Page 749 - Basic Electrical Engineering

will also rotate by 45° in the anticlockwise direction. In the next step the BB′
coil will be energized while coil AA′ will not be supplied with any pulse. The
stator magnetic field will now be along the stator field poles BB′. The rotor

magnet will rotate by another 90° in the anticlockwise direction to align with
the stator field. The dc pulse to the stator field poles can be sequenced such

that in every step the rotor will rotate by 45°. By changing the sequence of
supply to the stator field windings, the rotor can be made to rotate in steps in

the clockwise direction. The step by which the rotor will rotate can be chosen
by a proper design, i.e., by choosing the proper number of stator and rotor

poles.
Stepper motors can be rotated to a specific angle in discrete steps, and
hence such motors are used for read/write head positioning in computer

floppy diskette drives. Stepper motors are also used in computer printers,
optical scanners, and digital photocopiers to move the optical scanning

element. The quartz analogue watches contain the smallest stepping motors.

9.11 REVIEW QUESTIONS

1. Explain the constructional details and principle of working of a split-phase-type single-phase
induction motor.
2. Explain double revolving field theory and show that a single-phase induction motor without
the auxiliary winding will not develop any starting torque.
3. Explain the need for connecting a capacitor in the auxiliary winding of a single-phase
induction motor.
4. Explain how an alternating magnetic field can be considered equivalent to two revolving
fields.
5. Draw and explain the complete torque-speed characteristics of a single-phase induction motor.
6. Explain ‘the speed of a single-phase induction motor is somewhat less than an equivalent
three-phase motor’.
7. Show the constructional details and explain the principle of working of a shaded pole
induction motor. How do you determine the direction of rotation?
8. Explain the working principle of an universal motor, draw the torque-speed characteristic, and
mention its applications.
9. Explain the principle of working of a one type of single-phase synchronous motor.
10. What is a reluctance motor? How does it attain synchronous speed? Draw and explain its
torque-speed characteristic.
11. Explain the construction and working principle of a hysteresis motor.
12. How do we make a single-phase induction motor self-starting?
13. Explain the working principle of a stepper motor. Mention two applications of such a motor.