Abstract: A rotary transformer having a stator and a rotor is energized by a power supply at commercial power frequency; sensors are located on the rotary part of the machine and provide sensing output signals, at discrete threshold levels, to a threshold circuit which, in accordance with the sensed signals, provides loading on the transformer secondary in discrete steps; the loading on the secondary is reflected into the primary and sensed to provide, selectively, alarm or stop-motion signals if operating parameters, typically temperature, as sensed by the sensors, reach levels necessitating an alarm or even a stop-motion signal.