Phase Imbalance & Vibrating Cables - Help Reqd 2

[Tim4269]

This one has now eluded me!

3 control panels each seperatly feeding multiple blowers with 132kw Star Delta startup motors. Three problems noted to date:

The cables to the motors vibrate (100hz on 50hz supply deffinatly not mechanical vibration)

The motors have a phase imblance - 1 winding takes 10% less current but - Supply voltages are balanced at the motor terminals - motor windings have just been checked and are all ok - motor resistances are identical.

I have run out of ideas (and so have ABB drives!) any help or suggestions would be appreciated

Thanks

Tim

 

[jraef]

Funny, I see only 2 problems (unless the 3rd is that you have no answers to the other 2 ).

Possible source of vibration:
Your cables are not adequately braced. The magnetic forces created from startup current, even using YD start, are significant. Each cable will be exerting a field that will "vibrate" at 50Hz, 2 adjacent cables will vibrate at 100Hz. This happens all the time, but the force is rather weak (compared to the typical stiffness of copper wire). With inadequate bracing of the cables it may become noticable. I have seen cables on a 350HP motor starter whip around and bang against the enclosure side walls when not braced.

Possible source of imbalance:
1) Bad termination. If you have a weak connection on a terminal, it will present additional resistance in the circuit. Voltage would be the same on the line (upstream) side, but would show lower on the load (downstream)side of the termination.
2) Coiled / looped conductor. I have seen several installations where the user did not want to cut a piece of cable (just in case they needed it longer someday) and coiled it up or looped it in the box. This can create an inductor (choke) in the circuit. Might explain excess vibration as well. On one istallation the user left the entire reel of wire in the circuit ON THE REEL so that he didn't have to cut it, thinking he would return it when the project was done. Unfortuately he could not get enough starting torque out of the motor!

"Venditori de oleum-vipera non vigere excordis populi"

 

[Tim4269]

Thanks for the reply,

The 3rd problem which I managed to forget to include in the post is that the contactos feeding the blowers are burning out even though thay are oversized.

I have checked the resistance of the windings including the feed cables and found them to be the same, the voltage across the windings are also very similar.

Additioanlly there are 8 blowers all doing the same on the same phase (red). The windinging have beentested and found to be ok with no earth leakage, however when tested there is an 8 amp earth leakage from the motor casing to the terminal box, but not back to the control panel.

I am at the point of considering stripping down one of the motors - although this will cause mass disruption!

Ti,

 

[electricpete]

Contactors burning out? Does that mean the main contacts or the control coils?

With contactor problem and current unbalance I would have suspected contactor as source of volt unbalance... but you said you verified voltages balanced at the motors.

If voltage is truly balanced at the motors, that would seem to rule out everything except motor problem.

Some more tests you can try:
1 - Do pdma tests including indutive imbalance. Might possibly show something not shown in resistance tests.
2 - Do a surge test. Sometimes miswiring in the motor will show up with different pattern among phases. Be careful what voltage you apply and beware this is a potentially destructive test.
3 - Swap power leads and see if the current unbalance problem stays on the same motor lead. Based on your comment of balanced voltages it should stay at same motor lead, but I think it may be worth trying just to make absolutely sure you have ruled out power system as source of the problem.


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[electricpete]

by the way, you probably reached the same conclusion, but I suspect the cable vibration is a result of twice line frequency vibration of the motor which is related to the current imbalance and will go away when the source of the current imbalance is corrected.

What does ABB drives have to do with it? Star delta start so I assume it's not a vfd.

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[ccdubs]

I have been experiencing a problem very similar to the phase imbalance one described above.

I put the problem down to system imbalances.

My situation differs as I have a synchronous generator. When not online system line voltages may be as much as 3-5% unbalanced (largely due to all single phase loads being connected to Red).

When online the generator maintains a voltage balance at the generator bus but I have currents that are imbalanced by 10%.

Can this problem (as with the original post) be caused by the system/grid? e.g., Tx impedances, loads connected down the line, etc.

I definitely know that imbalanced systems are of concern internationally due to decreased life expectancy of 3 phase machines.

 

[Tim4269]

Further progress

The Motors have been tested upto 2kV and found to be balanced, the voltage at the terminals with the motors running is balanced so assuming that we have not managed to cheat the laws of electricity its either a problem with the motors internal windings that manifests itself with the motor running (i.e. heat) or an obscure power supply - Swapping the phases around should demonstrate which.

The contactors burning out is actually a problem with either the contacts themselves or the connections getting hot (to the point that it melts the insulation) all of the components are within their ratings and when the covers are removed from the contactors the actual contacts are not too bad.

As a reply to ccdubs if the system is drawing an unbalanced load then the generator will provide an unbalanced load, as to damage to the generator I would not like to venture a guess as to how much it would shorten its life.

 

[light1]

Have you checked the power supply quality, namely, voltage and current waveforms? If there is a second harmonic content, it will support 100Hz vibration on 50Hz power supply. It may also be a combination of power quality problem including imbalance and harmonics.

 

[ccdubs]

Tim4269:

I have checked with the generator supplier about the unbalanced currents and they assure me it is nothing to worry about in terms of generator lifetime.

Please let us know if swapping the phases proves if it is a system phenomenon. I am interested to know how common this is.

I swapped the phases of the generator and results showed that the same phase had the low current but for equivalent power the magnitude of the currents was different. In my case this could be due to single phase sensing for power factor correction.

 

[jraef]

I like the tip light1 made. Especially when you mentioned the burning contactor terminals. This is a known phenom in the contactor world when high harmonic currents are present. It can be exacerbated by using high strand-count cable as well, i.e. welding cable.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Tim4269]

I have not managed to get the phases swoped over yet due to other constraints, I have however previously checked the harmonics

3.7% peak 5th with long durations of over 3% at weekends
0.8% peak 7th

Which is not really enough to justify what is happening, however I do have suspicions about the network and some switching they did recently that could have given us a period of high harmonics.

High frequency spectrum has been checked upto 500mhz - the local radio stations and police band as present but nothing of any great magnitude (all below 0.1V rms)agin though this does not mean that the distortion was not present in the past

Breaking news - the power factor correction unit on the panel section has caught fire - I'm of the the doctors for a sick note!!!

Tim

 

[rbulsara]

I am in business of advising clients to solve the problems and not as much to anyalyze to death...

so here are my two cents:

explanations given for cable vibrations are valid.

The contact burning is normally related to high currents and mechanical issues with large contactors.

In fact, based on my past life (early in the career), I figured that star-delta starter for LV (less than 600V)motors, 75 HP and larger, are always a head ache.

In these time and age, I would receommend you try the following before stripping down any motor.

1. If your service and distribution can sustain it, use DOL starter. This may exacerbate cable vibration during the start though, but will reduce contactor burn outs.

OR (and recommended)
2. Replace the Y-Delta starter solid state reduced voltage starter, eliminating all three contactors. You cant burn what you don't have. Solid state RV starters are very affordable now a days.

 

[ScottyUK]

Hi jraef,

It can be exacerbated by using high strand-count cable as well, i.e. welding cable.

I've never come across this before. What is the cause of the problem with multi-strand cable? Normally we put a lug on multi-strand before terminating to the contactor. Is it something that afflicts direct termination methods only?




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

 

[jraef]

You got me. Common sense tells me that a higher strand count would be better (i.e. more surface area), but that has not been my experience. When I used to work for contactor manufacturers it was a constant headache. The terminal lugs would get hot enough to melt, yet the contacts would be fine. My unfounded guess-geneering puts it at the fact that the lugs are not designed to properly compress the thinner strands and higher count adequately, so normal thermal expansion / contraction caused it to work loose creating excess resistance. Nobody ever paid me to look into it any further, it was cheaper to give the customers new contactors. Then I got out of that end of the biz. We use flex cable where I work now, but we use compression lugs and a hydraulic cripmer, no problems.

"Venditori de oleum-vipera non vigere excordis populi"