DC Motors

On Thu, 19 Feb 2004 02:49:13 GMT, JK brought forth from the murky depths:

It's probably that little red one over there on the left. No, higher. There, that one. Have it tested.

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Dang, I deserved that. Shouldnt type when I'm tired.

What I was hoping for wasa some guidance on what sub system could be haywire. The local electric motor repair guy does'nt trouble shoot thesecontrollers but he tested the motor and it is ok. I have already looked for cold solder joints, broken foil patterns, burnt resistors but was hoping that someone with more knowledge could point me in the right direction.

regards, Jim

You should be able to identify the main IC chip in the controller. It is a black thingie with pins soldered into the board. Check the description on it and call local sources for a replacement. Replacing it is about all you can do without diagnostic information and test equipment.

All of my control systems experience is with little bitty clean things, not great big dirty things so I'm not familiar with off-the-shelf controllers.

But no one else is answering, so I'll stick my oar in the water.

Is there a speed sensor on the motor (like, four wires to the motor instead of two)? If so, the controller should be holding the motor right on speed until something lets the smoke out of the motor, and you should be looking at that part of the controller.

Are there just two wires to the motor, and it normally slows down a little bit under load, but now it's doing it a lot? If so, then the controller is designed to supply a constant voltage to the motor, but is now having trouble doing so.

It looks from the Grainger website that the box takes in 120VAC and supplies DC to the motor. My guess is that the controller rectifies the AC to DC with a full-bridge rectifier, and controls the average DC level either by chopping the rectified DC or by using a thyristor rectifier. If it's rectifying and chopping in two steps you'll see four diodes (two terminals each, may be round, may be square) and one or more transistors (three pins each, one for the motor, one for the rectifier circuit, one from a small snarl of circuitry that eventually comes from the speed control knob), or you'll see a big diode bridge (four terminals) and the transistor. If it's a thyristor rectifier you'll see the AC going straight to four thyristors (three terminals, just like a transistor), with control signals coming from a big snarl of circuitry that leads back to the speed control.

If all my guesses are correct so far it sounds like one leg of the rectifier is out. If the controller uses thyristor rectification, it could be a blown thyristor. If the controller rectifies then chops the DC to the motor then one of the chopper transistors could be bad, or one of the rectifier diodes. In any case the part in question will have failed open, because if it failed closed the motor would either run at full speed always or there'd be a burnt trace.

Without having the box in front of me and a voltmeter I can't tell you much more. You might want to try to identify the power flow through the box and if the other controller you have is identical, do some side-by-side comparisons of the voltages inside (with all due caution for the fact that

100-ish volts is sometimes painful and scary, and sometimes fatal -- keep one hand in your pocket to avoid that direct-across-the-chest path).

Any electrolytic capacitors? Typically the first thing to fail from age.