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Locked Rotor Detector circuit for small single phase induction motor

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Has anyone here designed a locked rotor detector circuit for small single phase induction motor?

I'm thinking a Hall effect current sensor and the cheapest uC with an A/D that you can actually purchase with the present supply line issues.


I suppose that would do. Or even dumber: current transformer and latching relay?


Depending on the small 1 phase motor type, there may not be much difference ( only a few %)  between magnitude of run current and start/locked current. The phase angle changes a bit.
I took some readings , made vector diagrams some years ago of a capacitor run motor, but can't find the dir I put it in.
I'll post up if I find it.

Power factor at locked rotor would likely be pretty awful. Feed the motor with several tens of volts from a transformer and put a scope on it and see how many degrees the current lags the voltage. If it is significantly different from normal running then you might have a doable method. Although... at normal running speed and low load the power factor will be low too, so maybe not so good.

Is it the kind of single phase motor that has a separate starting winding and a centrifugal switch? If so, could you get inside the motor and bring out an extra wire from the starting winding to give an indication of when the switch is on because of very low rpm?

Is your application to detect if the motor stops revolving, or to detect if it never starts in the first place? Knowing if it is only one of those and not both might simplify the solution.


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