What modifications would necessary if a motor is required to
operate on voltage different from that for which it was originally
designed?
Answers
Explanation:
When motors are supplied directly from the power network, the supply power frequency is constant, while voltage and current change during motor starting. During motor acceleration to synchronous speed (synchronous motors) or close to synchronous speed (induction motors), the current would initially rise to multiple times the rated current and cause voltage drop. Lower voltage while supply frequency is constant means lower V/Hz ratio and lower flux which affects the torque. Once the motor accelerates, the voltage recovers to close to rated value and the torque available at the motor shaft is at the rated value. The speed of the motor is then constant and synchronous (synchronous motors) or close to synchronous (induction motors). With motors connected directly to power network, the speed is dictated by the fixed network frequency and cannot be controlled. To manage the speed when necessary, additional mechanical systems are used: dampers, valves, gear boxes, brakes, etc. Mechanical systems reduce the overall system efficiency. In addition, as explained previously, induction motors consume reactive power, so maintaining the power factor may be a challenge with induction motors. Synchronous motors do not cause issues with the power factor, they can actually help.