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General
• The induction machine is used as a
motor and as a generator. However,
it is most frequently used as a
motor. It is the Workhorse of
industry.
• Majority of the motors used by
industry are squirrel cage induction
motors.
• Both three-phase and single-phase
motors are widely used.
• The induction generators are
seldom used. Their typical
application is the wind power plant.
Basic principles:
• An emf is induced in the
conducting bars as they
are “cut” by the flux while
the magnet is being
moved.
• E = BVL (Faraday’s Law)
• The emf induces or
produces a current
I,
which in term produces a
force,
F.
• F = BIL Lorentz Force
Stator construction
– The stator of an induction motor is similar to
a stator of any synchronous motor.
– Laminated iron core with slots
– Coils are placed in the slots to form a three
or single phase winding
• Squirrel-cage rotor construction
– Laminated Iron core with slots
– Metal bars are molded in the slots
– Two rings short circuits the bars
– The bars are slanted to reduce noise
Wound-rotor
• The picture shows the
rotor of a large woundrotor
motor
• The ends of each phase
is connected to a slip
ring.
• Three brushes contact
the three slip-rings to
three wye connected
resistances.
INDUCTION MOTORS
Rotating Magnetic Field
• In ac machines, the three-phase currents ia, ib
and ic, each of equal magnitude, but differing in
phase by 120°, produce a magnetic field of
constant magnitude that rotates in space. Such a
magnetic field produced by balanced three phase
currents flowing in thee-phase windings is called
a rotating magnetic field (RMF). Existence of a
RFM is an essential condition for the operation
of a ac rotating machine.
Three phase motors. Development of equivalent circuit
• The induction motor consists of a two magnetically connected systems:
Stator and rotor.
• This is similar to a transformer that also has two magnetically
connected systems: primary and secondary windings.
• The stator is supplied by a balanced three-phase voltage that drives a
three-phase current through the winding. This current induces a voltage
in the rotor.
• The applied voltage (
V1) across phase A is equal to the sum of the
– induced voltage (
E1).
– voltage drop across the stator resistance (
I1 R1).
– voltage drop across the stator leakage reactance (
I1 j X1).