Answer & Solution
Answer: Option
D
Solution:
Understanding Synchronous Motors
Synchronous motors are special because their speed is directly tied to the frequency of the AC power supply and the number of poles in the motor.
The Key Formula
The synchronous speed (Ns) is calculated using the formula: Ns = (120 * f) / P, where:
* Ns is the synchronous speed in RPM (revolutions per minute)
* f is the frequency of the AC power supply in Hertz (Hz)
* P is the number of poles
What the Question Asks
The question states that the number of poles (P) is made *half* of its original value.
Let's See the Impact
If you decrease P to half its original value in the formula Ns = (120 * f) / P, the overall speed (Ns) will *increase*.
For example:
* if initially P=4 then Ns = (120 * f) / 4 = 30f
* if P becomes half i.e. P=2 then Ns = (120 * f) / 2 = 60f
* So from above you can see that speed becomes double.
Why the Other Options are Wrong
* A: remains same as the original value: This is wrong because changing the number of poles *directly* impacts the speed.
* B: decreases to half the original value: This is wrong because reducing the number of poles *increases* the speed.
* C: tends to becomes zero: This is not correct synchronous motor will run at a particular speed which depends upon frequency and number of poles.
The Answer
Therefore, the correct answer is: D: increases to two times the original value.
