Induction Motor

Induction Motor or Asynchronous Motor

An induction motor or asynchronous motor is an AC electric motor in which the electric current in the rotor need to produce torque is obtain by electromagnetic induction from the magnetic field of the stator winding.An induction motor can therefore be made without electrical connections to the rotor.

History

In 1824, the French physicist François Arago formulated the existence of rotating magnetic fields, termed Arago’s rotations. By manually turning switches on and off, Walter Baily demonstrated this in 1879, effectively the first primitive induction motor

The first commutator-free single-phase AC induction motor was invent by Hungarian engineer Ottó Bláthy; he used the single phase motor to propel his invention, the electricity meter.

Operation Principle

the AC power supplied to the motor’s stator creates a magnetic field that rotates in synchronism with the AC oscillations. Whereas a synchronous motor’s rotor turns at the same rate as the stator field. an induction motor’s rotor rotates at a somewhat slower speed than the stator field. The induction motor stator’s magnetic field is therefore changing or rotating relative to the rotor. This induces an opposing current in the induction motor’s rotor. in effect the motor’s secondary winding. when the latter is short-circuit or close through an external impedance.

The rotating magnetic flux induces currents in the windings of the rotor, in a manner similar to currents induced in a transformer‘s secondary winding

The induced currents in the rotor windings in turn create magnetic fields in the rotor that react against the stator field. The direction of the magnetic field created will be such as to oppose the change in current through the rotor windings, in agreement with Lenz’s Law.

The cause of induced current in the rotor windings is the rotating stator magnetic field, so to oppose the change in rotor-winding currents the rotor will start to rotate in the direction of the rotating stator magnetic field.

Construction

The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the magnetic field. windings are distribute in slots around the stator. with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases.

Many single-phase motors having two windings can be viewed as two-phase motors.since a capacitor is use to generate a second power phase 90° from the single-phase supply and feeds it to the second motor winding. Single-phase motors require some mechanism to produce a rotating field on startup. Cage induction motor rotor’s conductor bars are typically skew to avoid magnetic locking.

AC type Induction Motor
Induction motor

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