Chapter 17 - Induction Motor Speed Control

Question 1

Explain 3 ways the synchronous speed of a motor can be changed?

Answer 1

1. Changing the Frequency.
2. Changing the Number of Pole Pairs by Stator Switching.
3. Changing the Number of Pole Pairs by Pole Changing.

Question 2

Synchronous Speed Equation?

Answer 2

1. Ns = (60 x f) ÷ p

• Ns = Synchronous Speed
• f = frequency
• p = number of Pole PAIRS

Question 3

Explain the difference between Stator Switching and Pole Changing?

Answer 3

• Pole changing is done by changing the configuration of the winding’s of the Stator.
• Stator Switching has 2 separate Winding’s which are controlled by the control box and one or the other is energized dependent on desired speed.

Question 4

Explain how Pole Changing is achieved?

Answer 4

• The 2 Speed are achieved be re-configuring the winding’s of the Stator
• So a winding may produce a 4 pole rotating field when connected one way and then re-configured to produce 2 poles another way
• The Service method of this speed control is the Star-Double Star Connection.