The document discusses induction motors. It explains that an induction motor works by electromagnetic induction, where the alternating current in the stator produces a rotating magnetic field that induces current in the rotor and causes it to turn. It describes the basic components of induction motors including the stator, rotor, and housing. It also discusses how varying the frequency of the alternating current supply can be used to control the motor's speed.

2. What is induction motor
OAn induction motor is an AC
electric motor in which the electric
current in the rotor needed to produce
torque is obtained by
electromagnetic induction from the
magnetic field of the stator winding.

3. Working principle of induction
motor
O When ac supply is given to the stator
winding of induction motor, the
alternating current starts flowing through
the stator or main winding. This
alternating current produces an
alternating flux called main flux. This
main flux also links with the rotor
conductors and hence cut the rotor
conductor.

4. FARADAY’S LAW OF ELECTROMAGNETIC
INDUCTION
According to the Faraday’s law of electromagnetic induction, emf gets
induced in the rotor. As the rotor circuit is closed one so, the current starts
flowing in the rotor. This currents called the rotor current. This
rotor current produces its own flux called rotor flux. Since this flux is
produced due to induction principle so, the motor working on this
principle got its name as induction motor. Now there are two fluxes one is
main flux and another is called rotor flux. These two fluxes produce the
desired torque which is required by the motor to rotate.

5.  Induction motors are used worldwide in
many residential, commercial, industrial,
and utility applications.
 Induction Motors transform electrical
energy into mechanical energy.
 It can be part of a pump or fan, or
connected to some other form of
mechanical equipment such as a winder,
conveyor, or mixer.

7. It consists of two parts:
1. Stator - It is the stationary part of the
motor.
2. Rotor - It is the rotating part of the
motor.

9. STATOR

10. Stator has three main parts:
Outer Frame – It is the outer body of the of the
motor. It protects the inner part of the machine.
Stator Core – Built up of high grade silicon steel.
Carries the alternating magnetic field
Stator winding – Has a three phase winding.

11. CORE
WINDINGS
SLOTS

13. Effect of 3 Phase Current Passing Through a
Stator Winding:
When a 3 phase AC current passes through the
winding It produces a rotating magnetic
field (RMF). As shown in the figure below a
magnetic field is produced which is rotating in
nature. We will see how this is produced in the
next section.

15. The Concept of a Rotating
Magnetic Field
To understand a rotating magnetic field, we
consider a simplified 3 phase winding with just
3 coils.
A wire carrying current produces a magnetic
field around it. Now for this special
arrangement, the magnetic field produced by 3
phase A.C current will be as shown at a
particular instant.

17. The components of A.C current will vary with
time.
Due to the variation in the A.C current, the
magnetic field also varies in orientation and its
magnitude remains the same.
The speed of rotation of the magnetic field is
known as synchronous speed.

19. The Effect of RMF on a Closed
Conductor
Assume we are putting a closed conductor inside
such a rotating magnetic field.
Since the magnetic field is fluctuating an E.M.F will
be induced in the loop according to Faraday’s law. The
E.M.F will produce a current through the loop.
The situation has become as if a current carrying loop
is situated in a magnetic field. This will produce a
magnetic force in the loop according to Lorentz law, So
the loop will start to rotate.

21. ROTOR

22. There are two types of rotors which are
employed in 3 – phase induction motor.
Squirrel Cage Rotor.
Phase Wound/ Slip Ring Rotor.

23. SQUIRREL CAGE
ROTOR

24. It consists of a laminated cylindrical core
having semi closed circular slots at the outer
periphery.
Copper or aluminum bar conductors are
placed in these slots and short circuited at
each end by copper or aluminum rings called
short circuiting rings.
The rotor winding is permanently short
circuited and it is not possible to add any
external resistance.

26. The rotor slots are not parallel to the shaft
but skewed to
Reduce humming .
 Provide smoother torque for different
positions of rotor.
Reduce magnetic locking of stator and
rotor.

28. PHASE
WOUND
ROTOR

30. It is also called SLIP RING ROTOR.
 Consists of a laminated core having semi
closed slots at the outer periphery and
carries a 3-phase insulated winding.
 The rotor is wound for the same number of
poles as that of stator.
 The three finish terminals are connected
together forming a star point and the three
star terminals are connected to three slip
rings fixed on the shaft.

31. Construction (Enclosure)
 The enclosure consists of a frame (or yoke) and
two end brackets (or bearing housings). The stator
is mounted inside the frame.
 The rotor fits inside the stator with a slight air gap
separating it from the stator. There is NO direct
physical connection between the rotor and the
stator.

32.  The enclosure also protects the electrical and
operating parts of the motor from harmful effects of
the environment in which the motor operates.
Bearings, mounted on the shaft, support the rotor
and allow it to turn. A fan, also mounted on the
shaft, is used on the motor shown below for
cooling.

33. Stator
Rotor
Air gap

34. The Working of an Induction Motor
A 3-phase AC current passing through a Stator winding
produces a rotating magnetic field.
current will be induced in the bars of the squirrel cage and it
will start to rotate.
We can note variation of the induced current in squirrel
cage bars.
This is due to the rate of change of magnetic flux in one
squirrel bar pair which is different from another, due to its
different orientation. This variation of current in the bar will
change over time.

35. which is the most commonly
used one in induction motors.

36. RMF produces a
torque on rotor as
in the simple
winding case.

37. Poles and speed
O Every ac induction motor has poles, just like a magnet. However,
unlike a simple magnet, these poles are formed by bundles of
magnet wire (windings) wound together in slots of the stator core.In
most cases, you can look inside the motor and count the number of
poles in the winding; they are distinct bundles of wire evenly spaced
around the stator core.
O The number of poles, combined with the ac line frequency (Hertz,
Hz), are all that determine the no-load revolutions per minute (rpm)
of the motor. So, all four-pole motors will run at the same speed
under no-load conditions, all six-pole motors will run at the same
speed, and so on.
O The mathematical formula to remember in helping make this
calculation is the number of cycles (Hz) times 60 (for seconds in a
minute) times two (for the positive and negative pulses in the cycle)
divided by the number of poles.

38.  Therefore, for a 60-Hz system, the formula would be:
60 x 60 x 2 = 7,200 no-load rpm ÷ number of poles.
 For a 50-Hz system, the formula would be:
50 x 60 x 2 = 6,000 no-load rpm ÷ number of poles.
 Using this formula, you can see that a four-pole motor
operating on the bench under no-load conditions runs at 1,800
rpm (7,200 ÷ 4 poles). Note that when an ac motor is loaded,
the spinning magnetic field in the stator does not change
speed. Instead, the rotor or moving part of the motor is
restrained by the load from “catching up” to the field speed.
 The difference between the field speed of 1,800 rpm in this
example and the rotor speed of approximately 1,725 rpm is
called the “slip.” Slip varies with the load over a narrow
operating range for each motor design.

39.  Motor Speeds, Both Loaded and Unloaded
 Our spinning four-pole motor, then, operates at 1,800 rpm in
this example under no-load conditions and approximately
1,725 rpm under load. Motors of this speed are commonly
found in belted applications such as blowers, fans, air-handling
equipment, compressors, and some conveyors .A
two-pole motor operates at 3,600 rpm (7,200 rpm ÷ 2)
unloaded, and approximately 3,450 under load. Two-pole
motors often are found in pump applications, such as sump
pumps, swimming pool pumps, and water re circulating
equipment.
 One thing for the service technician to keep in mind in the
field is that the higher the rpm, the noisier a motor may sound
to the untrained ear. It is beneficial to become aware of the
different speed-related sounds motors make.
 found in ceiling fans.

40. O Six-pole motors run at 1,200 rpm unloaded (7,200 ÷ 6)
and between 1,050 and 1,175 rpm loaded. They are often
used for air-handling equipment, direct-drive
applications, window fans, furnace blowers, room air
conditioners, heat pumps, and other equipment where
the relatively slower motor speed makes for quieter
operation. All can come in either totally open, totally
enclosed, or combination models, adding to their
versatility.
O To satisfy consumers’ desires for quieter motors,
manufacturers have developed eight-pole motors. These
operate at 900 rpm (unloaded) and approximately 800
rpm under load. They are being used in applications
where customers expect quieter operation, such as room
air conditioners and outdoor heat pump applications.
O Less-common pole configurations include 12-pole
motors (600 rpm) that are used in applications requiring
slow speeds, such as washing machines, and 16-pole
motors (450 rpm unloaded), often found in

41. electricity is induced in rotor by magnetic
induction rather than direct electric
connection , That's why the name induction
motor is used.
To aid such electromagnetic induction,
insulated iron core lamina are packed inside
the rotor.

42. Thin layers of iron lamina which are
packed in rotor

43. The Speed of Rotation of a Rotor
Both the magnetic field and rotor are rotating.
To find the speed of the rotor let's consider
different cases.

44. Consider a case where the rotor speed is
same as the magnetic field speed.
Since both the magnetic field and the rotor
are rotating at same speed, relative to the
rotor, the magnetic field is stationary.
The rotor will experience a constant
magnetic field, so there won’t be any induced
e.m.f and current. This means zero force on
the rotor bars, so the rotor will gradually slow
down.

45. But as it slows down, the rotor loops
will experience a varying magnetic field,
so induced current and force will rise
again and the rotor will speed up.
In short, the rotor will never be able to
catch up with the speed of the magnetic
field. It rotates at a specific speed which
is slightly less than synchronous speed.

46. Slip
The difference between the flux (Ns) and the
rotor speed (N) is called slip.
 % Slip = (Ns – N) × 100
Slip speed = Ns – N

48. Energy Transfer in the Motor
In an induction motor, electrical energy is enters
via the Stator and output from the rotor, the
mechanical rotation is received from the rotor.
But between the power input and output, there
will be numerous energy losses associated with the
motor. Various components of these losses are
friction loss, copper loss, eddy current and
hysteresis loss.
Such energy loss during the motor operation is
dissipated as heat, so a fan at the other end helps
in cooling down the motor.

49. A cooling fan is used to remove heat
liberated by motor

50. SPEED CONTROL OF AN INDUCTION MOTOR
The speed of an induction motor can be easily
controlled by varying the frequency of the 3-phase
supply.
To maintain a constant (rated) flux density, the applied
voltage must also be changed in the same proportion as
the frequency (as dictated by Faraday’s law).
This speed control method is known as Volts per Hz.
 Above rated speed, the applied voltage is usually kept
constant at rated value; this operation is referred to as
constant HP. At low frequencies (i.e. speeds), the voltage
must be boosted in order to compensate for the effects of
the stator resistance.

51. Advantages and disadvantages of
induction motors
“Advantages”
 They have only one moving part, the rotor,
which makes them low-cost, quiet, long-lasting,
and relatively trouble free.
DC motors, by contrast, have a commutator and
carbon brushes that wear out and need replacing
from time to time.
 The friction between the brushes and the
commutator also makes DC motors relatively noisy
(and sometimes even quite smelly).

52. “Disadvantages”
Since the speed of an induction motor depends on the
frequency of the alternating current that drives it, it turns at a
constant speed unless you use a variable-frequency drive.
 the speed of DC motors is much easier to control simply by
turning the supply voltage up or down.
Induction motors can be fairly heavy because of their coil
windings.
 Unlike DC motors, they can't be driven from batteries or
any other source of DC power without using an inverter.
That's because they need a changing magnetic field to turn
the rotor.