Integrated synchronous motor protection and control

This paper discusses different options of integrating protection and control functions of a synchronous motor in a single electronic device. A basic concept of synchronous motor design and operation is presented along with a comparison to the induction motor. Synchronous motors have been used for large drive applications at constant speed, such as in oil rig pumping, cement mills, ship propulsion, etc. The main advantage of a synchronous motor is that it can produce reactive power and improve the stability of the power system in general. This paper also presents control and protection features specific to synchronous motors. The control functions relate to the starting sequence, synchronization and power factor regulation. Essentially, the rotor current and voltage need to be controlled in order to secure a steady motor operation. Typically, the output of an external direct current amplifier supplies the rotor coil current, so its gain and response time influence the parameters of the entire control loop. Monitoring and protection of the rotor is very important. Special care has to be taken when designing a field undercurrent protection feature. The monitoring and protection features of the stator are very similar to those of an induction motor. The evaluation includes describing the main hardware components that need to be added to assure a successful design.