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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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    Hi Chris

    The coils are probably fine. The varnish used in older items is often very dark, but you can tell if overheated as it tends to flake off easily if burnt, and it smells pretty bad. Most of the current is controlled by the winding inductance, not the resistance, so those figures sound fine. The start winding is probably thinner wire as it does not operate very long, and so will not overheat. I presume the start capacitor is OK, this along with the start winding inductance phase shifts the winding current to be as close to 90 degrees as possible, allowing the first bit of rotation which then continues although only the run winding has current. These motors have fairly poor response to excessive torque, is it a suds pump or similar?

    Regards

    David

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  • 0

    Hi Chris

    The coils are probably fine. The varnish used in older items is often very dark, but you can tell if overheated as it tends to flake off easily if burnt, and it smells pretty bad. Most of the current is controlled by the winding inductance, not the resistance, so those figures sound fine. The start winding is probably thinner wire as it does not operate very long, and so will not overheat. I presume the start capacitor is OK, this along with the start winding inductance phase shifts the winding current to be as close to 90 degrees as possible, allowing the first bit of rotation which then continues although only the run winding has current. These motors have fairly poor response to excessive torque, is it a suds pump or similar?

    Regards

    David

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