Thermal Over Load

I don't know for sure, but possibly a thermistor. A thermistor is a resistor that's resistance varies with temperature. I work with robots and all the servo motors have thermistors that let the robots controller (computer) know if the motors are hot.

∫_window I(t) dt [??]

haps this link may help?

GE Multilin

The microprocessor relay/PLC output card relay does not drive motor directly, it drive motor contactor or VFD. There is auxiliary contact on motor contactor that gets wired back into PLC via descrete input card. Motor temperature input gets wired into analog input card on PLC.

Motor protection on a protective relay without external temperature sensors is accomplished by monitoring motor current. In the case of a simple bimetal thermal overload relay, the bimetal sensing elements are chosen to approximate the minimum cool down time of a typical motor, but are not terribly accurate. In the case of microprocessor based motor protective relays, they monitor the current and create a motor model to compare it to. That model can range from being just a mimic of a bimetal element, to a self-adaptive model that tracks and averages successive starts. The better the relay, the more accurate that motor model becomes. Some of the higher level functions allow the user to program a known cool down rate, a known maximum number of starts-per-hour, and a minimum time between starts based on motor manufacturer data points.

Thanks Mr.Jraef,

as you said the if temperature is calculated by monitoring current(which is a imaginary value),then isn't it sufficient to have tripping on over current rather than thermal overload.