John Wrote

“Most of these small motor only have a thermal switch embedded? in the motor windings. If the windings get too hot?the switch?opens the circuit to the motor. The problem with the thermal switch is temperature lag - the switch warms at a slower rate than the windings in the hottest spot of the motor. Larger motors use a thermal replica relay which uses a resistor and thermal switch to approximate the heat in the motor.“

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Of course the thermistors or RTDs are no better at lag, and have the same placement issues.

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And when talking about the “larger” motors here, well I usually do not see advanced protection like

the thermal replica relays [or electronic motor relays] applied till we are talking about motors rated in 100s or 1000s of HP.

[Even then the protection is really an estimated educated guess at what the worst case? temp in the motor is.]

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As John indicates, though protecting against stall can be problematic,

mostly because every time you start, that IS a stall condition.

A stall condition that you don’t want to trip for.

I will however disagree that overloads do not protect against stall.

They just will not always do so without allowing for some insulation damage.

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