the disclosed leonard type elevator system comprises a reversible converter for controllably driving a dc motor , and an ac voltage detector and a dc voltage detector connected to the ac and dc sides of the converter respectively for detecting the voltages developed on the respective sides of the converter . when a difference in detected voltage between the two detectors exceeds a predetermined magnitude due to a decrease in the ac source voltage , a comparator produces an output serving to disconnect the dc motor from the converter . also the comparator may produce such an output when the converter receives regenerative power from the dc motor .