Electric motor problem

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GlennW
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Electric motor problem

Post by GlennW »

I have a 1/2 hp, single phase, capacitor start motor that decided not to start. It's a GE 1725 rpm cradle mount, 56 frame, 115/230V wired for 230V and has never had a problem.

I was using it, switched it off, and when switched back on, just fairly quietly hummed. I figured that it may be the start capacitor, so I gave the belt a pull and hit the start button and it acts like it only runs on the start circuit, as it cycles on and off.

I went after the start capacitor (with the switch off) and got a nice crisp POP and an arc when I shorted the terminals with a screwdriver, so it's charging up nicely!

I checked the two power legs for power and am getting 126v on each. Tried each leg to the frame with an ohm meter and they are both open and it does not trip a breaker when attempting to start it.

I have another identical motor on another machine, and started to switch start capacitors, but this one seems to be pretty healthy.

I checked the start/stop switch and am getting 120+ volts to each leg at the motor when it tries to start.

Does anyone have any suggestions as to what to check next? :?

Thanks!
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
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wsippola
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Re: Electric motor problem

Post by wsippola »

Since the Cap seems to be charging, perhaps it's the points feeding the cap to the start coil? Give it a spin to start it, the points will open with the centrifugal switch, perhaps that will dislodge any dirt there. Failing that, I'd clean them.

I'm no expert, but that's where I would look next.

Wayne

Just re-read your post and saw it will run on the start circuit only. Next guess is a bad connection to the field coil, allowing the voltage to register but not any real current. I have seen that on other items - showed voltage but wasn't getting any real power.
scmods
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Re: Electric motor problem

Post by scmods »

Glenn,

The first thing I would check is the start switch and it's circuit. I don't know why everybody goes for the cap right away, maybe its because its outside and easy to get to. Your test with the spark and significant "pop" is probably as good a test as any, and is a verification of it's ability to hold a charge, so scratch that off the list, for now.

Pull the access plate on the back end of the motor and take a good look at the terminal board. Take a sniff and see if there's any evidence of phenolic cooking, it has a distinctive smell. Look for discoloration around the terminal mounting area or warpage. Either is evidence of excess heat, probably from a loose connection. Remember, a burnt start switch qualifies as a bad connection.

If, as you say, the motor is connected for 230 volts, the windings will be connected in series, the series midpoint connection will be the connection point for one side of the start winding and one of the two "hot" lines will be the other side of the start winding, as the start winding is 110 volt only.

Remember, a motor can only fail 3 ways. An "open " winding is a broken wire or bad connection. A "short" is an unintended cross connection within the winding. A "ground" is an unintended cross connection to the motor frame. The short is the most difficult to locate in a static condition, although if the motor can be coaxed into running, a current test will indicate excess current draw in a shorted motor.

Something else to consider is a failure of the thermal overload cut out. Failure here is somewhat rare, and will manifest itself as an open winding. These are bundled with the winding ends if self-resetting, and against the motor case if accessible for manual reset.

If you have access to a "megger", connect one lead to the midpoint junction on the terminal board and the other to the case, wind it up and note if the internal resistance is low. This can often find low level grounds that don't really show up with a VOM, but will cause excess current and internal overheating.

I would also take off the back end bell and check for lubricant contamination that can lead to degradation of the varnish on the winding ends, and coking of the start switch contacts.

Electricity really baffles a lot of people, as you can't see it and must rely on instruments.

If I was a betting man, I'd put my money on the start switch, but that's more of an educated guess without a look and a sniff.

Keep us posted and go for it. Look at it this way, it doesn't work now, what have you got to lose?

Take care and good luck

Bill Walck
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GlennW
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Re: Electric motor problem

Post by GlennW »

Thanks for the suggestions!

It occurred to me while driving home, that I neglected to remove the belt and see if the motor started and ran, as I may have a machine problem. :oops:

That's first on the list in the morning.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
SteveM
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Re: Electric motor problem

Post by SteveM »

scmods wrote:I don't know why everybody goes for the cap right away, maybe its because its outside and easy to get to.
My mechanic says "you check the easy stuff first, even if it's not the most likely stuff".

That saves you from going back and kicking yourself after spending two hours diagnosing something, only to find out it wasn't plugged in :-)

Steve
hammermill
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Re: Electric motor problem

Post by hammermill »

Get the voltmeter back out and check l1 to l2 for 240v in and out of the switch. Measuring to ground tells you very little.

I also suspect the switch or power feed
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GlennW
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Re: Electric motor problem

Post by GlennW »

Got it running this morning, but then my computer died when I was going to update this thread...

I was going to remove the belt and try it, but when moving the belt there was very little resistance so I didn't bother. Instead I gave the belt a pull and hit the start button again and it went through about five start stop cycles and then there was a little "pop" and I saw a small flash on the terminal board and it was off and running. Upon further investigation I found a screw that was about 3/4 turn loose on the terminal board. It wasn't L1 or L2, but a screw securing a jumper and a spade terminal with a red wire attached to it. The only red wire I could find were connected to the start capacitor, so who knows, but it's fixed and running!

The computer is up and running again too.

Can't wait to see what breaks tomorrow!!

Thanks!!

And, bye the way, the only other time I had a capacitor start motor just hum and not start, the start capacitor was bad. :wink:
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
hammermill
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Re: Electric motor problem

Post by hammermill »

well you are off and going, good observation on a loose screw :lol:
scmods
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Re: Electric motor problem

Post by scmods »

Glenn,

Glad to hear of your successful outcome. The color and description of the wire and connection sounds like part of the start circuit. The terminal bolt very likely connected to one side of the start switch on the underside of the terminal board.

I apologize to the board members if my assessment of the usefulness of capacitor examination was overly critical. Its just not up there on MY diagnostic procedure, based largely on experience and that other than mechanical damage, water involvement, and dielectric failure from voltage spikes, there's not much to go wrong with them.

I agree with the general philosophy of checking the easy things first, and most flow diagrams for diagnostic purposes start with things like "is it plugged in" and "does it have fuel", which is the fundamental of easy, such a diagnosis may not directly lead to proper solution. For instance, lets think out loud here and speculate as to how this terminal bolt could become loose. Firstly, it may have been improperly tightened when initially set up. As everything was new and clean it could work fine for a significant time before age oxidation, arc burning, or contamination degrades the compromised connection to the point of failure. Secondly, local overheating due to some defective connection may bake the terminal board, causing erosion of adjacent material to the point where the connection is loosened to the point of failure. This is where the "sniff test" comes into play, as it leads the diagnosis in a different direction. Going for the "easy" stuff does not make a distinction as to whether it was a primary or consequential failure, and may lead to stopping the diagnostic process too soon.

I realize that I'm starting to get obscure here, but that's the way I approach things. I suspect that its the result of autopsying all of my motor failure casualties. Nothing beats a good look and a cleaning. Nuff said.

Bill Walck
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GlennW
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Re: Electric motor problem

Post by GlennW »

scmods wrote: For instance, lets think out loud here and speculate as to how this terminal bolt could become loose. Firstly, it may have been improperly tightened when initially set up.
Hello Bill,

I was curious about that as well!

It is on a 1972 Sunnen MBB-1680 honing machine and I imagine that it is the original motor. I've had it about ten years now and have never had a problem with it. There is no sighn of degradation or discoloration around the jumper or screw, and no odor. It is a cradle mount motor, so vibration isn't a big deal. Perhaps the Phenolic board just collapsed slightly after years of tension on it from the screw as, well as possible thermal effects. Or possibly just not tightened properly when assembled. If it makes it another 42 years without a problem, I'll be satisfied!

Typical Sunnen quality. :D

Image

I've yet to purchase a machine and wish I had bought a less expensive one, but I have bought a couple and wished I would have stepped up to a better one after using them a while!

Thanks for the input!
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
scmods
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Re: Electric motor problem

Post by scmods »

Glenn,

Thanks for the application info. How many times did that motor start in 42 years? One can only speculate.

You certainly have the right idea about quality. Funny how the same names keep showing up on good serviceable old equipment.

I hope some of the younger guys get the message.

Bill Walck
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steamin10
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Re: Electric motor problem

Post by steamin10 »

The last company I worked for, used a thermal imaging camera on alll of their drive cabinets, once a year. After the line has been running 24 hrs, they took their reference pictures.

We then spent weeeks, trouble shooting the hot spots seen on the images. Relays, contacts, terminals. All higher resistance areas that reflected failing current flow, and the resulting heat. Hot running relay coils were replaced, so the electrical failure rate was way low.

Yes, I start at the caps, this is an easy fix and cheap too. Starter switches, and failed or stuck centrifugal operators are next. Motors exposed to damp conditions, have terminal boards that corrode, and switches that burn contacts. Most people toss a no-start motor. Usually a little work, can make them live happy again, for a long time.

A freind gave me a dual speed Crapsman grinder buffer, from the 80's. He said it was worn out, and I could have it for parts. The 2 speed switches were notorious for failing. So with several units of the same model, I wired them around the switch for either hi or low speed and avoided the $25 switch, that fails again. Anyway, plugging it in it ran in fits and starts, depending on the angle to the floor. Thats odd, I thought. Pulling on the cord stopped operation all together. So I pulled the case screws, and sure enough, the power cord had been flexed enough to break down the wiring, making it intermittent under the rubber cord protector. I offered it back, but he had bought another, so I had 3 of the exact type. They have been long out of production, one died with the motor shaft gear stripping on the driven bevel gear. So its parts, providing a handle half for another unit, and a back case where sombody broke the lock button out, changing a grinding stone. (guilty)! These big heavy units have seen many hours of grinding putty, fiberglass, wood, burnishing metal, and all sorts of disc cutting. They make short work of welds in reachable corners, and take off the residue of unwanted weld strings on projects. I dont know what I would do without one. I collected an orbital polisher for auto waxing, so my low speed unit will be set back to hi, and so it goes. $15 at the pawn is good for me to polish the trucks.

When you know the right questions to ask, even the most sticky problems can be solved. Sometimes you just have to go there, and probe the darkness and doubts, until the dawn comes.
Big Dave, former Millwright, Electrician, Environmental conditioning, and back yard Fixxit guy. Now retired, persuing boats, trains, and broken relics.
We have enough youth, how about a fountain of Smart. My computer beat me at chess, but not kickboxing
It is not getting caught in the rain, its learning to dance in it. People saying good morning, should have to prove it.
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