is there a possible ct fault that gives false high current 4

[electricpete]

Out of curiosity...

Is there any credible fault in a CT or associated wiring that could make the secondary current seen by one of the loads increase above actual?


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[itsmoked]

You make a good arguement skogs. Of course you are entirely violating the human's need to know.. NOW!

 

[electricpete]

Well, the motor was put into service. There was an attempt to recreate the problem at exact same flow conditions and changes at my request. I don't know the exact numbers. I gave some paramters (for one thing current lower at the lower flow than higher flow) and they were met... in other words no abnormal unbalance and everything close to what would be expected.

My theory now seems less relevant. The extra info is that this pump has very heavy axial vibration 0.3 ips broadband that increases during low-flow operation.

We have another pump with similar setup (horizontal sleeve bearing motor, Thomas Shim pack coupling) which has axial movement of the motor shaft visible during low flow operation 1/8" back and forth. The explanation is apparently that small movements of the pump under influence of low-flow turbulence cause broadband excitation which excites the resonant frequency of the system... motor attached to pump through Thomas Shim pack coupling which acts like a spring. Resonant frequency very low about 1 hz ... you can seee it move.

I have seen that phenomenon with my own eyes on this other motor. I have never looked for it on the motor that tripped (only noticed this vib pattern after the trip as well) but it seems reasonable similar setups could have similar motion.

Now what does slow shaft axial vibration do to the current. Here is a thread with some info indicating it can be a substantial change. thread237-128033

But one thing is the increase would be reactive current... would have to get higher than the real current to make a difference. Would have to be a tremendous increase. Probably not possible. Oh well, back to reality.

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[DanDel]

'pete, if both the alarm relay and the trip relay are connected together in series with the same CTs, then any current, whether constant or intermittent, would be seen by both relays (unless there was a fault bypassing one relay circuit). My original suggestion of a loose connection or intermittent shorted turns would not explain why one relay picked up and the other didn't, unless the intermittent fault is in the relay secondary between the two relays.
I certainly think that manually restricting flow in your system can cause some kind of current increase, but both relays would see the same thing, especially on an O/C trip.
I think your problem is what you suggested first, a defective relay. I've had some unexplained nuisance trip problems with those ITE Circuit Shields.
Ever hear of Occam's Razor?

 

[BJC]

Have you got the right pump curve? Has the pump been reworked? It is not uncommon for spare impellers to be cast to a standard size and machined for particular pumps. I cna easily see it happening to a company that gets bought out or merged etc. A quarter inch increase in impeller diameter can be significant on the curve.