causes of intermittent motor trips associated with the MOTOR 5

[electricpete]

OK, here is the situation.

2-pole motor ~ 50hp fed from ungrounded 480 vac system.

Motor tripped on instantaneous overcurrent a few months ago.

As part of investigation - Meggered/bridged motor (sat). Did a start while monitoring current on O-scope (sat). Checked molded case breaker instantaneous trip (sat - within expected bounds based on nameplate LRC and up to factor of 2 increase in 1st peak due to dc component). Breaker was replaced at that time.

Motor has not shown any unusual vibration or other characteristics.

Now this weekend - Motor tripped again on start (instantaneous). Motor meggers and bridges sat again. Voltage is normal.

They called me up at home and want to know: "Is there anything about the motor that can make it susceptible to causing a trip?"

My response is: No. The highest instantaneous peak varies in a random manner based on phase at closing (as we all know). None of the previous tests have shown anything abnormal. I have never heard of a 460vac motor fault that would lead to an instantaneous trip, and then motor subsequently operates fine. I think you need to check your setpoints and make sure they are high enough.

Their response is: the setpoints have been fully checked and are well above that theoretical worst case. We are on the verge of changing out the motor instead of re-revaluating the setpoint. We want to know what is it about the motor that can cause variation in starting current beyond what is predicted by analysis using 2* factor and nameplate data.

Can you help with this question? Any credible faults on the motor that cause an assumed (by others) abnormally high instantaneous current during start?

(note that is the narrow focus of my question - I am not interested in talking about the setpoint).

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[electricpete]

Breaker is Culter-Hammer HMCP 100R3C

Currently set on G.

Do you think there is justification to bump it to H?

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[electricpete]

"Do you think there is justification to bump it to H?"
(that is assuming of course that there is not a problem with coordination with upstream breaker)

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[cdevine]

Your last couple of posts electricpete make it clear that you are not dealing with connected load issues. In fact if you could decouple the pump you may end up with the same trips.

Circuit breaker sensitivities extend to harmonics and higher ambients also. I suspect that the higher ambients only impact on the thermal trip elenment. Electronic trip elements that are true RMS sensing overcome crest factor issues.

If you are looking to increase the magnetic trip setting a check of downstream cabling for short circuit performance is the only other thing I would suggest. You may weld a contactor as well. On balance its worth a shot.

Chris Devine

http://www.icande.com.au,

http://www.redbusbar.com

 

[electricpete]

During two monitored starts of this motor, we saw something resembling contact bounce (~1/4 cycle) while during one monitored start of sister motor, we did not see that (see last slide of my post 2 Sep 08 9:18).

Do you think this can possibly cause an increase in instantaneous inrush?

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[itsmoked]

Oh heck yes!

That can result in an out of synch reconnection of a running motor. Several in rapid succession. Same thing that causes Wye/Delta starter systems to throw occasional head scratching instantaneous trips. Only in your case you'd get several shots at a fully out of synch reconnection on each start.

I'd either;
1) With the motor locked out, mess with the start circuit and try to figure out why that's happening.
or
2) Swap contactors and watch the problem switch motors.
or
3) Replace the contactor.

One issue is the switching circuit that is running the contactor coil power. If it has issues any contactor would have the problem.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Without seeing the contactor it is hard to say, but my first reaction would be to inspect for loose, weak, broken, or burned springs in the contactor.
I am referring to the springs that maintain contact pressure when the contactor is closed.
I don't fully agree with Keith but I think that he is on the right path.
I see two phases establishing a voltage division and phase angle for two windings (assuming a wye connection) and then the third phase abruptly changing both the phase angles and voltage and current division.
I like Keith's idea of rolling starters or motors and see which the problem follows. I am not convinced the problem is in the motor. I still suspect a possible contactor problem. The late make on one phase may be a problem or not. But I suggest looking for mechanical and/or tracking problems in the starter.
Does one coil operate all three poles of the starter or is each pole powered independently?
I agree with Keith's suggestions.

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

 

[davidbeach]

I had the same thoughts as Keith, but then had second thoughts about how long the contactor is open as it chatters. If the motor remains within about 30[°] of the system while the contactor is open you won't get a lot of inrush. The fact the motor is starting does increase the current but not above that of the initial inrush. I can see chattering causing all sorts of problems, and it needs to be fixed, but am having a hard time envisioning an instantaneous trip out of a chattering contact.

 

[itsmoked]

What about the inrush? Not the starting current. The chatter is going to happen at the initial closure. Before the motor is turning more than a few RPM. Can the motor at this point be considered as transformer? If so then it would be the same as a chattering contactor to a transformer. Could he be getting those 15X transformer like inrushes?

davidbeach; I recognize the low angle reducing things but at a few RPM isn't the angle going to be changing really fast? 30 degrees is represented by only 1.3ms. Bouncing contacts are going to be bouncing at about 50ms? Meaning a more random situation? Granted their won't be much EMF available to send back into the system.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I am wondering about one contact coming in 1/4 cycle late. The first two phases establish an inrush current (yes, with a wye connected motor, less than normal) and then 1/4 cycle later the third contact closes. I imagine another inrush and a forced phase shift of the established current(s).
Can this cause above normal currents? I don't know but I wonder. Is there anything here that may push one or more windings into saturation?

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

 

[davidbeach]

480V system, instantaneous is probably magnetic and therefore much faster than an instantaneous from a relay. With that assumption, my thought is that we have to find something that can increase the peak current magnitude. Things that merely prolong or proliferate the peak current magnitude but don't increase it are unlikely to cause and instantaneous trip. That's the problem I am having with chattering contacts.

At slips close to 1, the condition at the initial start, there is no rotationally induced voltage in the motor field and closing angle doesn't matter, you just get more of the starting level current, but that isn't enough to trip the instantaneous. At slips closer to 0 you begin to have to be concerned with the internal voltage and how it synchronizes with the line voltage but now it takes a while for the phase angles to get too far out of sync because the rotor is moving at near synchronous speed.