A novel stopping method for induction motors operating from variable frequency drives

Many applications involving induction motors that are controlled using variable frequency drives require the ability to stop quickly. These applications include emergency stops, quick stopping of fans, centrifuges, presses, etc. The technique that is widely accepted in the industry for achieving quick stopping makes use of brake resistors in series with a power semiconductor switch. The switch-resistor combination (brake-unit) is applied across the dc bus. The fastest decelerating time achievable depends on the size of the resistors and the switch employed. In this paper, the authors propose a novel method of achieving quick stopping times without the use of any brake-unit. The stopping times achievable depend on the inverter rating, motor parameters, and the inertia of the load. The proposed method uses the concept of achieving large slip condition between the rotating motor and the commanded drive output frequency. Control of current through the motor during the process of stopping is achieved by modifying the V/f pattern. Experimental test results with and without this method on a large inertia motor-load combination show that the proposed stopping method is able to reduce the stopping time significantly compared to normal decelerated stop without the need for a braking unit.