24-07-2012, 01:22 PM
INDUCTION MOTOR
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induction
or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than a commutator or slip rings as in other types of motor. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are rugged and have no brushes. Single-phase versions are used in small appliances. Their speed is determined by the frequency of the supply current, so they are most widely used in constant-speed applications, although variable speed versions, using variable frequency drives are becoming more common. The most common type is the squirrel cage motor, and this term is sometimes used for induction motors generally.
History
Squirrel cages were invented by Mikhail Dolivo-Dobrovolsky.
The idea of a rotating magnetic field was developed by François Arago in 1824,[1] and first implemented by Walter Baily.[2] Based on this, practical induction motors were independently invented by Nikola Tesla in 1883 and Galileo Ferraris in 1885.[3] According to his 1915 autobiography Tesla conceived the rotating magnetic field in 1882 and used it to invent the first induction motor in 1883;[4] Ferraris developed the idea in 1885.[5] In 1888, Ferraris published his research to the Royal Academy of Sciences in Turin, where he detailed the foundations of motor operation;[6] Tesla, in the same year, was granted U.S. Patent 381,968 for his motor. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky a year later.
Operation
In both induction and synchronous motors, the stator is powered with alternating current (polyphase current in large machines) and designed to create a rotating magnetic field which rotates in time with the AC oscillations. In a synchronous motor, the rotor turns at the same rate as the stator field. By contrast, in an induction motor the rotor rotates at a slower speed than the stator field. Therefore the magnetic field through the rotor is changing (rotating). The rotor has windings in the form of closed loops of wire. The rotating magnetic flux induces currents in the windings of the rotor as in a transformer. These currents in turn create magnetic fields in the rotor, that interact with (push against) the stator field. Due to Lenz's law, the direction of the magnetic field created will be such as to oppose the change in current through the windings. The cause of induced current in the rotor is the rotating stator magnetic field, so to oppose this the rotor will start to rotate in the direction of the rotating stator magnetic field to make the relative speed between rotor and rotating stator magnetic field zero.
For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field ( ), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. As the speed of the rotor drops below synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the windings and creating more torque. The ratio between the rotation rate of the magnetic field as seen by the rotor (slip speed) and the rotation rate of the stator's rotating field is called "slip". Under load, the speed drops and the slip increases enough to create sufficient torque to turn the load. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as induction generator, or it can be unrolled to form the linear induction motor which can directly generate linear motion.