N2 Electrical Trade Theory|The Easy Way! 57


Exercise 4.1


1. Where are the main windings of an alternating current induction motor situated?


2. Explain what you understand by the stator of an ac motor. 3. Explain what you understand by the rotor of an ac induction motor.


4. Make a neat sketch to illustrate the construction of the rotor windings and end-rings of a squirrel-cage induction motor.


5. What is the difference between a wound rotor and a squirrel-cage rotor of an induction motor?


6. Explain how the energy is transferred from the stator to the rotor in an induction motor.


7. With reference to an induction motor, what is the purpose of slip-rings? 4.3 Rotating magnetic field


Te operation of a three-phase induction motor is dependent on a rotating magnetic field. Tis rotating magnetic field is established by the stator currents through the stator windings.


A simplified stator winding layout of a three-phase, two-pole, star-connected motor is shown in fig. 4.2 (a). When the winding is connected to a three-phase supply, the phase currents vary in time phase as shown in fig. 4.2 (b), and a pulsating flux is set up by each of the three windings.


A brief but accurate description of how a rotating magnetic field is produced in a three-phase motor, follows:


Te currents through each of the three stator windings in a three-phase motor differ in phase by a third of a cycle (1200). A pulsating magnetic field is set up by each of these windings. Since each magnetic field follows aſter the next, they combine to set up a resultant field which moves around the stator surface at a constant speed.


Tis resultant flux is called the rotating magnetic field and the speed at which it rotates is known as synchronous speed.


Each set of phase windings sets up a pair of poles (north and south).


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