At what condition does a synchronous motor operate?

A synchronous motor operates at a constant speed that is directly synchronized with the frequency of the alternating current (AC) supply. This means that, regardless of changes in load, the motor maintains a consistent rotational speed as determined by the electrical frequency. This characteristic is pivotal for applications where precise speed and timing are essential, such as in industrial processes and power generation.

In synchronous motors, the speed can be calculated using the formula: Speed (RPM) = (120 × Frequency) / Number of Poles. This relationship allows engineers and operators to predict and control the motor's performance effectively. Maintaining this synchronous speed results in high efficiency and stable operation, making synchronous motors advantageous in many applications.

The other conditions provided do not accurately describe the operational behavior of synchronous motors, as they imply variable or fluctuating speeds that do not align with how synchronous motors are designed to function.