Question

V/f is maintained constant in the following case of
speed control of induction motor:
(1) Below base speed with voltage control
(2) Below base speed with frequency control
(3) Above base speed with frequency control
(4) None of the above

Answer 1

Answer:

Below base speed with frequency control

Explanation:

Answer 2

V/f can be maintained constant in the case of below base speed with frequency control. Therefore, option(2) is the correct answer.

• The induction motor's synchronous speed is given by $N_{s} =\frac{ 120f}{P}$ . The base speed and frequency are inversely proportional. By decreasing frequency, we can control the base speed.
• Flux is inversely proportional to frequency and it is proportional to the voltage by the formula Ф =$\frac{E}{4.44* K *T *f}$ where E is emf induced, k is wind constant and T is a number of turns.
• Hence, when we decrease frequency flux increases and we decrease voltage, and flux decreases.
• This will work in opposition to each other keeping flux constant.