What could cause a lathe to have low torque and only develop approximately half its rated RPM?

Question

My Jet 1642 lathe is only developing about 500 RPM instead of the 1200 it should in low range. It also has significantly less torque, making it difficult to turn anything even with a light touch & sharp tools.

It has a 3 phase inverter, and I suspect one of the phase legs have failed.

Is it possible that the inverter is contaminated with fine dust? I turn a lot of tropical hardwoods and frequently sand to a very high grit and wonder if the silica common in them could somehow be interfering with the inverter circuitry. Since the lathe is long since out of warranty, I'm considering opening it up and blowing it out with compressed air to see if that helps and possibly cleaning the circuit board with alcohol.

Has anyone else experienced this, and if so, how were you able to resolve it?

Update: It's now three years later, the lathe is behaving completely normally and I've done nothing beyond cleaning it really well. I suspect it may have been water intrusion since the lathe sits right up against my garage door. I have put in a plywood panel to prevent direct spray, but that's really just a guess as to what the actual problem might have been.

Answer 1

You didn't say how much work your lathe has seen, but here's what the user manual has to say about the symptoms you mentioned:

Motor or spindle stalls or will not start

• Excessive cut. Reduce cut depth.
• Worn motor. Replace motor.
• Broken belt. Replace belt.
• Worn spindle bearings. Replace bearings.
• Improper cooling on motor. Clean sawdust from motor fan.

Motor fails to develop full power.

• Power line overloaded. Correct overload condition.
• Undersize wires in supply system. Increase supply wire size.
• Low voltage. Request voltage check from power company and correct low voltage condition.
• Worn motor. Replace motor.

I suppose some of the above may also apply to the inverter--for example, as you suggested, maybe the inverter os overheating and needs to have the dust blown out.

In addition to the above, the belt could possibly slip and cause low torque and maybe low RPM if it's worn, dirty, and/or not tensioned (also if it's the wrong size belt or if it has stretched).

Answer 2

You can test the voltage on each leg of the inverter with a multi-meter to confirm your suspicious. Between any two phases you should get the equivalent of the square root of 3 multiplied by the voltage. So if your supply voltage is 120V, you should get ~208V between any two phases.

One layer of a printed circuit board is a solder mask (this is what gives the board its color), so dirt on the board itself should not short out the individual traces. Blowing dust off of it won't hurt, but I doubt it will resolve the problem. If something did short out, that component is likely damaged.

Assuming you do trace the issue to the inverter, I would start with a visual inspection of the board, look for burnt or bulging components. Smell the board too - if you get a strong smell of "burnt" and/or ozone, something is likely fried. Whether or not you can fix it is probably outside the scope of this site.

If the inverter is OK, check and test all of the electrical connections - maybe you'll get likely and something has just come loose.

Might also be worth reaching out to Jet to see if this is a common symptom of a particular problem they know about.