CHK ATT 4

Polyphase Induction Motor



Polyphase Induction Motor

Task One

The polyphase induction motor is the most commonly used industrial motor, finding application in many situations where speed regulation is not essential. It is simple and relatively inexpensive, and the absence of sliding contacts in the squirrel-cage machine reduces maintenance to a minimum. There are two general types of polyphase induction motors: the squirrel-cage type and the wound-rotor machine. Both motors have an armature or stator structure similar to that of the alternating current generator, consisting of a hollow cylinder of laminated sheet steel in which are punched longitudinal slots. A symmetrical polyphase winding is laid in these slots which, when connected to a suitable voltage source, produces a travelling MMF wave in the air gap, rotating at a synchronous speed equal to: RP Msync = 120 fp (1) where f is the frequency and p the number of poles for which the stator is wound. The squirrel-cage type of rotor is made up of sheet steel laminations keyed to the shaft and having slots punched in the periphery. The number of slots in the rotor is never a multiple of the number in the stator, thereby preventing rotor locking under light load conditions. The rotor conductors in most machines are made of aluminum alloy either molded or extruded in place in the slots, with end rings being cast as an integral part of the structure and connecting all bars at both ends. The air-gap length between rotor and stator is kept as short as manufacturing tolerances will allow in order to minimize the magnetizing current necessary for the production of normal air-gap flux. A simple twopole, three-phase, squirrel-cage induction motor is diagrammed in Fig. 1.

The wound-rotor induction motor has a rotor similar to that of the squirrel-cage machine except that the short-circuited squirrel-cage winding is replaced by a three-phase insulated winding similar to that on the stator. This winding is usually wye-connected with the terminals brought out to three slip rings on the shaft. Graphite brushes connected to the slip rings provide external access to the rotor winding which is connected to a rheostatic controller, the purpose of which is to insert additional resistance in each rotor phase to improve the starting characteristics. In practically all induction motors, either the rotor or the stator slots are skewed one slot width as shown in Fig. 1(a). The purpose is to smooth the flux transition from one slot to the next, thereby reducing harmonics in the torque characteristic and improving the operation.

Basic operation of the induction motor

As previously shown, the phase displacement between the voltages applied to the stator windings produces a travelling MMF or rotating magnetic field in the uniform air gap. This field links the short-circuited rotor windings, and the relative motion induces shortcircuit currents in them, which move about the rotor in exact synchronism with the rotating magnetic field. It is well known that any induced current will react in opposition to the flux linkages producing it, resulting ...