Can a megger be used to test single-phase motors? (Motor diagnosis)

[chipwitch]

I have a 5-hp 220v motor that has been sitting for 10 years and the motor starter cuts out almost instantly.  The motor only turns a jerk (1/4 turn, if that).  I took the cover off and found two caps.  I assume a start (600 uF) and a run (50 uF) cap.  Both sets of wires on both caps go into the motor housing (no obvious common ground in the junction box).  Both caps visually look good and an ESR tester correctly identified them.  The one thing that bothers me is that the start cap is only rated for 125V!  Maybe that's acceptable since it is used so briefly, but...

I bought a megger (uni-t 505) a few years ago but have never used it or any other.  My main problem is knowing what to connect it to?  I tried connecting to earth and to each of the capacitor terminals (caps off, of course) in succession.  I get between 500 and 700 megohms on each of the leads though it will sometimes flash OL (open line?) the megohm result.  Don't think that should matter?  Is that it?  Anything else I need to check with the megger?

Also, this is on a drum sander.  It has a 5hp drum motor which moves freely and a 1/4hp conveyor motor.  Both are fed from the same motor starter.  This used to function just fine before going into storage.  I took the cover off the motor starter and could smell a faint burning smell, like a burned out motor, but when I disconnect the 5hp motor, it would engage the relay and run the 1/4hp motor just fine.  Mind you, I tried to start the motor a half dozen times or so before I took the cover off.  The line in was slightly warm to the touch, though I couldn't detect anything warm on the load side.

Lastly, and maybe this is the problem, I was trying to run this off a 6500W generator that I use to feed my house during hurricane outages.  Of course I have to manage distribution, but it will run tv, coffee maker, couple fridges, water heater, etc. all close to 6kW at times.  I know the start up could be enough to cause the generator to stall, but it kept running during my attempts to start the sander.  I don't think the voltage got too low to keep the contactor engaged, but I didn't test the voltage under load.  I did test it without load and was surprised that it was at almost 280V.

I could connect to mains, but my only source is an outdoor panel which I just replaced the main breaker in a few months back (though it sees very little load).  So, I'm a little gun-shy to try a motor that I thought should run fine and isn't on that panel.  Anyway, motors and starters aren't my strong suit.  Your help is much appreciated as always.

[james_s]

Before you even mess with the megger, have you checked for continuity between the motor frame and any of the winding terminals? Does the rotor spin freely by hand? It's also possible the starter has failed and is tripping when it shouldn't.

[chipwitch]

Before you even mess with the megger, have you checked for continuity between the motor frame and any of the winding terminals? Does the rotor spin freely by hand? It's also possible the starter has failed and is tripping when it shouldn't.

Motor spins freely as stated in the OP... I didn't "spin" it as it is still pulleyed to the two drums, but I can rotate it easily.  Starter tripping when it shouldn't is one of the things I was wondering about... A higher voltage line side I wouldn't think would cause a trip.  The contactor holds fine with the 1/4hp load.  Maybe that's not enough to trip it if it's marginal?

As for testing winding to ground, no.  But I did test across the various combinations of the 2 pair of cap leads.  I got about 1.5ohm when connecting the VOM in place of the caps (each one).  Plus, I got 0.1 ohm from two of the wires (one from each cap) which I took to mean they are wired common at the winding.  Hope that helps.

[james_s]

I would expect if an insulation failure is present that it would be from a winding to ground. In my experience induction motors are extremely reliable though, I've seen capacitors fail and I've seen centrifugal starting switches fail but I don't recall ever encountering a bad winding. If you're trying to run it from a generator that could possibly be part of the problem, a lot of generators put out a nasty waveform, I would expect it to work but maybe the starter is fussy? Most motor starters I've dealt with won't fault trip that quickly anyway, usually there's a fuse that will blow if something is shorted, but the overload protection is usually a thermal thing that will heat up and trip after a period of time if the motor is overloaded.

[chipwitch]

I would expect if an insulation failure is present that it would be from a winding to ground. In my experience induction motors are extremely reliable though, I've seen capacitors fail and I've seen centrifugal starting switches fail but I don't recall ever encountering a bad winding. If you're trying to run it from a generator that could possibly be part of the problem, a lot of generators put out a nasty waveform, I would expect it to work but maybe the starter is fussy? Most motor starters I've dealt with won't fault trip that quickly anyway, usually there's a fuse that will blow if something is shorted, but the overload protection is usually a thermal thing that will heat up and trip after a period of time if the motor is overloaded.

So maybe it's worth the risk of connecting to mains to see if it behaves in the same way?  I don't have a suitable plug and receptacle on hand so I'd need to wire direct to a breaker temporarily to test it.  That's another reason why the generator.  Hubbell twist-lock receptacle on the genny that I had a plug for.

[james_s]

Well in a pinch I've been known to remove the plug and poke the wires of something I wanted to test into the dryer socket. I do so with the breaker off and then turn it on. I do think I would test it that way first, as well as closely inspect the motor starter to make sure it's wired correctly and in good mechanical shape.

[tautech]

If nothing's been changed WRT to the wiring for the motor and it's been stored well one would suspect the gen's breaker is not up to the task of supplying the considerable 'motor start' current surge.
That's why the make dedicated D curve motor start breakers.

Just suss out the motors' label and the listed run current and be sure to have ~50% higher capacity breaker to handle the start surge.
Good luck.

[Silveruser]

There is likely a centrufical switch buried deep in the motor. This should connect the start windings to get it spinning then disconnect when up to speed. Sitting for a long time could mean the contacts are dirty or corroded. If it's safe you may be able to help it along with a little spin but make sure you don't jam you hands up. If you have some compressed air you could try blowing any debris out of the motor if there is access but it's likely sealed. If the caps are indeed good the motor likely has to come apart.  Also keep in mind that motor is going to want lots of current for a few seconds to start maybe 100A or more surge. You should hear the generator bog down if the start windings are connected.

[Paul Moir]

My mill only runs clockwise (nice, huh?).  When I need to run backwards, I disconnect the start capacitor, hold down the start contactor and spin the motor backwards by hand to start it.  You could do something similar to rule out the starting circuit.  It also reduces the peak current drawn while starting, though starting a ~4kW motor with a 6.5kW generator shouldn't be a problem.  Unless those are shop-vac kilowatts. 

[james_s]

I don't think the centrifugal switch is the issue here, if that is stuck the motor will trip the breaker, but it will start and get up to speed for at least a few seconds before it trips.

[chipwitch]

I don't think the centrifugal switch is the issue here, if that is stuck the motor will trip the breaker, but it will start and get up to speed for at least a few seconds before it trips.

That was my thought as well.  It was almost as if it was tripping the contactor the instant I pushed the button.  Kind of like trying to turn on a circuit breaker that has a dead short on it, only it was a couple hundred milliseconds slower than a dead short. 

I'm going to wire direct to the mains panel in just a bit.  I'll post back the results.

[chipwitch]

My mill only runs clockwise (nice, huh?).  When I need to run backwards, I disconnect the start capacitor, hold down the start contactor and spin the motor backwards by hand to start it. 

Couldn't you just use a DPDT switch to reverse polarity of the winding?  Besides, by "mill" I assume you're talking about a metal machining mill?  Knee-mill?  Bridgeport?  Any I've seen have a mechanical reverse (in addition to electrical).  I can't think of an occasion where I ever needed to run backwards.

[chipwitch]

Grrr.... I HATE when I do this!

I was so sure the generator was sufficient to run the sander and it wasn't a simple matter to connect it to mains, that I disassembled the whole thing searching for the problem that wasn't there. 

Oh well, at least I can honestly say the thing has been disassembled (partially) and thoroghly inspected.

Thanks to all for your help.  It is not working properly on mains power.

[amyk]

What does it do on mains, does it still trip the breaker in a second or sit there buzzing until you turn it off? If it's the latter, try giving the shaft a turn.

[james_s]

I suspect he meant to write that it is *now* working on mains power.

[duak]

A few thoughts here:
- assuming it's the original start cap and it's rated for only 125 V, the run winding could be split or tapped to act as an autotransformer to provide a reduced voltage to the start winding.  Dual voltage single phase motors often do this so that only one start winding and one start cap is required.  It threw me the first time I encounted it.
- induction motors generally draw about five time their run current (FLA - Full Load Amps) when started.  The time it takes to drop depends on the RPM of the motor and the load.  There isn't a lot that can be done other than to reduce the voltage applied to the motor.
- reversing the motor is possible but does require access to enough winding leads to reverse the electrical phase between the run and start windings.  Note that unlike a three phase motor, a single phase motor usually can't do a plug reverse.  Rather, it pretty much has to come to a stop before restarting in reverse.
Cheers,