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Fixing a small motor

Chris Pearson

I want to fix an ancient 1/8 hp motor (Carter Electrical Co Ltd). As far as I can tell, the centrifugal switch has worn out, so that will be fun making a new one.

My question is what resistance seems reasonable for the coils. IR is good (well into the tens of megaohms) but they are a little dark and I would like to be confident that there has been no internal shorting.

The main coil is about 18 ohms and the starting coil 33 ohms. My maths tells me that the running current should be around 0.4 A. I cannot remember a lot about motors, but I realise that the impedance of the coils will be higher at 50 Hz than the (DC) resistance and that back EMF reduces the current. I hope therefore that the resistance values are about right.

Grateful for any reassurance!

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    OK, what you are concerned about then is not the winding resistance, that only sets in-rush, nor the exact value of static inductance too much, that has more to do with the presence or absence of the rotor and the air gaps in the iron work, but the Q - this is the parameter that takes a dive in the event of a single shorted turn, but is also the hardest thing to detect if the motor is not turning. (only once the motor is more or less up to speed and generating do you know if the current draw is meaningful).

    One  can measure Q (essentially ‘ringing’ the coils, either with a pulse and looking at the after shocks, or with  steady AC at the resonant frequency) but it is ‘involved’, even for an electronics lab, and you would be easier with a good one to just compare resistances to, which is probably where you came in.

    If there is good reason to suspect the windings, then I might be  tempted to put it together without the centrifugal bits and just spin it up.

    Mike.

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  • 0

    OK, what you are concerned about then is not the winding resistance, that only sets in-rush, nor the exact value of static inductance too much, that has more to do with the presence or absence of the rotor and the air gaps in the iron work, but the Q - this is the parameter that takes a dive in the event of a single shorted turn, but is also the hardest thing to detect if the motor is not turning. (only once the motor is more or less up to speed and generating do you know if the current draw is meaningful).

    One  can measure Q (essentially ‘ringing’ the coils, either with a pulse and looking at the after shocks, or with  steady AC at the resonant frequency) but it is ‘involved’, even for an electronics lab, and you would be easier with a good one to just compare resistances to, which is probably where you came in.

    If there is good reason to suspect the windings, then I might be  tempted to put it together without the centrifugal bits and just spin it up.

    Mike.

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