Question

A 4-pole, 50 Hz induction motor has a synchronous speed of:

• A. 1000 rpm
• B. 1500 rpm
• C. 3000 rpm
• D. 750 rpm

Hint:

For a standard 50 Hz grid system, synchronous speeds for different pole configurations are fixed and worth memorizing:
2 poles $\implies$ 3000 rpm
4 poles $\implies$ 1500 rpm
6 poles $\implies$ 1000 rpm
8 poles $\implies$ 750 rpm

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the synchronous speed of a 3-phase induction motor, which is the speed at which the stator-generated rotating magnetic field revolves around the stator core.

Step 2: Key Formula or Approach:
The synchronous speed ($N_s$) of an induction motor in revolutions per minute (rpm) is given by the relation:
\[ N_s = \frac{120 f}{P} \]
where:
$f$ = supply frequency in Hz
$P$ = number of stator poles

Step 3: Detailed Explanation:
• From the question, we are given the following machine parameters:

• Number of poles, $P = 4$

• Supply frequency, $f = 50$ Hz

• Substituting these values into the synchronous speed formula:
\[ N_s = \frac{120 \times 50}{4} \]

• Performing the multiplication in the numerator:
\[ 120 \times 50 = 6000 \]

• Dividing by the number of poles:
\[ N_s = \frac{6000}{4} = 1500 \text{ rpm} \]

• This calculation shows that the magnetic field rotates at a constant speed of 1500 rpm.

Step 4: Final Answer
The synchronous speed of the given 4-pole, 50 Hz induction motor is 1500 rpm.