Evaluation of synchronous motors on grinding mills

Synchronous motors continue to be a viable alternative to induction motors because of efficiency advantages. Long used for providing leading kVARs for PF improvement and helping 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 synchronous motor 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 paper reviews various Mill drive configurations and improved synchronous motor characteristics when used with more advanced Current Source Inverter (CSI) drives. It will also review the different synchronous motor excitation types and resulting performance characteristics with VFD control for new or retrofit installations.