Introduction To Induction Motor: Experiment 11

Introduction to Induction
Experiment 11 Motor
Name:________________________ Date:_________________________
PERFORMANCE OBJECTIVES:
Upon successful of this completion of this experiment, the students will be able to:
 Understand the basic working principle of 3-phase induction motor
 Identify the constructional parts of a induction motor
EQUIPMENT:
Induction Motor Squirrel Cage (SM 2644)
THEORY:
Induction motor:
Induction motors are most common type of electrical motors. It is widely used because of
its simple construction, economical cost and ruggedness.
As the name suggests, the motor works on principle of electromagnetic induction. In a way it can
be called as rotating transformer because of the close similarity in principle of operation.
A schematic diagram of an induction motor is shown above. Pairs of electromagnetic
poles are housed in a casing called stator. The e electromagnetic poles are wound with
conductors to produce magnetism which is called stator windings. A rotating part called rotor is
placed in the annular gap of the stator by suitable mountings in such a way that the rotor can
rotate freely. The rotor may be either wire wound or may simply be bars of metal. In the latter
case, the rotor is called squirrel cage type.
Construction:
The stator consists of wound 'poles' that carry the supply current to induce a magnetic
field that penetrates the rotor. In a very simple motor, there would be a single projecting piece of
the stator (a salient pole) for each pole, with windings around it; in fact, to optimize the
distribution of the magnetic field, the windings are distributed in many slots located around the
stator, but the magnetic field still has the same number of north-south alternations. The number
of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.)
Induction motors are most commonly built to run on single-phase or three-phase power,
but two-phase motors also exist. Single-phase power is more widely available in residential
buildings, but cannot produce a rotating field in the motor (the field merely oscillates back and
forth), so single-phase induction motors must incorporate some kind of starting mechanism to
produce a rotating field. Three-phase motors have three salient poles per pole number. This
allows the motor to produce a rotating field, allowing the motor to start with no extra equipment
and run more efficiently than a similar single-phase motor.
There are three types of rotor:
a) Squirrel-cage 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 those solid copper
or aluminium strips can be shorted or connected by a ring or sometimes not, i.e. the rotor can
be closed or semi closed type. The rotor bars in squirrel-cage induction motors are not
straight, but have some skew to reduce noise and harmonics.
b) Slip ring rotor:
A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are
connected to slip rings. When these slip rings are shorted, the rotor behaves similarly to a
squirrel-cage rotor; they can also be connected to resistors to produce a high-resistance rotor
circuit, which can be beneficial in starting
c) Solid core rotor:
A rotor can be made from solid mild steel. The induced current causes the rotation.
Principle of operation:
A 3-phase power supply provides a rotating magnetic field in an induction motor.

The basic difference between an induction motor and a synchronous AC motor with a
permanent magnet rotor is that in the latter the rotating magnetic field of the stator will impose
an electromagnetic torque on the magnetic field of the rotor causing it to move (about a shaft)
and a steady rotation of the rotor is produced. 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.
By way of contrast, the induction motor does not have any permanent magnets on the
rotor; instead, a current is induced in the rotor. To achieve this, stator windings are arranged
around the rotor so that when energized with a poly phase supply they create a rotating magnetic
field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in
the rotor conductors. These currents interact with the rotating magnetic field created by the stator
and in effect cause a rotational motion on the rotor.
However, for these currents to be induced the speed of the physical rotor must be less
than the speed of the rotating magnetic field in the stator (the synchronous frequency ns) or else
the magnetic field will not be moving relative to the rotor conductors and no currents will be
induced. If by some chance this happens, the rotor typically slows slightly until a current is re-
induced and then the rotor continues as before. This difference between the speed of the rotor
and speed of the rotating magnetic field in the stator is called slip. It is unit less and is the ratio
between the relative speeds of the magnetic field as seen by the rotor (the slip speed) to the speed
of the rotating stator field. Due to this, an induction motor is sometimes referred to as an
asynchronous machine.
Synchronous speed:
To understand the behavior of induction motors, it is useful to understand their distinction

from a synchronous motor. A synchronous motor always runs at a synchronous speed- a shaft
rotation frequency that is an integer fraction of the supply frequency. The synchronous speed of
an induction motor is the same fraction of the supply.
It can be shown that the synchronous speed of a motor is determined by the following
formula:
Where ns is the (synchronous) speed of the rotor (in rpm), f is the frequency of the AC supply (in
Hz) and p is the number of magnetic poles per phase.
Slip:
The slip is a ratio relative to the synchronous speed and is calculated using:
Where
s is the slip, usually between 0 and 1
nr = rotor rotation speed (rpm)
ns = synchronous rotation speed (rpm)
Remarks :
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Introduction To Induction Motor: Experiment 11

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Introduction to Induction

Induction Motor Squirrel Cage (SM 2644)

A 3-phase power supply provides a rotating magnetic field in an induction motor.

To understand the behavior of induction motors, it is useful to understand their distinction

s is the slip, usually between 0 and 1

nr = rotor rotation speed (rpm)

ns = synchronous rotation speed (rpm)

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