Efficiency considerations for low frequency operation of induction motors

The possibility of replacing a low-speed gear drive by a direct-drive induction motor operated at reduced frequency is explored. The primary issue is the efficiency reduction caused by the need to operate the machine at higher values of slip at the lower frequency. The results show that optimal-efficiency low-frequency operation can readily be obtained by adjusting the V/Hz of the inverter to achieve minimum motor current. For the 5 hp system, used as an illustration, a reduction in efficiency of about 5 points at 20 Hz was predicted despite the fact that optimal-efficiency, 20 Hz operation was compared to nominal operation of the less-efficient (smaller-size) gear-drive motor at 60 Hz. It was also found that the inverter kVA required by the larger direct-drive motor was about 50% larger than the gear drive system even if the larger motor is rewound to make optimal use of the inverter.<>