motor failure 2

[humbleuser]

I don't hang out in this forum anywhere near enough as I should. This is great technical site. However, I do have a question that I'm hoping some of the more experienced mebers can answer. Keep in mind that I'm not a trained electrician; I'm more of general maintenace type of guy

I have 25 HP G.E. motor (230/460 3-phase) attached to an injection molding machine. I saw this machine running and making parts prior to me buying it. The place where it came from had it wired for 460. I brought it and rewired it for 230. The motor ran for about 5 minutes, somewhat noisy, before it started smoking and tripped the breaker. The shop that is rewiring it told me that I had the proper wire connections to the motro. I asked him what caused it to burn out? He's not sure. He says the bearings aren't in bad shape. he thinks that some of the 'varnish coating' on the windings may have worn out. It's possible that some condensation on the worn area may have caused the motor to short out. Is this possible? Thanks in advance.

 

[itsmoked]

Get a good electrician to help you restart that bugger. Tell him you want the protection checked.
You want the currents checked.
You want to make sure the rotation is correct!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Oh, hey there's a thought! Reversing the motor rotation on a hydraulic pump could be nasty. Depending on the hydraulic system design you may have been trying to start or run it against a closed system, which would have been essentially locked rotor. That would have made noise!

1989, I doubt it would be a delta-connected motor. You can do a simple check though with a continuity tester. Disconnect the incoming leads. If it's Y connected you will have continuity between 7, 8 & 9. If Delta, you will have continuity between 7, 2 & 5.

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http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[stardelta]

The rewind shop should be able to tell you exactly how and why the winding failed, if they cant I wouldnt have much faith in them (sorry to be negative). They should at the very least be able to identify from the condition of the failed winding, water and/or moisture ingress, earth faults either through insulation failure or core rubs, overload and single-phase conditions, shorts between turns and shorts between phases. I would go back to them and ask them to be more specific and which of these fault conditions applied in the hope it can shed some light on what has happened. Another thing to consider, the pump, did it rotate the wrong way when first started? Maybe it has a screw-on impeller that has loosened and screwed off while running the wrong way due to inertia. This in turn may have caused a partial siezure and overload. However my money is on an incorrect connection made when it was converted from the higher voltage to the the lower, lets be honest its the only significant change that has occured. An incorrect connection would lead to a reverse phase condition in the motor winding and a motor of that size would shake and vibrate like mad in these circumstances, this may explain the noise issue. In addition its doubtful it would reach full speed under load, it would have a reduction in power and would certainly draw unbalanced and excess phase currents leading to a burnt out winding. At the risk of sounding condescending so I say it with respect, dont ever assume that just because a motor spins everything is ok and just walk away. Full load phase currents and speed need to be checked while running as an absolute minimum and compared to the information on the nameplate. We dont see many NEMA spec`dual voltage motors of that size here in the UK, so my knowledge on this topic is limited but another consideration, again at the risk of sounding condescending. I have seen complications arise with NEMA spec motors due to the figures `6` and `9` being confused on the lead markings, if that has happened in this case it would explain everything.

 

[l3city]

jraef: "Improper fuse size would not make the motor smoke!"

Can you explain what you meant by this? Is your statement for this specific "humbleuser" situation. I'm so confused now, all this time I tought using OL fuses helps protect motor from damages.

 

[itsmoked]

He means putting the wrong size fuses in will not make the motor smoke. The wrong fuses could allow the motor to smoke.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

But! Having the wrong fuses could allow one fuse to fail and then the single phased motor to smoke.. Seen it many times. So maybe the wrong fuses could make a motor smoke.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Thanks for jumping in Keith, yes that's what I meant.

But in this case, had he failed to change fuses when he went from 480 to 240V they would have been 1/2 the required size and would have blown almost immediately, which would have prevented the motor from smoking. Your point about the single phasing causing the motor to burn out is valid, but in this case 30 seconds is far too short of a time for it to start smoking from that cause IMHO.

http:/

http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[l3city]

jraef:

fiiiuuuuf, thanks for clarifying!!

 

[waross]

I have seen a lot of hydraulic pumps that were so noisy that you could not hear the motor. Are you sure that the noise was not pump noise? The people most able to suggest the cause of burnout are the winders. Was the motor in a heated area a day or two before being started? If the motor was in an unheated area, then condensation is very possible.
respectfully

 

[ScottI2R]

Jraef,
Correct me if I'm wrong, but shouldn't the fuse rating INCREASE?

Scott

In a hundred years, it isn't going to matter anyway.

 

[ccjersey]

Those small dual voltage delta connected motors do exist here in NEMA land, farm crew burned up a brand new one just a few months ago. "we connected it up just like the old one".

 

[jraef]

ScottI2R,
Yes, it SHOULD have. I was pointing out that IF he did not increase the fuse size when changing from 480 to 240, his 480V rated fuses would be too small and blow immediately.

 

[BL1]

Just a thought.Just wondering actually.Nema design letter
and the service factor designation? Were those on the name plate as well?

 

[moira]

Hello, HUMBLEU.: When you buy an unknown motor you should: 1) Make a uncoupled running test; see how it sounds, see how it turns by hand ( should have very little physical resistance, it should take some time to coast to a stop), look for cracks, loose joinery, any signs of abuse, for a running UNCOUPLED load reading - You want no more than 35% of the nameplate rate FLA at the tested voltage!; should not develop objectionable heat after several minutes. Naturally, you want a good $$$ price!! If the motor doesn't pass the tests---YOU DONT WANT IT!!! Too many pretty girls in the world; MOTORS TOO! When you hook up and REWIRE in your own place, make the same test! Readings should be about the same, proportionally to applied voltage. Before you hook up your load ( making your product) know that the motor works fine! About this 230 volt thing; are you sure it's not 208VAC???? Even at 230 VAC you are going to have an aprox. 2X increase in current. If your problem wasn't passable in the beginning added heat caused by current could have been the killer. If you have the bad motor, get an idea with a MEGGA of just how the windings are still held together; sure to check the center connection! If you only have 208, 3 phase VAC and the motor nameplate isn't marked "suitable for 208 VAC" you should not have used this motor! 10 % difference in required voltage here. why are you changing motors?? How is the condition of the motor starting relay and feeders?? Hope this is of some help, MOIRA

 

[machmech]

humbleuser

I have to ask what exactly did the noise sound like? I think it would help trouble shoot the cause or at least give everyone a better idea, A hum or sizzle to me is electrical and chatter or clank = mechanical. Smoke is the end result.

Chuck

Getting older is inevitable
Acting your age is optional

 

[bigbillnky]

humbleuser

The windings of a correctly-connected motor that failed from single-phasing will be easy to spot. If it was wye connected(most likely), two of the three stator windings will be charred. If it was delta connected, only one will be charred. If the motor failed due to an overloaded condition OR a mechanical problem, all three windings will be charred.

I would suspect that the motor or load was either damaged in transit or improperly reassembled/aligned, or a faulty connection existed and caused the failure. A poorly made connection or a high-resistance grounded fault will allow a motor to start, but will create a noise similar to true single-phasing. The motor will perform poorly and fail in short order if the control does not trip the motor off line.

Have a qualified electrician examine ALL the control components for proper size, rating, and OPERATION before reconnecting the motor. Also, thoroughly check the mechanical components of the machine. Keep in mind a bad bearing when cool can seem fine, turning effortlessly and having little or no play. Once it reaches load and speed it will soon start to heat up and lock down. A non-contact infrared temp gun is a great tool for finding this type of bearing problem.


Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[machmech]

bigbillnky

I have to question your description of potential bearing problem as this is very unique in my experience. The bearings I have had heat up and lock down self destruct very soon and cool down/recovery not likely. We use self aligning, precision thrust, and standard roller.

My questions - What type of bearing have you experienced this condition with ? And how often ?

Chuck

Getting older is inevitable
Acting your age is optional

 

[humbleuser]

The shop insisted again that I had the connection correct. I'm thinking the bearings on visual inspection looked fine but under rotation and load they were not. The motor was rewound and the bearings will arrive/be installed on Monday.

I believe I mentioned that the pump had been rebuilt a few months ago. I don't know exactly what was wrong with the pump; I'll find out on Monday. For those not familiar, the pump is bolted onto a mounting frame that is bolted onto the motor face. It's pretty difficult for the motor/pump to be misaligned. A sleeve coupling is used; and the hubs were snug on their respective shafts.

I think it's the bearings that caused it. Just my $0.02 worth.

 

[stardelta]

You are paying for the motor to be repaired and as such you are entitled to a reliable repair, as they seem unable to provide you with an accurate fault diagnosis I would be questioning there competance. A winding failure attributable to a bearing problem has 2 forms. The winding can be overloaded by a partial bearing seizure but given its 25hp the bearings will display obvious signs of damage. If a bearing collapses, the motor will suffer a core-rub, the effects of which are both catastrophic and very obvious. Ensure the repair company check and ensure the bearing housings and journals are not worn and are within the tolerances specified by the bearing maker for the relevent sise. Ask for evidence this has been done. Worn components will rapidly lead to premature bearing failure, just looking at them is not enough, they must be measured using accurate instruments and allowances made for the locked and floating ends. The shaft needs to be checked for run-out and idealy the rotor check-balanced.

 

[waross]

With a hydraulic pump, someone may have inadvertently changed the setting on a bypass valve or a pressure relief valve. Bad valve settings can make a lot of noise
I think the two most important issues to check are;
1> Voltage. Make sure you are on a 240 volt three phase system.
2> Pump settings. Make sure that none of the pressure switches or pressure relief valves are out of adjustment. You may have a solenoid controlled unloader valve or dump valve that is not opening properly.
Either a voltage problem or a pump loading problem will show up as excess current so check your motor currents.
respectfully