What is the reason for using a rheostat on a synchronous motor?

Using a rheostat on a synchronous motor primarily serves the purpose of varying the power factor to meet load conditions. In synchronous motors, power factor can be adjusted by changing the excitation of the motor. By adding a rheostat into the excitation circuit, operators can increase or decrease the level of excitation, thereby controlling the reactive power supplied by the motor to the system.

As the load changes, the motor's ability to maintain operational efficiency and stability relies on its power factor aligning with demand. When a motor operates with a leading power factor (capacitive), it can help to correct the overall system power factor when connected to a grid, enhancing stability and efficiency. The use of a rheostat in this context allows for finer adjustments to ensure that the motor operates optimally, regardless of fluctuations in load.

This approach is particularly valuable in applications where the load can vary significantly, enabling the synchronous motor to handle changes without the need for complete system shutdown or the replacement of motor components.