Starter thermal overload protection system

Abstract

A system for protecting the starter of an internal combustion engine from overheating as a result of cranking for an excessive period of time when the engine is difficult to start. Override circuitry is electrically interposed between a manually operable starter switch and the engine starter to apply a prescribed ON and OFF cranking sequence to the starter and thereby limit the duty cycle even though the operator continuously operates the switch. Preferably, the overall maximum duration of the start attempt (the duration of the ON and OFF cranking sequence) is limited by termination circuitry. The ON and OFF cranking intervals and, optionally, the overall maximum duration of the start attempt are particularly selected so as to allow sufficient time for motor winding heat to dissipate into the starter housing. This limited cranking duty cycle protects the starter from overheating. Also, when employed in combination with a thermostatic switch mounted on the starter housing and arranged to prevent starter energization when a predetermined temperature is reached, the limited starter ON time and intermediate resting periods permit the housing and the thermostatic switch to more closely follow winding temperature. As a further aspect, in order to maintain optimum operation of the protection system as starter load conditions vary, starter current is sensed and the starter ON/OFF duty cycle is appropriately modified in response.