or zero.
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