Harbor Freight 7x10 lathe motor control issue

[ChazzC]

Check the wiring diagrams at LMS:

Wiring Diagrams

Some wiring diagrams

littlemachineshop.com

There are several different models of HF 7x10 listed, but they describe the characteristics to help you identify which you have. You can also call LMS and ask for help: unlike many resellers, LMS will help you even if you don’t buy anything.

 

[homebrewed]

That resistor is part of the motor controller's feedback loop. It senses the current flowing through the motor, in an attempt to maintain the RPMs under load. It works reasonably well, considering that there's no direct RPM sensing involved. The controller is a PWM type, where the duty factor of the 60Hz line is varied with some SCRs. That portion is much like an old-style light dimmer.

The resistor looks like a wirewound on a ceramic body. They are pretty tough, I've seen them glowing red (in a completely different application) rather than burning out. The resistor itself actually may be OK, it's likely that one or both of the solder joints on the PCB opened up. But if it was me I'd replace it. Make sure the replacement has the same or higher power dissipation as the original.

The burnt resistor probably is a result of a problem elsewhere in the circuit. I'd suspect one or more of the SCRs used to control the motor. It sounds like the motor is working so you probably aren't looking at replacing the motor.

I second the suggestion regarding oldUHFguy. I haven't used his services but have read many good comments about his work.

 

[markba633csi]

The resistor may just be undersized on it's wattage rating-. I agree there may also be an open trace/pad nearby.

DC motors with SCR controllers are usually pretty reliable as long as the motor brushes are not worn out-

 

[TheCase]

I asked him to remove the circuit board. He found there was a wire that came loose. Soldered it back on and all is well.

I would still like to know what the specs on that fried component. I have a feeling that may be the reason the wire came loose.

Hopefully someone that has this board might help identify is some day.

 

[markba633csi]

Was that a wire (as in stranded, insulated wire) or a component lead on the circuit board itself? Was it one leg of the burned resistor?
If you can find a picture of that board online, you could read the color code for the value
You could also measure the value if you have a low-ranging ohmmeter
I'm guessing around 0.025 to 0.05 ohm at about 10 watt (but you could go higher on the wattage) depends on the motor HP
Using this KB chart as a rough guide: