Single phase motor start cap. failure?

We have a hydraulic pump cabinet we build, using a vendor supplied 0.5 hp, 110v AC single phase motor with a start capacitor. One of these had trouble in the field (hot dry Nevada desert location), blowing the 10 amp slow-blow fuse wired to it. Realizing we probably undersized the fuse (the original customer for these used 220v power service), we upped it to a 15 amp slow-blow fuse…and it blew that one too. Replaced the motor and had the unit returned to us.

The original unit now has a new motor and a software change to limit the number of motor starts per hour, as we suspected (and later confirmed, we thought - see below) that the motor was being subjected to a higher duty cycle than it could take. It’s been operating just fine through the hotter days of the summer, so “problem fixed” we thought.

Upon return of the unit about a month later, the motor started and ran just fine back home in the cooler and probably slightly wetter pacific northwest. Motor start and run amperage (measured via a current clamp and oscilloscope, and also using peak-capture on a DVM) were identical to a fresh factory motor - about 6.2-6.5 amps running, and 25 ish amps starting according to the DVM that measured rms voltage.

Based on measured motor run hours on the unit, we knew that the motor had been subjected to about 10x the operating time as an identical unit mounted in the same room. This led us to suspect the start capacitor had failed (open?), due to thermal runaway stemming from a high duty cycle, leading to high motor starting amps that blew the fuse.

But…how did the capacitor heal itself? Is there some other mechanism I’m missing that would cause high motor run or start currents, that goes away when the motor cools off or sits idle for awhile?

I’ve been trying to replicate the fault by running a high duty cycle on the motor mounted in an identical cabinet. With no load and a 30-second start/run cycle, the capacitor temperature rises, but no change in start or run amps, and no tripping occurs. We increased the duty cycle (start/stop every 4 seconds) and loaded the hydraulic pump to its limit, and got the cabinet, motor and motor cap temperatures up over 50C…but still can’t see a rise in start or run current, and no tripping.

Any thoughts?

Capacitors can partly fail. That being an internal fuse. Just replace the whole thing.

1 Like

“Partly fail”? hmm.

“Just replace the whole thing” That’s what we did, but I’m just trying to confirm the failure mechanism on the returned motor by testing…and failing to do so. I tried running the test overnight last night (thinking it may be a time @ temperature thing and we needed to bake and dry out the capacitor), still no failure (tripping) this morning - a thermistor on the capacitor can shows no temperature rise, no runaway thermal event. It is possible/probable that we just can’t get the box as hot as it got in the desert. Or maybe they had much crappier power or low voltage?

I suspect that the motor may have been running on a long, undersized extension cord.
It sounds like a low voltage issue.

1 Like

Could be, WaRoss, I would have to measure at their facility. I’m measuring 120v rms with a Fluke meter here at work on our test bench…but there may be some voltage droop when the motor starts, will check that today.

Friday I managed to catch the motor having tripped the breaker (finally). Things definitely got hot (motor case temps up to 180F) - but it seems the motor stalls before this happens (tested by resetting the breaker - motor switches on but doesn’t rotate…and doesn’t trip the breaker even though it’s drawing the full 26+ amps of the start circuit). This actually appears to have happened several times during the long overnight run - I can see temperature spikes where the motor (outer case) temperature rises abruptly, but the oil temperature drops, indicating there is no oil flow. During those overnight runs, the motor eventually restarted on its own. I think it stalls because the oil in the pump loses lubricity at the high temperatures, or because there is gunk in the oil/pump, (or maybe the seals seize up due to the heat?) - will have to tear it down to find out. If the motor windings were shorting, or the capacitor weakened due to high temperatures, one would not expect the problem to go away on its own.

Some more data gained at least - and some ideas for the next iteration of controller software:

  • verify pump pressure rise when motor starts (and stop if there is no pressure rise, indicating motor is not turning)
  • monitor oil and possibly motor case temperatures and limit operation if temperatures exceed some limit

Okay, more data today. Restarted testing on the same returned unit this morning, using a 15s on/5s off duty cycle. The motor stalled about an hour in, but restarted within about a minute or two, then stalled long enough (2-3 minutes?) to trip the breaker at the 1.5 hour mark. Temperatures were all fairly low, having just started after laying dormant through the weekend. So, the motor stalls are not apparently directly related to operating temperatures…saga continues. Plan to tear down the pump later and do what limited forensics I can.

Before the test started, I measured current and voltage transients again, looked normal. The voltage droop at start got down to a minimum of about 108-106 Vac, from a nominal 122 Vac running, 124 Vac motor off. Is that low enough to cause concern? Start amperages seem high for a typical cap start motor (25-27 amps) relative to running amps (7.2 amps nameplate, 6.5 amps measured running at full load). Also, the motor start transient amps and time to start never vary much as the temperature climbs, and the stalled motor shows the same amperages…so not a capacitor problem, right?