Motor current rises 1

[rovineye]

I have a two speed, two winding fan motor, 460V, 100/25 HP. It is run as high and low speed, plus in reverse at low speed. It runs fine in low speed both directions, and can shift to high speed without issue.

When running at HS, the current increases over time, but not above name plate amperage. It normally runs at 80 amps but will increase up to about 110 amps or so. The current is also unbalanced on one phase by about 10%. Voltage is very balanced.

The motor will not start in HS from standstill , but it should. Not sure what happens when started at HS, I couldn’t find out if it tripped on OL or what. The controller contractors smoked before I was involved.

The motor was replaced before I was involved and before the contactors smoked. Cables, motor and circuits meggered OK.

They are rebuilding controller now, and I am gathering my thoughts on what to check. Thinking that maybe somehow the LS and HS windings were energized at the same time, not sure what would happen then. Also wondering if there is failure mode with electronic overloads that could act like a CT with open secondary?

This is a new installation.

Any ideas?

 

[edison123]

There are certain ratio of stator slots to rotor slots for any speed that would not allow the motor to start. In your case, I suspect the HS winding fell into that category.

I once stupidly rewound a 8/4 pole for a 4 pole star/delta connection and found, at a cost, that I forgot to check that ratio and that motor wouldn't start in 4 pole connection.

 

[edison123]

My bad - It is a two winding motor. So, ignore my first post, which was for siingle winding, two speed operation.

As for the current unbalance, did you check the voltages right at the motor terminals. May be there is a loose connection there. If the voltages at the motor terminals are balanced, then do a surge test on the motor to check for turn shorts.

 

[rovineye]

I have not checked voltage at the motor. I would expect the motor replacement would have corrected any loose connection there. It is possible they received two bad motors as well, but unlikely.

I have an identical motor/controller near by and I think I will swap motor feeders and see if the problem follows the motor. At that time I shoot some thermal images in the controller and look for hot spots.

 

[waross]

If the overload settings were made under the assumption that the motor would always start first in low speed and then switch to high speed, the overloads may have tripped and interrupted starting current. I used to run into an IEC design of contactor that had a habit of blowing arcs out the front of the deion plates where the arcs would join to produce a three phase fault. This was normal switching, not faulted motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Another thought since you didn't state how long the current windup takes.

As the flow gets established it could take a bit of time for it to really smooth out at which time the mass flow will reach it's highest point - the motor the highest load.

If the fan is slightly wrong high speed could be too much load. All they need is the back pressure to be too low.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Bill,

Can you remember which manufacturer's contactors performed this firwework show? I've never seen anything like that occur in a correctly assembled contactor. If there are nasty surprises lurking then it's better to be aware of them.

The closest I've come was an interpole flashover on a big DC contactor caused when someone decided that the arc chutes were for decoration and left them off. That made a bit of a mess.


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If we learn from our mistakes I'm getting a great education!

 

[rovineye]

I asked more specific questions of the ships crew, and had some answers this AM.

I had been told all 3 contactors smoked. That didn't make much sense to me as the issue seemed to be only with HS. Well, first they blew the HS contactor, which they then swapped with the LS, and smoked it, and then swapped with the reverse, and smoked it. SO yes, they blew all 3 contatctors, but really blew the HS 3 times!

I thought maybe they were just smoking the contactor coil because it wasn't pulling in or something, but no, it was the contact assembly (ScottyUK, Cuttler Hamer, and I agree, highly unusual).

Finally found out that when trying to start the fan in HS from standstill it was the overloads that tripped.

The rising current I believe is a red herring. As it turns out they were trying numerous things at the same time to try to get this fan running. Machinists were adjusting blade pitch while electricians were replacing contactors. Arrrgh.

waross, yes the circuit and components are designed to start at HS from stand still. That is the most common mode for the automation.

In the end, when starting the fan for us, an unusual humm was heard. Both in the controller and faintly from the nearby switchboard. We had already looked at the rosebud contacts where the CB attaches to the switchboard and they were fine. We swapped out the breaker and the humm went away and the current at HS evened out (was balanced). The fan was able to start at HS from stand still.

OK, other than my usual frustration of not sufficient or incorrect fault symptoms, there are still a few things that bother me.

Why did the voltage appear balanced if there was a connection problem at the breaker? I am guessing they were not checking the voltage under load.

Why did the contactor smoke? Maybe they troubleshot this thing literally to death. Having the contactor open many times in short duration during starting current could have done it.

Ahhh, the joys of long distance troubleshooting.

 

[waross]

Hi ScottyUK;
It was a long time ago, and rather than trust my memory It would be safer if I described them.
The contactors were roughly cube shaped. The defining feature and the cause of the problems was the deion plates. There were six small windows, three across the top and three across the bottom. The deion plates were stacked in these windows. A heavy arc would blast right out the front of the windows. Sometimes the plasma would spread out and join up. The phase to phase arc would take out the fuses. Replace the fuses and go back to work. Well, actually, call the service company (us) and wait for a serviceman to arrive.
The application was an overhead bridge crane. The motors would by about 10 HP to 25 HP. Control was by push buttons on a pendant. The plant was near a river and the humidity was a little higher than the surrounding area.
We eventually replaced the contactors with Allen Bradley contactors and never had another flashover.
Years later I saw a similar series of failures with similar contactors. That one is also the reason that I sometimes get a little testy when someone appears about to ignore established standards.
It was during the first energy crisis. There were government subsidies and tax breaks for energy saving equipment. The application was a series of fans. 60 HP to 400 HP. They used wound rotor motors and rectified and inverted the rotor current back to the line, rather than using dampers for flow control. It was not my area, but we were hearing about spurious fan controller failures around the plant.
When the electricians were connecting the controller for the 400 HP drive, the foreman queried the use of a 250 HP contactor to control a 400 HP motor. The engineer in charge stated the following.
1> I have a PhD in engineering and I know what I am doing.
2> We have performed extensive tests on these starters and have determined that they can handle 400 HP without overheating.
3> You should know your place and say in it.
Word of that exchange quickly spread through the plant.
The drive was up and running and the stop button was pushed.
It seems that the jerk (his actual nickname was much ess flattering) had not thought to test the contactor's ability to interrupt full load current.
The plasma joined upon the load side of the contactor and crowbarred the motor. It instantly became a short circuited induction generator. The motor de-cellerated,- quickly! The fan that it was driving did not de-cellerate as fast as the motor. We found pieces of the coupling all over the room.
Total damage;
Actually the contactor survived with smoke damage.
The electronic control board was toast.
The motor shaft was bent.
The fan shaft was bent.
Beware of contactors with two rows of three deion windows.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Bill,

No kidding? I can think of a few bigger European types which fit that description - Telemecanique's LC1 series for a start - but they have always performed well even when breaking LRC on a stalled motor. There, read it and remember it: I said something positive about a Schneider product for once!

Events like Bill describes is yet another reason to avoid open door testing.


Glad you got sorted Rovineye. Feedback regarding the cause of a problem is always appreciated - it might just be one of our plants where it happens next.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Although I don't recall the brand of contactor, I am aware of Telemacanique. The contactors in both instances WERE NOT TELEMACANIQUE.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Bill: "I have a PhD in engineering and I know what I am doing"

Someone we used to know here? Sounds very familiar...

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Hi Gunnar.
It was about 30+ years ago. I have never seen the man since and don't remember his name. Honestly, no-one on this forum has ever reminded me of him. But I now owe him a small dept of gratitude for a lesson learned by observation.
During the recovery after Hurricane Mitch, we were under a lot of pressure. One day there was a special emergency and the pressure increased. One of my helpers tried to tell me something but with a language barrier and "Get the street lights back on before dark"! I didn't pay much attention. Thankfully the helper went to the other helper who was bilingual. Thankfully again, I put aside my superior knowledge and took the time to listen. It saved me a lot of grief. He was trying to tell me about some serious hidden damage to one of our cables. The special emergency may make a story for another thread some day.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

That scary story deserves a LPS.

 

[jraef]

Great story, and I think I know who Gunnar is thinking of. I believe he left this forum prior to Bill discovering us.

Back to original thread...

Why did the voltage appear balanced if there was a connection problem at the breaker? I am guessing they were not checking the voltage under load.

That would be my first thought as well. A bad connection can sometimes be deceiving if not tested correctly. Under load, the resistance increases at that bad connection and causes the voltage drop. But without the load, no resistance, no drop, no problem! Very common issue in the world of bad breakers, but bad breakers themselves are not very common to most people so it's not usually in one's repertoire of test procedures. So my educated guess at the scenario would be that the load at low speed was so low that it did not cause the problem (or it wasn't noticeable). When switched from low to high, the added current began the process and caused the imbalance. One of the effects of a severe imbalance is the creation of negative sequence currents and torque, which means you start to pull more amps for a given amount of torque. Faster fan, higher torque requirements; less shaft torque available from the motor, more current draw.

But when you tried to START in high speed, the starting current had a severely more dramatic effect on the voltage drop across the bad breaker connection. That would translate to a marked increase in all of the above issues.

Why did the contactor smoke? Maybe they troubleshot this thing literally to death. Having the contactor open many times in short duration during starting current could have done it.

That is a possibility and I have seen things go down that way for sure; hence the term "smoke test". But another maybe more likely cause is that the voltage drop caused by the aforementioned bad connection was on one of the phases feeding the control power source, so the contactor started chattering as soon as you when into high speed. If left to itself for even a few seconds, the contacts can melt or even vaporize. Sometimes the chatter is so fast that nobody hears it because of all the other ambient noise going on.

Ahhh, the joys of long distance troubleshooting.

Yep, welcome to OUR world!


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[rovineye]

jraef,
I would hope they would have noticed relay chattering. They had been looking inside the controller and taking clamp-on readings inside. It is a dead quiet space when the generator isn't running (it wasn't). They eventually heard the breaker humm in the switchboard about 20 feet away.

We commission a few ships a year and each has a few hundred 3 phase breakers. I would say on average we find 3 to 5 molded case breakers with problems. Almost all are discovered during thermal imaging. Most of those would not have led to imminent failure. Is a 1-2 % rate too high? I don't know.

We had not reached the time for thermal imaging on this ship, so I believe we would have caught it eventually. The breaker issues numbers I cited do not include issues with breaker UV trips, which have a higher failure rate for us.

 

[mshahwan]

if you have a current inbalance, disconnect the motor, the test the winding resistance ,A-B,B-C,C-A, if one of these tests is different then your motor winding insulation is degraded and you need a new motor.
hope this helps.

also if your motor starts fine , then the current increases over time, your motor is undersized, or not rated for the duty cycle, so your ending up with over heating problem which usually does not affect all phases equally, hence the inbalanced current on one phase.