The polyphase induction motor is the most widely used ac motor due to its low cost, simple and extremely rugged construction, high reliability, high efficiency, reasonably good power factor, low maintenance cost and simple starting arrangement.
It differs from other types of electric motors in that there is no electrical connection from the rotor winding to any source of supply. The necessary current and voltage in the rotor circuit are produced by induction from the stator winding. This is the reason that it is called the induction motor.
CONSTRUCTION
The induction motor essentially consists of two parts
(i) Stationary part known as stator and
(ii) The revolving part, known as the rotor.
1. Stator. Stator of a 3-phase induction motor is similar in construction to that of a 3-phase synchronous machine. The stator winding, also sometimes known as primary winding, is placed in slots on the inner periphery of the core. The core and windings are enclosed in cast iron frame. The stator winding is usually arranged for 3-phase power supply, the phases of which may be connected either in delta or star as per design of the machine. It is would for a definite number of poles as per requirement of speed-greater the number of poles lesser the speed and vice-versa for a supply of given frequency.
2. Rotor. The rotors employed in 3 phase induction motors are of three types namely
(a) squirrel cage
(b) Wound rotor
(c) Double Cage.
(a) Squirrel Cage Rotor. Almost 90 percent of induction motors are provided with squirrel cage rotor because of its very simple, robust and almost instrucible construction. Squirrel cage rotor is constructed of a laminated core with the conductors 9copper or aluminum bars) placed parallel, or approximately parallel to the shaft and embedded in the core surface. At each end of the rotor, the rotor conductors are all short-circuited by continuous end rings of similar materials to that of conductors. The slots on the rotor are usually skewed in order to obtain a more uniform torque and reduce the magnetic humming noise while running and reduce the locking tendency of the rotor.
(b) Wound Rotor. Such a rotor is wound with an insulated winding similar to that of stator winding and for the same number of poles as that of the stator. The rotor winding is always three phase winding even when the stator is wound for two phases.
(c) Double Squirrel Cage Rotor. Such a rotor carries two squirrel cage windings embedded in two rows of slots. The outer slots contain a high resistance and low leakage reactance winding and the inner slots contain a low resistance and high leakage reactance winding. The outer winding bars are usually of manganese brass while the inner winding bars are of red copper.
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