460 VOLT MOTOR PROBLEM

[Smart $]

A meger can be used to check for winding faults as well, not just to ground.

...but can it check a temperature-related intermittent fault to itself???

For example, when the motor ran for one cycle, it warmed up. The motor may not have reached a steady-state temperature that it would in continued operation... meaning there is likely hot spots. As the hot-spot heat dissipates while not running, it will also raise the temperature around the hot spots, possibly triggering an intermittent fault in a winding conductor. If it cools down to where it was prior to initial run, the intermittent fault may clear itself and we're back to square one.

 

[Sahib]

Doesn't have to be be positive pump. What he described could easily be your typical domestic water well, which is usually a multistage centrifugal pump, and is started/stoppedbased on state of a pressure switch.

But such pumps will have little problem starting against pressure because they are not a positive displacement pump and there is no pressure differential across the pump inlet and outlet while stopped with such a pump design.

A common positive displacement type of pump that could easily start against pressure is an air compressor, but a good design in that kind of application allows for pressure to bleed off at the pump somehow when it is stopped so that when it starts the next time it doesn't have to overcome pressure when starting.

In addition to pressure switch operation, I also took into account other symptoms indicative of a positive displacement pump: low voltage when the pump attempts to keep running. If it were a centrifugal pump, its response to any full pipe blockage would normally be very reduced current (around 30% of normal running current) and so there should be no drop in voltage.

 

[kwired]

Maybe I missed it, but did you read the current? If starting current is 5 to 7 times running just during a normal start, sounds OK. If it stays high when running slow, you may have a winding problem or a connection problem. Did you take off the "pecker head" cover and check all of the connections? Check current on all 3 phases. They should be almost equal if voltages are equal. Megger only tests for grounds. Over current is what trips breakers. If all else fails, put on a recording ammeter to see what's happening during starting on each phase. Sounds like single phasing to me.

Could be single phasing. We don't know for certain if his 430 volt reading was for all three phases or just between two phases. He did say he checked both sides of the controller though, so I kind of assumed he has checked all three lines as well. If it were single phasing you would think there would be pretty significant voltage imbalance, but current should be checked as well.

 

[ptonsparky]

...but can it check a temperature-related intermittent fault to itself???

For example, when the motor ran for one cycle, it warmed up. The motor may not have reached a steady-state temperature that it would in continued operation... meaning there is likely hot spots. As the hot-spot heat dissipates while not running, it will also raise the temperature around the hot spots, possibly triggering an intermittent fault in a winding conductor. If it cools down to where it was prior to initial run, the intermittent fault may clear itself and we're back to square one.

I do not know if I have ever run across that problem and have a good hunch most of us never will, but to answer you posed question, if it opens you most likely won't find it. Now my Fluke has an excellent Ohms setting that may find that short if it stays faulted.

There is one test we do not mention very often and relates to opening the pecker head for a look see. If it smells like a failed motor, most likely it is.

 

[kwired]

There is one test we do not mention very often and relates to opening the pecker head for a look see. If it smells like a failed motor, most likely it is.

That is not a look see - that is a sniff test But is quite effective at determining if a motor is toast.

A look see is pulling end bell off the motor and looking directly at the ends of the windings.:happyyes:

 

[Smart $]

I do not know if I have ever run across that problem and have a good hunch most of us never will...

Hmmm... and that's just the way my luck goes. I have seldom worked with motors, and I've run across an intermittent open fault in a motor winding twice in my career.

 

[iwire]

Hmmm... and that's just the way my luck goes. I have seldom worked with motors, and I've run across an intermittent open fault in a motor winding twice in my career.

How is that even possible?

 

[ActionDave]

There is one test we do not mention very often and relates to opening the pecker head for a look see. If it smells like a failed motor, most likely it is.

Used the smell test as an indicator but like kwired I have always pulled off an end bell before making final diagnosis and informing the guardians of the motor that its time on earth has expired.

 

[hurk27]

If this is a new install then double check proper wiring for high voltage (460v) that the correct taps are put together and such, if it is an existing install that was running fine but then started having problems, check for single phasing all the way to the motor, do not check for single phasing by metering from phase to ground or you will just see the back fed voltage from another winding, go A-B, A-C, B-C for proper voltage, also check the current on all three phases, you could have a bad tip on the contactor that allows the first start up but after that one of them opens up after it has heated up, this can also happen on the overload heater elements if they are the heater type and not electronic.

If this is a positive displacement pump such as a pressure washer or other high pressure application then check the unloading valve to make sure it unloads the head pressure when the motor shuts down, this may be in the pressure switch or one in the head that uses the oil pressure to close it like on an air compressor, Is there a check valve after the outlet pipe? maybe it is sticking shut?

I'm leaning to a single phasing issue like the others specially if this is a centrifugal pump.

Also does this motor have a WYE start delta run starter? (6 or 12 lead) or is the motor a 12 lead but is wired in a WYE, if it is a 12 lead and you are using a DOL starter then make sure the motor is wired for delta.

Try to ohm out the phases to the motor right when it doesn't start but make sure you have removed the power, this could show up a winding opening up from heat, if it takes 10 to 15 Min's before it starts back up this sounds like a heat issue that is opening one phase to the motor or in the motor winding.

One more thing, if you find that when it is not starting and you confirm it has high current on all three phases then you might want to look for a mechanical issue such as seals or bearings binding from heating up after the motor has been running, it might take using three amp meters to catch this if it trips out faster then you can check each phase, but you must make sure you have high current on all three phases to be sure its not a single phasing issue.

 

[hurk27]

A common positive displacement type of pump that could easily start against pressure is an air compressor, but a good design in that kind of application allows for pressure to bleed off at the pump somehow when it is stopped so that when it starts the next time it doesn't have to overcome pressure when starting.

I have yet to see an air compressor that does not have an unloading valve or a bleed off on the pressure switch that removes the air pressure from the head when the compressor shuts down, some unloading valves are controlled by the oil circulation pump using this pressure or lack of to open the unloading valve when it stops, some use the air pressure from between the stages, constant run units use a pressure control valve to control the unloader but the motor never shuts down, the unloader just opens an extra intake or exhaust valve on most, I have had many unloaders or bleed off valves fail and stall a three phase compressor, even ones that only go up to 90 psi.

An refrigerant compressor is one positive displacement pump that doesn't unload/equalize right at the time it shuts down and is why a single phase units has to wait a few Min's before starting back up after the high and low side equalize, but a three phase unit can start back up right away but is not good for it as it adds to the start up time and current.

 

[Smart $]

How is that even possible?

Even at the time, all I could do was speculate. Couldn't even find it physically. Perhaps a stress fracture in the magnet wire? IDK. One of the motors was rewound, and worked flawlessly thereafter (at least while I was at that facility).

I've run across new insulated solid wire in NM that had a conductor that appeared to have two wire ends just butted together within the insulation, but yet no break in the insulation. You can ask the same question about it. I asked myself. Guess what answer I got...

 

[kwired]

Hmmm... and that's just the way my luck goes. I have seldom worked with motors, and I've run across an intermittent open fault in a motor winding twice in my career.

How is that even possible?

I guess I can see it being possible, but also see it being difficult to confirm - especially for the average maintenance electrician in the field.

Any weak connection supplying an inductive load seems to have a tendency to burn itself open in a way that is generally obvious. Maybe the chances of intermittent problem increases some with a small motor, and maybe even more if it is lightly loaded.
A break in a winding probably doesn't have too much chance of becoming an intermittent "open" but maybe the mechanical connection between a magnet wire and the lead to the peckerhead has a better chance of becoming an intermittently working connection.

 

[bure961]

I was called sunday by a mechanic who wanted the key for the lockout because they found what they though caused the problem. After disassembling the pump they found wood that they said could have caused the pump to lock up. After it was started and when it tripped out they reset the ol's and would run for awhile and the same thing would happen. They were resetting the ol after each trip. Seems like the wood was not the cause.

Now the bad news, the motor is toast. When I came in tonight to see the progress the pump did not start , starter pulled in that was it. Megged motor again and it is shot. Checked the feed to the pump from breaker to the starter line and load sides, all 3 phases are good, 90' of # 6 thwn. Also shut down mcc and pulled the bucket to check for a bad connection to buss.

When motor comes back I will run it with the mechanics there and leave it to them finish the job. Hopefully they will find something with this pump before then. When they do I will post the findings.

There was a lot of very good information from you all and I learned a few things here. Cannot say thank you enough for all the time and help you guys put in on this.

 

[Milltrician]

Good thread. I'm looking forward to what you find out caused the problem. As a lumber mill electrician, we occasionally encounter strange electrical issues that don't follow the logical "rules". Although I haven't encountered what Smart $ described, I wouldn't go as far as to rule it out. On a couple occasions we have had motors that passed insulation test and appear to be fine but would fault the drive. Finally just changed the motor and problem solved. Sometimes not a practical solution if you don't have shelves of spare motors.

I agree that it sounds like it was single phasing. Sounds like you covered all the bases. Only other thing I might have tried if possible is uncoupling the motor and running it. How did it look in the pecker head?

 

[ptonsparky]

Don't discount the wood connection. Repeated resets of motor overloads, without the operators finding out why, keep us busy replacing motors and my office help busy sending out invoices. $$$:thumbsup:

 

[kwired]

On a couple occasions we have had motors that passed insulation test and appear to be fine but would fault the drive. Finally just changed the motor and problem solved.

Depending on exactly how you did insulation test - one could easily miss a motor with damaged winding insulation resulting from IGBT reflected wave issues from a VFD, in particular on a motor used with a drive on a 480 volt system. The drive senses fault current at startup though the fault current level is low at this point and shuts down to protect the drive from higher fault current if it were to continue to accelerate that motor. You will not read any failures with a simple 1000V pass/fail type of meg test, and likley will need even higher test voltage because the motor sees higher then 1000V spikes from the IGBT reflected wave.

If motor is damaged from this it is probably a good idea to put a line reactor on the load side of the drive to smooth out the spike before it gets the replacement motor.

 

[Jraef]

Although what smart$$ described is a little far fetched as an open conductor in the stator half of the motor, it is more common than you might think on the rotor side. What typically happens there is that there is a broken connection to one of the collector rings, likely over several rotor bar connections. When the motor is cold, they connect and conduct, then as it heats up, they move further away from each other and you lose one or more rotor bar, which drops the motor torque precipitously. Once the pump is already running you may not notice the torque loss, especially if the pump is a little over sized. But attempting to RESTART when the motor is already hot results in an inadequate amount of torque to overcome inertia.

My guess is that whatever was supposed to prevent wood from getting into this pump has not been doing the job for a long time, and repeated jamming of the impeller eventually broke the rotor bar connections. After that it was only a matter of time before the repeated resetting of the OL trips without addressing the root cause led to a breakdown of the stator insulation, which became an unavoidable problem.

The point is, if this system was not designed as a "chopper pump" that could handle occasional chunks of debris in the liquid, then replacing the pump motor without finding the source of the possible incursion of that debris will play out as a repeat.

 

[meternerd]

A meger can be used to check for winding faults as well, not just to ground.

How? Winding insulation to ground I can understand, but winding to winding reads almost a dead short anyway. For an induction motor there's no wiring going to the rotor so what's to measure? Wound rotor motors are not too familiar to me. Usually only saw them on DC on the submarine.

Not questioning you, just curious for my own info. Thanks for any education.....

 

[ptonsparky]

How? Winding insulation to ground I can understand, but winding to winding reads almost a dead short anyway. For an induction motor there's no wiring going to the rotor so what's to measure? Wound rotor motors are not too familiar to me. Usually only saw them on DC on the submarine.

Not questioning you, just curious for my own info. Thanks for any education.....

Open all the winding leads. 1&4 should show low ohms, say .4. 2&5, and 3&6 the same. 8&9, 7&8, 7&9 about .8 for a 9 lead motor. None of the grouped windings should show continuity to the other. 12 lead motors would make 7&10, 8&11, 9&12 .4 ohm.

 

[meternerd]

Open all the winding leads. 1&4 should show low ohms, say .4. 2&5, and 3&6 the same. 8&9, 7&8, 7&9 about .8 for a 9 lead motor. None of the grouped windings should show continuity to the other. 12 lead motors would make 7&10, 8&11, 9&12 .4 ohm.

Makes sense. For new motors, we just ohm out the individual windings looking for continuity, then meg 'em at the starter after we hook 'em up. Majority were 9 lead Wye, part winding start. Soft starts came later. That way, we test for grounds before we fire it up. I've never found one with shorted or open windings, though. Just grounds. We're talking about smaller motors, though. 480V up to 500HP. The big boys (6000 and 9000 HP, 13,800 Volt nuclear pumps) were a whole different story.

Thanks.....