MOTOR PROBLEMS: HELP!!!!!!!! 3

[vmac100]

Dear all,
I have a 3phase, 400V, 75kW FLYGT pump used in wtare storm water evacuation in our office area. The pump starts on star and runs on delta.We have been having issues with the contactor controlling the pump. The contactor is manufactured by TELEMECHANIQUE and has the following name plate parameters: 3phase, 480 to 600V, 37kW/60hP, Aux Cont A600, P600, Continuous current 110A.

This contactor gets melted/welded on the blue phase after one or two starts and we are wondering what the cause is.

Please all ideas will be appreciated.

Regards,
Omonigho

 

[Skogsgurra]

The obvious answer to that is that a 37 kW contactor shouldn't be used with a 75 kW pump.

Gunnar Englund

http://www.gke.org

 

[vmac100]

Thanks Gunnar.
However, we are using contactors of the same specifications on a pump with the same specifications and it has not gotten burnt or welded. Any take on that?

 

[waross]

How do the voltage, current and megger readings look?
yours

 

[UKpete]

vmac100, have you got all your numbers correct? 60hp is actually 45kW. Also, if you have a star-delta starter, you should have two contactors:

http://www.lmphotonics.com/star_delta.htm

One of them can have a lower rating than the other I think.

 

[UKpete]

Correction, three contactors. Do you have unbalanced currents?

 

[waross]

UKpete
On the contactors that I see there is a difference in rating methods between IEC and NEMA. I assumed that the KW rating was IEC and the HP rating was NEMA and I didn't expect them to be equivalent.
yours

 

[machmech]

Just a guess, but check for a loose connection.

 

[Marke]

Hello vmac100

1. There should be three contactors in the star/delta starter. Two of the contactors should be rated at a minimum of root three of the motor rating and the third, (star contactor) should be rated at half of the motor rating.

2. The timer used to control the contactors must have a suitable dwell delay between the star contactor opening and the delta contactor closing. If these operations are too fast, you could effectively close into a short circuit. I would expect this to primarly cause damage to the opening star contactor.

3. It is very easy to connect the star/delta starter incorrectly. The motor will not start if it is not wire correctly. Does the motor start when operated without damaging the contactor?

4. If the motor is over corrected in star, high voltages can be generated which can cause problems to the motor and switchgear. Is there power factor correction fitted to the motor/starter? If so, how is it configured and controlled.?

5. If you are reusing the contacts rather than replacing them, it is possible that the contacts are no longer capable of the duty. swap the contacts onto another phase and see if the problem moves, or swap the delta contactor and main contactor and see if the problem moves.

6. Most timing problems would tend to result in a random phase where the failure would occur. The description is that of always on one phase, so I would suggest that there may be an insulation/termination problem on that phase, either within the motor, or the wiring.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[waross]

The problem may be in the motor or the contactor. If the problem is in the motor the current and megger checks should indicate it.
You may have a control problem in the starter. You should have both mechanical and electrical interlocks in the starter. Both can fail. A failure in a control circuit may cause problems elsewhere in the controls.
Mechanical interlocks. When one contactor is sealed in, the other contactor is prevented from operating. However I have seen interlocked contactors that would allow both contactors to start moving if they were energized simultaneously. On some makes and sizes, the inertia will carry the contacts to the point of contact after the mechanical interlock has stopped the primary movement. This can result in a line to line short.
This can be caused by a failed or failing auxilliary lockout contact.
Without mechanical interlocks, the chances of a failing component causing simultaneous operation of two contactors is even more likely.

 

[Skogsgurra]

1. Has vmac verified that there is a star/delta starter?

2. I still think that it is dumb to run a 75 kW/400 V motor with a contactor rated 37 kW at 480 V. To me, it sounds like the contactor was abused about 2.4 times.

3. Have no idea why such a small contactor works on an identical pump. Are you sure it is identical - and also fully loaded?

Gunnar Englund

http://www.gke.org

 

[jraef]

Gunnar,
He said up front that it was started Star-Delta.

vmac110,
The contactors can be rated for root 3, or 56%, of the motor power as Marke pointed out, so 75kw x .56 = 42kW minimum and those contactors are rated for 37kW. The answer is that someone undersized those contactors. It doesn't seem like much, but in general Star-Delta starting can be really hard on contactors because of switching transients, so using even marginally sized ones is a bad idea, and using undersized is going to be guaranteed trouble at some point. Even the ones you say are identical yet functioning are below marginal and will not last long.

A common mistake among do-it-yourself Y-Delta starter builders is to read that the contactors can be reduced to 56% of the motor rating, and screw up in thinking "reduced BY" 56% of the motor, so they size the contactors at 44% of the motor, which in this case would be 75kW x .44 = 33kW, which makes it look as if that is what they did here.

That doesn't rule out the possibility of other problems, but no matter what, those contactors are too small.

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

 

[Skogsgurra]

Hmm.. I see that now. :~/

Gunnar Englund

http://www.gke.org

 

[aolalde]

vmac100
Certainly the problem seems to be undersized contactors.
Handling the contactor size by the kW or HP could be tricky. In reality is the current in amperes that mostly stresses the contacts.
37kW at 480 Volts (the rating you stated) means approx 60 amperes rated contactors.
To make this application worst, your voltage is only 400 Volts. That will result at full load approx. 145.5 amperes at 400 Volts and 75 kW.
You need contactors rated at least for 145.5/1.73 = 84 amperes (58% of FLC).


.

 

[Marke]

Just to through my oar back in, I agree that the contactor appears to be under sized, but:

1. Why is it the same pole failing each time?

2. Why is it failing after 2 starts? I would expect it to
last for many more than that. Correctly sized, it is good for more than 100,000 operations. If the current is double what it should be, then the power dissipated in the contact is four times, but with a pump, the start time is very short, so four times the power dissipated for one quarter of the time would lead to the same temperature rise.
I am not convinced that the problem is purely due to undersized contactors.

Thoughts?
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

I agree Mark, that's why I said it doesn't rule out other issues. It really is almost an aside, but likely a contributing factor and nees to be addressed no matter what else is found.

Most likely, if the OEM who built up the starters was capable of making that sizing error, they probably were capable of making an interconnection error as well. I would suspect something along the lines of what waross posted. I have seen quite a few panel builders who consider it adequate to use electrical interlocks alone, no mechanical linkage, and that is another source of potential trouble.

Another common problem is in a misalignment of the mechanical linkage, causing a binding of the contactor armature and sticking of one set of contacts. Older ABB contactors were notorious for that if you used their field retrofit mechanical interlock kit because the drilling tolerances for the mounting plate were critical. Depending on the age of the Telemecanique contactor design in question, that may be a problem here as well. And even the snap-on types being used now can bind if not installed properly.

aolalde, those contactors are rated for 80A inductive, not 60, but still too small because as you pointed out they need to be 84A minimum.

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

 

[waross]

Look for a broken return spring in the star contactor.
It really sounds like the delta contactor is coming in before the star contactor is clearing. The contactor may be undersized, but marginally. The points that I note are;
1> Fails after one or two starts.
Similar starters don't fail.
2> Always fails on blue phase.
Points to trouble on blue phase.

In my humble experience, welded contacts usually indicate a short circuit. Not always but usually.
If a motor is only 2 or 3 times too large for a starter, there will be burning and pitting of the contacts, and short life, but they will not usually weld. Most of the damage is done not on closing but on opening when the contactor finds that it doesn't have sufficient clearances to interupt the current. I have seen badly welded contacts and they were almost always the result of a fault current rather than an overload. I am more afraid of stopping a loaded motor with an undersized contactor than starting one.

One other possible cause the first time is a broken contact pressure spring in one pole of the starter. After the first or second time the broken spring is usually noticed and replaced with the contacts.
A question, Does the contactor fail quietly or go out with a flash and a bang?
Overloaded contacts may go quietly, or they may hiss and sizzle. There may be some arcing noise. A line to line fault will create a flash and bang noticible to anyone within 100 feet.
yours