Question:
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : A synchronous motor always runs at synchronous speed.
Reason (R) : Its rotor speed is locked with the stator rotating magnetic field.
In the light of the above statements, choose the most appropriate answer from the options given below :
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : A synchronous motor always runs at synchronous speed.
Reason (R) : Its rotor speed is locked with the stator rotating magnetic field.
In the light of the above statements, choose the most appropriate answer from the options given below :
Show Hint
Synchronous motor:
\[
N=N_s=\frac{120f}{P}
\]
Rotor locks magnetically with rotating stator field, so slip is zero.
Updated On: May 22, 2026
- Both (A) and (R) are correct and (R) is the correct explanation of (A)
- Both (A) and (R) are correct but (R) is not the correct explanation of (A)
- (A) is correct but (R) is not correct
- (A) is not correct but (R) is correct
Show Solution
Verified By Collegedunia
The Correct Option is A
Solution and Explanation
Concept:
A synchronous motor operates on the principle of magnetic locking between:
• rotating magnetic field of stator,
• magnetic poles of rotor. Due to this magnetic locking: \[ N=N_s \] where:
• \(N\) = rotor speed,
• \(N_s\) = synchronous speed.
Step 1: Understanding Assertion (A). Assertion: \[ \text{A synchronous motor always runs at synchronous speed} \] This is correct. Unlike induction motors, synchronous motors:
• do not operate with slip,
• rotate exactly at synchronous speed. Hence Assertion is true.
Step 2: Understanding Reason (R). Reason: \[ \text{Rotor speed is locked with stator rotating magnetic field} \] This is also true. The rotor magnetic field locks with the rotating stator field and rotates together with it.
Step 3: Understanding why synchronous speed remains constant. The rotating magnetic field speed is: \[ N_s=\frac{120f}{P} \] Once rotor poles lock with this field: \[ N=N_s \] Hence rotor cannot run slower or faster under normal operation.
Step 4: Checking explanation relationship. Reason correctly explains why synchronous motor always runs at synchronous speed. Therefore:
• Assertion is correct,
• Reason is correct,
• Reason correctly explains Assertion.
Step 5: Selecting the correct option. Hence the correct answer is: \[ \boxed{(1)} \]
• rotating magnetic field of stator,
• magnetic poles of rotor. Due to this magnetic locking: \[ N=N_s \] where:
• \(N\) = rotor speed,
• \(N_s\) = synchronous speed.
Step 1: Understanding Assertion (A). Assertion: \[ \text{A synchronous motor always runs at synchronous speed} \] This is correct. Unlike induction motors, synchronous motors:
• do not operate with slip,
• rotate exactly at synchronous speed. Hence Assertion is true.
Step 2: Understanding Reason (R). Reason: \[ \text{Rotor speed is locked with stator rotating magnetic field} \] This is also true. The rotor magnetic field locks with the rotating stator field and rotates together with it.
Step 3: Understanding why synchronous speed remains constant. The rotating magnetic field speed is: \[ N_s=\frac{120f}{P} \] Once rotor poles lock with this field: \[ N=N_s \] Hence rotor cannot run slower or faster under normal operation.
Step 4: Checking explanation relationship. Reason correctly explains why synchronous motor always runs at synchronous speed. Therefore:
• Assertion is correct,
• Reason is correct,
• Reason correctly explains Assertion.
Step 5: Selecting the correct option. Hence the correct answer is: \[ \boxed{(1)} \]
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