30-11-2012, 12:50 PM
Induction Motor
Electric Motors
Electric motors and generators are referred to as electric machines. Electricians are most
frequently concerned with electric motors, due to their extensive application.
The electric motor must be one of man’s most useful inventions. In the manufacturing
industries they are used in large numbers, to drive lathes, drilling and milling machines, augers,
conveyors, cranes, hoists, lifts, fans and steel rolling equipment. In the process industries they
are used to pump liquids and gases. They are used in transport to start engines, operate
windscreen wipers, open and close windows and power electric vehicles. In domestic situations
they are used in washing machines, clothes dryers, cookers, fridges, freezers, vacuum cleaners,
food mixers, audio / video equipment, cameras, clocks etc.
Electric motors are popular because they are compact, reliable, and cheap, need little attention,
and are convenient to use. They can be provided in a wide range of sizes and can be designed to
have different characteristics for various applications. Also, there is a readily available supply
of electricity. The electric motor may be regarded as an energy converter. It is supplied with
electrical energy and provides mechanical energy as an output as shown in Figure 1.
Three Phase ( Squirrel Cage ) Induction Motor.
This motor is very robust and, because of its simplicity and trouble free features, it is the type
of motor most commonly employed for industrial use. There are only three essential parts,
namely the
stator frame
, the
stator windings
and the
squirrel cage rotor
The Stator Frame
The stator frame may be constructed from cast iron, aluminium or, rolled steel. Its purpose is to
provide mechanical protection and support for the stator laminated metal core, windings and
arrangements for ventilation.
The Stator Windings
Three symmetrically placed windings ( one for each phase ) are fitted into slotted laminations,
which are made from high-grade alloy steel to reduce the effects of eddy currents. Generally the
six ends of the three windings are brought out into a terminal box. Figure 3 shows the stator
windings fitted into the slotted laminations. The six coloured leads from the winding ends are
also visible.