Any bright ideas to bolster my diagnosis about this electric motor?

November 23, 2013 1:17 PM

Troubleshooting an ancient 3hp split phase 240 VAC motor, I found a likely problem, which I fixed, and the fine old beast still trips the breaker. Join me in my diagnosis, and suggest if further work is warranted.

Neighbor brought me a largish (75 pound) single phase 3 HP motor that refuses to run. It formerly powered his planer.

Usual drill is to clean off debris, dust, and rust of anything I get for repair, which I did. For fun, I popped open the cans that hold the start and run caps. Both were 120V 708uF caps and testing them with an Amprobe AM160 DMM, they were good and within tolerance. One of them had a clearly cold solder joint on one lead, which I presumed to be the culprit. It was obviously there from day 0, when the motor was made, which I estimate to be 50+ years ago. I cleaned up the parts and securely soldered it properly, then reassembled the unit.

The only other problem was some worn insulation on the supply wires.

I wired it up to shop 240 VAC and when I start it, it trips the breaker. Where I am, the breaker is in another room, and for safety, in case it actually started, I used the breaker as as switch. It's a 40Amp circuit. It hums mightily and trips in 5 or 10 seconds. The motor is too large to get warm, the wires don't FEEL abnormally warm, and nothing burst into flames.

I measured the winding resistance at 0.4 ohms, more or less. That's DC resistance, of course. Way lower than I'd expect.

My theory is that the loose cap was the starter cap, that it was out of circuit and that the motor burned up due to not moving at all when VAC was applied before I got it. Second theory is that the centrifugal switch is open, and the start cap isn't even in the circuit. It's a chore to crack this mother open, so before I do, I'd like a motor-savvy elektrishun opinion.

I don't do motor work much, and when I do, it's DC motors. SEW, MI RITE R WUT?

No one fixes motors any more, so I am about it for my neighbor. He has had a rash of motor failures recently, 3 on a spa and this one. I think he's beginning to doubt my abilities. (The spa motors endured 20 years outside in Vermont. They are a rusted mess and the pumps locked. Windings are open.)

We are a cheap species here. Help greatly appreciated. I may open a business with this guy as my only client if I can get it going, and I will share the creds with Metafilter and all glory will accrue to Matt H for his splendid web site idea.)

posted by FauxScot to Technology (9 answers total)

Does it have a centrifugal starter switch? If so, the symptoms would fit if it had failed in the open position.

Does the motor have a starter switch box that could incorporate an external starter relay that you are omitting from your test circuit?
posted by the Real Dan at 2:03 PM on November 23, 2013

Response by poster: real dan.... if i spin the motor up manually and then power it up, i can prove the starter coil is out (via no contacts) and that the run windings are good.

any experience fixing the start contacts? these things should kinda last forever. there is a small amount of internal dust. are the contacts usually at the non-shaft end and accessible on motors in this size/age range?

thanks!
posted by FauxScot at 3:14 PM on November 23, 2013

Usually there will be a centrifugal switch activated a little lever which is pushed by the fly weights on the motor. The switch will be located at one end of the motor (all the ones I've deal with it the switch at the non shaft end) and on a TEFC unit which you likely are dealing with the switch will be inside the end bells.

These switches aren't super reliable and especially if the motor sits for long periods of time between uses (and considering the age) it's pretty common for them to stick and either burn up or not function requiring one to change them.

PS: there is at least one motor rewind shop in Burlington and an old single phase 3 horse motor like you have is well worth getting serviced in most cases as it'll be significantly cheaper than an equivalent new motor. And if the shop is good will run literally better than it did when new (bearings and insulation have gotten better in the last 50 years.)
posted by Mitheral at 3:46 PM on November 23, 2013

There's a place in Milwaukee that fixes motors--I bring them there occasionally. They do a TON of business. I'm sure there's somewhere local to you.
posted by Slinga at 6:26 PM on November 23, 2013

I don't think you can test those caps with a pocket DMM. It can't apply more than a few volts can it? I'm not saying they are bad (and if so that they are the only problem) but I think your test is probably inconclusive.
posted by fritley at 6:44 PM on November 23, 2013

Can you get to the wires that lead to the start switch and test them with your ohmmeter?

For that matter... can you jumper across the presumably-faulty start switch and see if the motor starts properly in that case (obviously don't run it any longer than necessary to tell that it started in this case)?

That would be additional evidence that it's worth taking apart the motor to get to the start switch, I think.
posted by hattifattener at 1:21 AM on November 24, 2013

Response by poster: Well, I found another cold solder joint on the other cap, fixed it, no change.

Spun it partially up to speed with another motor before applying power, no dice. If there was an open circuit on the start section, that's ruled out. I was uncertain as to the motor's direction of rotation, so I did that test both ways. Convinced myself that both caps were good. For a while, I measured 40 ohms from both lines to ground, but that seems to have gone away. I was unable to crack it open, and decided to call that motor repair shop in Burlington mitheral suggested tomorrow morning.

Unit was full of crap, which I blew and vacuumed out... looked like a combination of rodent transported material and dust/operational debris stuff, with some insulation fragments.

Line to line dc resistance is 0.3 ohms. I can only assume some shorted lines, but am calling it a day.

Thanks for the help and suggestions, all. For posterity, I'll leave the question open until I get a report from the motor shop.
posted by FauxScot at 2:20 PM on November 24, 2013

Troubleshooting an ancient 3hp split phase 240 VAC motor

I don't suppose there's any chance your power is three phase?
posted by jamjam at 2:46 PM on November 25, 2013

Response by poster: @jamjam.... no. my power source is residential. (i can create three phase power from a single phase input VFD, but i'd have to have a three phase motor to do it. it's a good thing to suggest and in an industrial setting, I might check.) friend took his working planer from one residence (with split phase 240) to another residence (same power) and I have just the motor in my place, also residential. no longer works at any of them.

@fritley ... those caps are 120V 708 uF nominal non-polarized. they are testable as to capacitance using a handheld meter with C function, and both read the same in and out of the circuit. no evidence of leakage, and the readings are highly repeatable on both. in the circuit where they appear, there's no chance of them seeing a full 120V for any length of time, if ever, so their WV wouldn't be a factor.

burlington folks were not interested in fixing it. quoted replacement at $600.

i did all tests on this thing that could be done, other than visual inspection. today, i found a local replacement for $300 so now I am free to destroy it to peer inside.

the replacement is identical except it is TEFC and has a slightly different base, but all other specs seem good. same shaft size, much lighter, reversable, nice motor.
posted by FauxScot at 4:01 PM on November 27, 2013