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Old 03-13-2017, 10:10 PM   #1
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Default Voltage Fluctuation

Hey guys,

Let me start by saying I'm a new journeyman, and I was raised in construction mostly doing offices so I have almost nothing for motor experience (should have gone industrial to learn more!).

Bumped into an issue I'm stumped on for the time being.
A 600V motor (water pump pushing water for one of the building's systems) is running but not pushing water. My boss (maintenance guy) gave me the best explanation he could of a problem the motor had seen.
"It lost a phase and hasn't worked right since. It blew one of the wires in the mcc. The wire and fuses were replaced by an electrical company."
Something did blow at the mcc, but I know none of the details. Everything in the mcc looks to be ok visually.
The motor sounds normal and runs but simply isn't running hard enough to move any water.
Now the problem...
Verified it was rotating correctly. I pulled the motor leads off at the peckerhead and pulled out my meter. On all three phases I could watch the voltage fluctuate from 80V - 130V.
I ran out of time today to do much investigating, but I did verify proper voltage on the line and load sides of the fuses in the mcc. I will be checking the contactor tomorrow.
Any insight as to what might be going on between the load side of the fuses and the peckerhead would be greatly appreciated. Common sense tells me that there is an issue with the contactor, but I do not see how a contactor would cause the weird voltages and would assume that the guys who replaced the blown wire at the mcc would have verified voltages. Although they may have measured the supply coming in to the mcc section, turned the motor on, saw the motor running, and walked away.

Also on a side note: There is a very small CT on one wire coming off the load side of the contactor. I have no idea what this is doing there and would appreciate an explanation if anyone knows.

Thanks and sorry I don't have as much info as you would probably like!
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Old 03-13-2017, 10:31 PM   #2
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Any chance that it's single-phasing ?
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Old 03-13-2017, 10:43 PM   #3
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telsa,

I could be wrong in my understanding, but if it were single phasing wouldn't I be seeing random voltages on the two connected phases and not a steady 80-130v on all three?
I've only seen a couple of RTU motors single phase due to bad fuses, but they also made some troubling noise. This motor is purring like a kitten.
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Old 03-13-2017, 10:58 PM   #4
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Let's take a wild guess that you have other 3-phase motors tied in to your MCC.

They can 'manufacture' the missing leg and bounce it back through the MCC... or any other cross connection.

Feeble connections can drive you crazy -- as they read decent voltage to a DMM while the actual ampacity is screwy -- choked.

The feeble connection could be inside a contactor -- across some heaters -- pitted faces -- on and on the failure modes go.

When this happens you're left with a low power 3-phase motor. It won't be as 'sick' as a single-phasing machine.

So, you need to make sure that your target motor is totally isolated, off-line...

Then test everything up from zero...

I'd meg the puppy straight off... with some concern that you've got internal damage.

A fall of potential test may be necessary across your contactor if the easy stuff doesn't pan out.
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Old 03-13-2017, 11:13 PM   #5
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Well hopefully its as easy as a fried contactor.
Long story on my current work situation, but I have no access to a megger or any fancy gear. I would put money down that there is some internal damage. Something in the motor's mcc section blows up, then the motor runs for who knows how long on low voltage.
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Old 03-14-2017, 01:02 AM   #6
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The CT might be part of an overload control feature to monitor power usage, could be for a current relay, etc. Hard to tell without more information. With damaged contact faces you will see a higher impedance that may fluctuate with arcing, and result in a fluctuating voltage drop, hence the fall of potential test Telsa referred to. Visual inspection will do well too, and will show carbon debris in the contactor which would prevent solenoid seating. Any significant damage will be noticeable. You may have a high impedance ground fault in the conduit as well. This is where an insulation test of the conductors would be helpful. Don't exclude the pump itself as a problem, it may be the root cause of anything electrical. Odds are, if the electrical system is properly designed and installed, it will not fail unless there are external factors contributing a problem- system age notwithstanding. I recommend checking another MCC bucket for power fluctuations to ensure that your system integrity is good from the outset. The bucket connection to the buss may also be the failure point. Any connection will be a potential problem. I have seen starters that lasted in service for years with a loose connection between the overload relay and the contactor which came from the starter manufacturer. Also, the starter coil maybe compromised. What is the system voltage?
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Old 03-14-2017, 05:45 AM   #7
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Check voltage phase to phase, not phase to ground.
Do a "fall of potential" test across the starter contacts and the overload relay.
Verify the the pump impeller is turning.
Check all fuses or breakers.
CHECK PUMP MOTOR FOR PROPER ROTATION.

You will need proper tools to go further:
Tach the motor shaft RPMs.
Megger the motor and motor leads.
Check amp draw on each phase.

Possibilities:
Motor running wrong direction.
Starter or overload failure.
Phase loss due to- bad fuse or breaker, high resistance at contactor, overload relay failure, broken wire, one winding open in motor.
Pump or bearing failure causing mechanical overload.
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Old 03-14-2017, 02:50 PM   #8
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Can't be a total phase loss or the motor would not start. 3 phase motors can RUN AFTER losing a phase, but will not re-start because there is not longer a phase "rotation" with only two phases. So they sit there and shake back and forth making a nasty sound that generally makes people freak out and hit the Stop button, or trips the breaker/blows a fuse if allowed to continue. Rule that out.**

But if you have high resistance on one phase, the lower torque on the third phase may be enough to get it to BEGIN rotating, then from that point on it is spinning, but has severely unbalanced currents. Current imbalance causes what's called "negative sequence current" to flow in the rotor, which in turn creates negative torque, meaning torque in OPPOSITION to the running torque. So now the motor consumes more current, but performs less actual work and is thereby unable to develop full torque at the shaft. In a centrifugal pump, this results in a loss of pressure and a "static" pipe fill; no more flow once the head pressure in the pipe equals the decreased pressure capacity of the pump. I've seen that happen when an ME missed a decimal point and undersized 3 x 600HP pumps, resulting in the water only getting to within 5 feet of the outfall into a tank. We could see the freaking water, but it couldn't make it to the top, even with 1,800HP trying to push it. The laws of physics cannot be violated.

So yes, I'd say it's a good bet that your previous single phasing event took place WHILE this motor was running, so the increased current caused the contact in the starter to burn. Now it has higher than normal resistance, resulting in a voltage drop on the phases connected to that pole. Since you lack the tools to perform better tests in the field, I would just start with replacing that starter. It's might be something else, but I'd give this a 75-80% chance of being the culprit given your explanation of the history and symptoms.

**OK, one possible way it MIGHT still be a complete phase loss, but it will depend on the piping system. IF you have what's called a "pump control valve" that opens on COMMANDING the pump to run (instead of relying on it ACTUALLY putting out pressure), then what can happen is that the pump is turned on and the valve is opened, but because the pump isn't actually spinning, the back-pressure flows through the pump control valve and spins the pump BACKWARD. Then because it STARTED spinning backward, the single phase power KEEPS it spinning backward, but you don't know that because you can't see it, you just know there is no flow.
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Old 03-14-2017, 05:38 PM   #9
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*OK, one possible way it MIGHT still be a complete phase loss, but it will depend on the piping system. IF you have what's called a "pump control valve" that opens on COMMANDING the pump to run (instead of relying on it ACTUALLY putting out pressure), then what can happen is that the pump is turned on and the valve is opened, but because the pump isn't actually spinning, the back-pressure flows through the pump control valve and spins the pump BACKWARD. Then because it STARTED spinning backward, the single phase power KEEPS it spinning backward, but you don't know that because you can't see it, you just know there is no flow.
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Trick $?|+.
Check frequency to see if this is happening.
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Old 03-14-2017, 06:51 PM   #10
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So they sit there and shake back and forth making a nasty sound that generally makes people freak out and hit the Stop button, or trips the breaker/blows a fuse if allowed to continue.
-The lighter side of JRaef

I agree that it is probably not (I'm trying to not be definitive) a total phase loss. I am going to go back to the fluctuating voltage. We need to establish that there is proper voltage from the source to the line side starter terminals before anything else. Once that base is established, and the output of the starter is bad, the starter can be isolated. I don't necessarily think this is a mechanical issue, but it is a possible root cause for a prior phase loss, etc.

1. Using a good flashlight, and your nose, carefully make a visual inspection of the bucket's components. Literally sniff the components inside the bucket for any signs of a burning odor. Sounds silly, but it can be very helpful.

2. Test incoming power at the line side of the bucket's OCPD, if possible. If not possible, test it at the load side of the bucket's OCPD. If either test fails, the problem is further upstream.

3. If test #1 passes, test voltage sequentially at every point from the OCPD in order to find the current problem component. Do this before checking any terminations.

4. After the problem component has been isolated, tighten any suspect terminations and then re-test voltages. If they are better, it is simple. If not continue troubleshooting.

5. Inspect the starter carefully. Open it and inspect the mechanism for damaged contacts, debris (carbonaceous debris is a sign of failure, i.e. something burned), peening of the armature, delaminated armature, discolored coil windings, cracked bakelite, etc. Check every connection whether it is a field connection or manufacturer's connection. Copper that is a dull red, or brassy looking is a sign of excessive heating. Any soot or frosting is a sign of excessive heating. Bakelite plastic that is eroded looking or soft and whitish is a sign of excessive heating. Tip: if it doesn't look like it is normal, it probably is not.

6. Visually inspect the load conductors where they are accessible. Lower voltage on one phase is usually a sign of higher current on that phase, which means higher than normal heating of that conductors insulation. One caveat, signs of overheated insulation are usually visible only near the source of the problem, but not always.

7. If the starter passes your visual inspection, disconnect the load and energize the starter. Test voltage at the line side, load side, and across each phase (testing one phase from line to load).

8. Check the nameplate data against the OCPD and overload size in the bucket. Check the wire for proper size, but be careful. There are times when the NEC allows conductors to be fused differently than the general rule from Art. 240. If anything is amiss that could be the root cause.

9. What kind of pump is it? If it is a centrifugal pump (non-positive displacement pump- there is nothing to keep the outlet physically separated from the inlet) how do you now correct rotation? Some have a volute shape like a seashell. If this is the case, the impeller should turn with the volute towards the outlet. If it is not volute shaped, the pump may be bi-directional. Check for an arrow cast into the pump. Remove the fan cover and try to manually sin the motor shaft to ensure it turns freely (this is probably the case as you said the motor was running).

10. Since you do not have access to a megger, test the motor windings with your ohm meter. This is always a preliminary test to meggering anyway. If it fails this test, there is no point in testing further. There shoud be no appreciable difference in reisistance between any of the sets of windings.

12. What kind of water is being pumped? Is it clean? If it is, you can rule this out as a problem. If it is dirty or contains debris, this may be the root cause as large debris can damage a pump's impeller.

13. Is there a valve after the pump outlet? Check to see that it is open. If it is a check valve, it may be clogged with sediment or stuck shut or partially shut. Is there a strainer before the pump inlet Remove the screen to see that it is clean.

Bottom line, you need to really invest yourself in this to find the root cause. It may be electrical, or it may be mechanical. My bet is on the bucket connection to its source. This is why I recommended testing power at another bucket in my earlier post. If the same issue exists at more than one bucket, the problem goes further upstream. Okay, so there was a prior single phase event. Good to know, but what caused that. Odds are, the real problem didn't go away by replacing parts.

Troubleshooting is always to isolate and correct the root cause. It must always be done from our field of expertise as a starting point, be absolutely must be done systemically. It must also always be done systematically as evidence leads.

Good luck Mr.Awesome. Let us no how it turns out.
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Old 03-14-2017, 06:58 PM   #11
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Quote:
Current imbalance causes what's called "negative sequence current" to flow in the rotor, which in turn creates negative torque, meaning torque in OPPOSITION to the running torque.
JRaef, thanks for this post. Something I didn't know. Cheers!
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Old 03-14-2017, 08:31 PM   #12
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LOL, the old "sniff test" is something I've used for years, never failed me. That acrid smell of burnt insulation (or lack thereof) can tell you a lot.

But in this day and age, sticking your head into an MCC bucket without having the entire MCC shut own can't happen any more. So to be clear, REMOVE the bucket, close the door or replace the starter, THEN sniff it when the bad starter is on the bench....
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Old 03-14-2017, 08:58 PM   #13
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Worth REPEATING Just IN case: VARMIT SAID Check VOLTAGES PHASE TO phase, NOT PHASE TO ground. You might want to confirm to us that is where you see 80...v

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Old 03-14-2017, 09:44 PM   #14
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There something else along the way .,,

Do you have any heavy cycling loads on the MMC ?? that may affect the reading of voltage and if you are on Wye or Delta system that may affect the voltage reading some.

But as other guys posted alot of very good points for troubleshooting.

As they posted that is well proven methold of trobleshooting and I have done simuair way as they posted.
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Old 03-14-2017, 10:26 PM   #15
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Thank you guys very much! For a guy with little motor experience, this thread is a treasure trove of information! I've been through alot of what has been said so it is nice to know I was on the right track. You have also given me some useful new information.
I'll address Corysan's thread as it covers some of the other points made...

1. Sounds silly yes but I was told in my first year by a wise old grouchy jman on service calls that your nose can save you time troubleshooting. I've since embedded it in my brain to smell things. No funk in this mcc section though.

2 - 4. I had tested the line and load side of the fuses yesterday. 347/600V respectively across the board there. I was able to get back to measurements today and will get to that in a moment...

5. Should the starter end up being replaced I fully intend to tear it apart and see what it looks like inside. I have replaced old panels that had rainbow coloring (red blue white you name it) from overheating and being leaked on over the years so I have also made a mental note to look for such things.

6. The load side conductors were all replaced after the initial explosion. They all look good.

7. Yesterday my tests were done at this location with the motor disconnected. 347/600V across the board.

8. Not done but will do. I assumed the repair guys sized everything accordingly but will check on this to be sure.

9/12/13. The impeller being part of the original problem causing something to blow has not been ruled out. The boss is thinking of tearing it apart (it is one of two pumps for the system so the other can run for now) to inspect it before they fire the system up in a couple of months. I think it is a pump for a water loop feeding their chiller unit and some coils around the building. That being said, would rotation really matter? Would it not just push water through the loop in the other direction just fine? I did consider that maybe two phases were flipped somewhere (ie: not in the order of red black blue) before the explosion and were not wired back up that way (new wires follow red black blue all the way through), but when we bumped the motor the boss claimed the rotation was correct. He could be wrong though. There is also no arrow on the shell nor any arrow stickers anywhere. Taking the pump apart will also allow for water inspection.
*Unrelated note: Wires should be orange brown yellow for the 600V system, but I have only seen this practice done a couple of times. Not sure why it isn't followed more often commercially.

10. I metered out the coils yesterday as well. I do not remember the resistance reading offhand but it was the same number across the board. It is a permanent wye connection with T1 T2 and T3 accessible. Meter also did not pick up on any shorts to ground.

I would love to vest more time into it and reinforce troubleshooting techniques while getting my hands on equipment I haven't used before, but it is up to the boss how much troubleshooting he wants to do vs. just replacing parts and hoping it works out. Even if the motor works fine from here on, I am very very curious as to why something blew up to begin with.

J,
I didn't know the "buckets" could be removed! The one mcc I touched during my apprenticeship was completely pre-wired, pre-fused, ol's set, everything. We just had to run our wires to the load side of the contacts (and did verify ol settings and such). I'm going to look up how these things are assembled.

Mike,
The 80-130V was phase to ground. I do not recall the phase to phase reading but it was off as well.

SO.... today's progress:
I went back to metering out the mcc section, motor still disconnected. 347/600V line and load side of fuses like yesterday. 347/600V line and load side of contactor I was not expecting. I feel puzzled, go back to the motor, 347/600V at the freakin' motor!
So the plan now is to leave it be until they are ready to prepare the loop for service. Then it will be turned on, rotation checked. If there is any other issues I told the boss to call up one of the contractors he rubs shoulders with to come by with a megger and tachometer to perform tests. I expressed that I would personally put money down on the contactor being pooched (that is the only thing I could see causing all three phases to act erratically at the same time and at the same voltage fluctuation when we know the supply is good) and if the motor checks out ok under the tests it will be replaced.
So it seems to be a waiting game! Unfortunately I am on a month long contract so I may not get to see the outcome of this.
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Old 03-14-2017, 10:28 PM   #16
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Originally Posted by frenchelectrican View Post
There something else along the way .,,

Do you have any heavy cycling loads on the MMC ?? that may affect the reading of voltage and if you are on Wye or Delta system that may affect the voltage reading some.

But as other guys posted alot of very good points for troubleshooting.

As they posted that is well proven methold of trobleshooting and I have done simuair way as they posted.
I don't think so. It is a small office tower. There are no monster air handling units or anything, all the equipment is relatively small.
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Old 03-14-2017, 10:54 PM   #17
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Rotation matters with some centrifugal pumps, which it sounds like this is. In the "volute" style, it will pump very little backwards. In the bi-directional style, it does not matter. Glad your stoked about the help. JRaef is right about safety, I guess I depend on the PPE between my ears too much...or not enough. Your choice!

Nice to know about the fuses as opposed to a circuit breaker. Do you have access to the blown fuses? If you cut the ends off the fuses and pull out the insides, it can tell you more or less what category of fault occurred. If the spring loaded section is open it was a sustained overload type of fault. This could be single phasing, mechanical overload, or a high impedance fault. If the short circuit section was open it was a low impedance, high amplitude fault. FYI
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Old 03-14-2017, 11:21 PM   #18
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The old fuses are long gone but I will keep in mind what you said for the future.
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Old 03-15-2017, 01:28 AM   #19
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Originally Posted by Mr.Awesome View Post
9/12/13. The impeller being part of the original problem causing something to blow has not been ruled out. The boss is thinking of tearing it apart (it is one of two pumps for the system so the other can run for now) to inspect it before they fire the system up in a couple of months. I think it is a pump for a water loop feeding their chiller unit and some coils around the building. That being said, would rotation really matter? Would it not just push water through the loop in the other direction just fine? I did consider that maybe two phases were flipped somewhere (ie: not in the order of red black blue) before the explosion and were not wired back up that way (new wires follow red black blue all the way through), but when we bumped the motor the boss claimed the rotation was correct. He could be wrong though. There is also no arrow on the shell nor any arrow stickers anywhere. Taking the pump apart will also allow for water inspection.
*Unrelated note: Wires should be orange brown yellow for the 600V system, but I have only seen this practice done a couple of times. Not sure why it isn't followed more often commercially.
Is that a Saskatchewan amendment? Because it's not in the CEC.
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Old 03-15-2017, 06:42 AM   #20
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Quote:
Originally Posted by varmit View Post
Check voltage phase to phase, not phase to ground.
Do a "fall of potential" test across the starter contacts and the overload relay.
Verify the the pump impeller is turning.
Check all fuses or breakers.
CHECK PUMP MOTOR FOR PROPER ROTATION.

You will need proper tools to go further:
Tach the motor shaft RPMs.
Megger the motor and motor leads.
Check amp draw on each phase.

Possibilities:
Motor running wrong direction.
Starter or overload failure.
Phase loss due to- bad fuse or breaker, high resistance at contactor, overload relay failure, broken wire, one winding open in motor.
Pump or bearing failure causing mechanical overload.
Don't put the cart before the horse, know that the pump is actually pumping and running in the proper direction. Then move on to further electrical troubleshooting.

Saying it is running smoothly and not pumping leads me to think of a simple problem being over looked.
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