Introduction: ...

Introduction:


The most commonly used motors in today’s industry are the induction motors, importantly since they are simple and robust construction, maintained easily and less expensive. One main function of induction motor is that it operates at constant speed from no load to full load; hence this function makes the induction motor to be used in various applications such as pumps, fans, compressors and conveyors which involve constant speed.
The induction motor is mainly built of two components. One is the stator, from the name itself it can be understood that it is stationary and it produces a rotating magnetic field when its coils are supplied with an AC (Alternating Current). The second one is the rotor which is inside attached with the shaft of the motor and it rotates when the rotating field from the stator gives a torque.
Stator:
The stator forms the main body of an induction motor and it consists of three phase windings. Depending on the starting methods used the windings can be connected in delta connection or wye (Y) or star connection. The stator which is the body of the motor is basically made of steel frames and inside this body is hollow and cylindrical cores made out of stacked laminations. A number of evenly spaced slots, punched out of the internal circumference of the laminations, provide the space for the stator winding.
Figure 1.1: Typical Stator [1.1]
Rotor:
The rotor is of two kinds of windings i.e. wound rotor which consists of slip rings and brushes and the other one is squirrel cage which is a brushless design. Squirrel cage motors are very common alternating current motors found around us in all domestic industries. The name squirrel cage is taken from its shape which consists of bare copper bars, slightly longer than the rotor which is made into slots. To short circuit the bars, the opposite ends are welded to two copper end rings. The bars and end rings are made of die-cast aluminium which can be found in small or medium sized motors. Normally there are very high currents at low voltage in the bars and end rings. In order to reduce the resistances in the motor, cast copper is used to increase the efficiency. Squirrel cage rotors are usually less expensive and robust than other rotors.
Figure 1.2: Squirrel-Cage Induction Rotor [1.2]
Wound rotors were usually used in the past when the need for variable speed in applications was required. In a wound rotor the numbers of poles are same as that of the stator and slip rings connecting with the motor shaft by wires are the windings. To change the motor’s slip rate, a controller like a variable resistor is connected to the slip rings by using carbon bushes. Windings are uniformly distributed in the slots which are wye or star connected. The three slip rings which turn the motor are connected to the terminals. Since the brushes and the slip rings require frequent maintenance, the wound ...