What defines synchronous machines?

• A. They operate at varying speeds based on load
• B. They are AC machines that operate at a constant speed
• C. They can only generate power
• D. They require no external power supply

Answer: (B)

Synchronous machines are indeed defined as AC machines that operate at a constant speed. This characteristic is fundamental to their operation because synchronous machines maintain a fixed relationship between the rotational speed of the rotor and the frequency of the alternating current in the stator.

In synchronous machines, the rotor is designed to rotate at the synchronous speed, which is determined by the number of poles in the machine and the frequency of the supply voltage. This means that regardless of the load applied to the machine, it continues to operate at that fixed speed, making it highly efficient for applications requiring precise timing and stability, such as in power generation.

The interaction between the stator's rotating magnetic field and the rotor’s magnetic field is what enables the machine to maintain its synchronous speed. As a result, any change in load does not affect the rotational speed, distinguishing synchronous machines from other types, such as asynchronous machines, where speed varies with load conditions.