Synchronous Motors on Grinding Mills: The Different Excitation Types and Resulting Performance Characteristics with VFD Control for New or Retrofit Installations

Synchronous motors (SMS) continue to be a viable alternative to induction motors because of efficiency advantages. Since SMs have long been used to provide leading kvar for power factor (PF) improvement and to help with the device switching in load-commutated inverter (LCI) drives, these machines are efficient prime movers for large mill applications. Both in motor efficiency and in low-base-speed configurations, the SM has advantages of lower current and higher efficiency. Often used in large mills in the cement and mining industry, these motors were driven by LCI variable-frequency drives (VFDs) or used with direct start with a clutch system that would synchronize the motor first to overcome low starting torques. This article reviews various mill drive configurations and improved SM characteristics when used with advanced drives. It will also review the different SM excitation types and resulting performance characteristics with VFD control for new or retrofit installations.