An induction motor (IM) is a type of asynchronous AC motor
where power is supplied to the rotating device by means of electromagnetic
induction.
The induction motor with a wrapped rotor was invented by
Nikola Tesla Nikola Tesla in 1882 in France but the initial patent was issued
in 1888 after Tesla had moved to the United States. In his scientific work,
Tesla laid the foundations for understanding the way the motor operates. 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 (74.6 kW) motor from 1976 takes the same volume as a 7.5 hp (5.5 kW)
motor did in 1897. Currently, the most common induction motor is the cage rotor
motor.
An electric motor
converts electrical power to mechanical power in its rotor (rotating part).
There are several ways to supply power to the rotor. In a DC motor this power
is supplied to the armature directly from a DC source, while in an induction
motor this power is induced in the rotating device. An induction motor is
sometimes called a rotating transformer 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 drives. Induction
motors are now the preferred choice for industrial motors due to their rugged
construction, absence of brushes (which are required in most DC motors) and the
ability to control the speed of the motor.
CONSTRUCTION:
A typical motor consists of two parts namely stator and
rotor like other type of motors.
1. An outside stationary stator having coils supplied with
AC current to produce a rotating magnetic field,
2. An inside rotor
attached to the output shaft that is given a torque by the rotating field.
Stator construction:
The
stator of an induction motor is laminated iron core with slots similar to a
stator of a synchronous machine.Coils are placed in the slots to form a three or single
phase winding.
Type of
rotors
PRINCIPLE
OF OPERATION:
- An
AC current is applied in the stator armature which generates a flux in the stator magnetic circuit.
- This
flux induces an emf in the conducting bars of rotor as they are “cut” by the
flux while the magnet is being moved (E = BVL (Faraday’s Law))
- A
current flows in the rotor circuit due to the induced emf, which in term
produces a force, (F = BIL) can be changed to the torque as the output.
In a 3-phase
induction motor, the three-phase currents ia, ib and ic, each of equal
magnitude, but differing in phase by 120°. Each phase current produces a
magnetic flux and there is physical 120 °shift between each flux. The total
flux in the machine is the sum of the three fluxes. The summation of the three
ac fluxes results in a rotating flux, which turns with constant speed and has
constant amplitude. Such a magnetic flux produced by balanced three phase
currents flowing in thee-phase windings is called a rotating magnetic flux or
rotating magnetic field (RMF).RMF rotates with a constant speed (Synchronous
Speed). Existence of a RFM is an essential condition for the operation of an
induction motor.
If stator is
energized by an ac current, RMF is generated due to the applied current to the
stator winding. This flux produces magnetic field and the field revolves in the
air gap between stator and rotor. So, the magnetic field induces a voltage in
the shortcircuited bars of the rotor. This voltage drives current through the
bars. The interaction of the rotating flux and the rotor current generates a
force that drives the motor and a torque is developed consequently. The torque
is proportional with the flux density and the rotor
An electric motor
converts electrical power to mechanical power in its rotor (rotating part).
There are several ways to supply power to the rotor. In a DC motor this power
is supplied to the armature directly from a DC source, while in an induction
motor this power is induced in the rotating device. An induction motor is
sometimes called a rotating transformer 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 drives. Induction
motors are now the preferred choice for industrial motors due to their rugged
construction, absence of brushes (which are required in most DC motors) and the
ability to control the speed of the motor.
CONSTRUCTION:
A typical motor consists of two parts namely stator and
rotor like other type of motors.
1. An outside stationary stator having coils supplied with
AC current to produce a rotating magnetic field,
2. An inside rotor
attached to the output shaft that is given a torque by the rotating field.
Stator construction:
The
stator of an induction motor is laminated iron core with slots similar to a
stator of a synchronous machine.Coils are placed in the slots to form a three or single
phase winding.
Rotor
is of two different types.
1. Squirrel cage rotor
2. Wound
rotor
Squirrel-Cage Rotor :
In the squirrel-cage rotor, the rotor
winding consists of single copper or aluminium bars placed in the slots and
short-circuited by end-rings on both sides of the rotor. Most of single phase
induction motors have Squirrel-Cage rotor. One or 2 fans are attached to the
shaft in the sides of rotor to cool the circuit.
PRINCIPLE
OF OPERATION:
- An AC current is applied in the stator armature which generates a flux in the stator magnetic circuit.
- This flux induces an emf in the conducting bars of rotor as they are “cut” by the flux while the magnet is being moved (E = BVL (Faraday’s Law))
- A current flows in the rotor circuit due to the induced emf, which in term produces a force, (F = BIL) can be changed to the torque as the output.
In a 3-phase
induction motor, the three-phase currents ia, ib and ic, each of equal
magnitude, but differing in phase by 120°. Each phase current produces a
magnetic flux and there is physical 120 °shift between each flux. The total
flux in the machine is the sum of the three fluxes. The summation of the three
ac fluxes results in a rotating flux, which turns with constant speed and has
constant amplitude. Such a magnetic flux produced by balanced three phase
currents flowing in thee-phase windings is called a rotating magnetic flux or
rotating magnetic field (RMF).RMF rotates with a constant speed (Synchronous
Speed). Existence of a RFM is an essential condition for the operation of an
induction motor.