Page 677 - Basic Electrical Engineering

Figure 8.6 Two magnetic fields always try to align with each other

Now let us consider an electromagnet, or a permanent magnet, which is

free to rotate, placed inside the rotating magnetic field as shown in Fig. 8.6.
The magnetic-field axis rotates in the clockwise direction and shifts its
position from 1 to 2, 3, 4…, and so on. The position of S-pole will be

opposite to the position of N-pole, as the poles rotate.
The rotor magnet, in trying to align itself with the rotating magnetic field,

will also rotate in the same direction at the same speed. When the rotor
rotates at the same speed as the speed of the rotating field, the rotor is said to

be in synchronism with the rotating field, and therefore this speed is called
synchronous speed. If the rotor is an electromagnet with dc supply given to

its windings, the rotor will rotate at synchronous speed when three-phase
supply is applied to the stator winding. Three-phase supply to three-phase
stator windings produce a magnetic field which rotates at a constant speed.

The rotor, when energized or excited by passing current through its windings,
becomes an electromagnet. The rotor, magnet, which is free to rotate, aligns

itself with the rotating magnetic field. It will continue to rotate at the same
speed as the rotating magnetic field. Such a motor will be called a three-phase

synchronous motor.
In a three-phase induction motor, however, only three-phase supply is

applied across the stator windings. No supply is provided to the rotor
winding. The rotor is made a closed winding either directly as in the case of