For an ideal synchronous motor supplied from a constant frequency source, what is the percentage speed regulation?

Introduction / Context:
Speed regulation of an electric motor indicates how much the speed changes between no load and full load conditions. It is usually expressed as a percentage of full load speed. Understanding the speed characteristics of different motor types is important for selecting the right motor for applications requiring constant speed or variable speed. This question focuses on the speed regulation of an ideal synchronous motor supplied by a constant frequency AC source.

Given Data / Assumptions:

• The motor considered is a synchronous motor.
• The supply frequency is constant.
• We assume ideal conditions without slip or significant frequency variation.
• Speed regulation is defined in the usual way as percentage change in speed from no load to full load.

Concept / Approach:
A synchronous motor runs at a speed exactly tied to the frequency of the AC supply and the number of poles in the machine. The synchronous speed Ns is given by Ns = 120 * f / P, where f is the frequency and P is the number of poles. For an ideal synchronous motor, the rotor locks in with the rotating magnetic field and runs at Ns regardless of load, as long as the motor remains in synchronism. Therefore, the speed does not drop from no load to full load, leading to zero speed regulation in the ideal case.

Step-by-Step Solution:
Step 1: Write the expression for synchronous speed: Ns = 120 * f / P. Step 2: Note that f and P are fixed for a given motor on a given supply, so Ns is constant. Step 3: In a synchronous motor, the rotor speed equals Ns when the motor is running in synchronism. Step 4: Define speed regulation as (N no load minus N full load) divided by N full load, all multiplied by 100 percent. Step 5: For an ideal synchronous motor, N no load equals N full load equals Ns, so the numerator is zero and speed regulation becomes zero percent.

Verification / Alternative check:
In contrast, induction motors exhibit slip, meaning that rotor speed is slightly less than synchronous speed and decreases further under higher load, leading to non zero speed regulation. This difference is often highlighted in textbooks to show why synchronous motors are preferred in applications that demand constant speed, such as certain synchronous condensers and power factor correction devices. This comparison supports the conclusion that synchronous motors ideally have zero speed regulation.

Why Other Options Are Wrong:
1%: This might be typical for high quality induction motors but does not describe the ideal synchronous motor. 25%: This is far too high and would indicate significant speed variation with load, which is not characteristic of synchronous machines. 0.5%: This suggests a small but non zero change in speed; for an ideal synchronous motor the theoretical value is exactly zero.

Common Pitfalls:
Learners sometimes confuse synchronous motors with induction motors and apply the concept of slip incorrectly. Another misunderstanding is to assume some small variation in speed must always be present. While real machines have minor deviations due to practical factors, examination questions about ideal synchronous motors take speed as perfectly constant and therefore assign zero speed regulation.

Final Answer:
For an ideal synchronous motor on a constant frequency supply, the percentage speed regulation is zero.