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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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    Run it up in series with a light bulb or other resistive load to serve as a current limit and visual fault indicator, and measure voltage and current across the windings. You can then deduce the XL. By pass the start switch for this, so it turns and never pulls out of start but do not keep it powered up for more than a few seconds. 

    Or to run it longer put the start switch to a real switch, like a spare light switch from the junk box to test it, and use your judgement as to when it starts spinning, flick the switch & disconnect the starter winding. Again, do not do this too often, allow time to cool between starts.

    Oh and clamp it down to the bench firmly.

     

    Mike

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  • 0

    Run it up in series with a light bulb or other resistive load to serve as a current limit and visual fault indicator, and measure voltage and current across the windings. You can then deduce the XL. By pass the start switch for this, so it turns and never pulls out of start but do not keep it powered up for more than a few seconds. 

    Or to run it longer put the start switch to a real switch, like a spare light switch from the junk box to test it, and use your judgement as to when it starts spinning, flick the switch & disconnect the starter winding. Again, do not do this too often, allow time to cool between starts.

    Oh and clamp it down to the bench firmly.

     

    Mike

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