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vacuum contactor transients

vacuum contactor transients

vacuum contactor transients

(OP)
I'm having some problems with 2 speed consequent pole constant torque motors dying from earth faults. They are rated at 12.5/25HP, from memory 15A FLC. They are on brand new shuttle cars, which are used in underground coal mining,
and nearly identical to other cars on site.
The only differrence is these cars are DC traction, as opposed to AC that we had previously and vacuum contactors instead of air break. The vacuum contactors are rated at a ridiculous 200A continuous.
So, my question is has anyone seen switching transients from lightly loaded vacuum contactors that would be causing these earth faults. The insulation is being broken down at varying points in the 6 motors that have gone down in the past 10 weeks. This is really hurting.
Anyone know where I can get information on this? I have spoken to Reliance and Toshiba without a lot of help.

Thanks for any help.

RE: vacuum contactor transients

moose2

I could not begin to answer your question, but you may want to ask this in the mining engineering section.  Mines on tracks are very common.  Maybe someone in the mine industry has seen the same problem.

If it is any help I do business with several track mines in the east US.  If you don't fine your answer please let me know and I'll get you in touch with a few people that may be able to help.

David

davidlsus@yahoo.com

RE: vacuum contactor transients

(OP)
I just had a thought this was not an isolated mining issue. And mining guys aren't that smart.... hehehe
Sorry I didn't mention that the motors drive a chain conveyor on the car and the cars are rubber tyred.

Thanks Dave, if i don't get any useful info from the gurus here then I'll contact you.

RE: vacuum contactor transients

moose2

I do business with these guy’s so I won't touch the IQ thing.   

Actually I wish that mines and their engineers were all I had to deal with.  They are willing to work with you when moving water.

Being a rubber mine I assume that you use quartz on the mine floor and have to pump water out of the mine.  What type pumps do you use?  In my case I have learned a lot about abrasion technology by dealing with mines.  High head, high flow pumps that can handle the quartz, not to mention the lock washers, rocks etc. is a problem.  

Note:
I do have "rubber" mine customers too.  Let me know if I can help.

David

RE: vacuum contactor transients

Moose2

I just remembered I have a friend in Toshiba engineering that may owe me one.  I’ll send him an email tomorrow and see what he has to say.

David

RE: vacuum contactor transients

(OP)
Yeah I have to careful what I say. Mining guys have a tough skin though. Sorry to confuse, we are a coal mine using rubber tyred shuttle cars for loading from the miner onto the belt. As for pumps, water is a huge issue at my mine. We have many dedicated pumping engineers and I do not enter their domain.

Thanks

RE: vacuum contactor transients

Moose2:

You need to send me an email I’m sure we could trade some high-tech war stories like my first experience with low top mines!!!  For that one I’m sure there were at least 20 guys lined up to chew me out.  I think some of them kept moving to the end of the line to get a second shot at me!!!

From all that I’ve seen around mines you have made a wise choice to avoid the water thing.  Getting the title of “water boy” on your business card is not a good thing!

You didn’t say if your motors were reliance and contactors were Toshiba or not.  I do have contacts in both companies that may help.

David
davidlsus@yahoo.com

RE: vacuum contactor transients

2 things come to mind.
1. Some vacuum contactors even from the factory have a bounce to their contacts. Check for this bounce by putting a cuttent through the contact and measuring the voltage drop across the contacts with an ocilloscope. Do this several times during the close and open operations. The spikeing if it exists is deadly to any type of electrical machine.
2. If I read you correctly this problem is occuring on the DC motors? Doesent make a difference. I would suggest that when you have these motors rewound that you request "drive class' information. This type of insulation is much tougher against electrical transients.
Doug

RE: vacuum contactor transients

Suggestion: It appears that additional improvements of motor drives in terms of filtering may be needed. To pursue this remedy over the car manufacturer is appropriate.

RE: vacuum contactor transients

(OP)
The motors in question are AC 2 speed consequent pole constant torque used to drive a chain conveyor, not traction.
To make matters more difficult they are in a hazardous zone which makes condition monitoring very difficult. 'H' class insulation is standard on these flame proof motors.
I will try to get a contactor on the bench to test out for bouncing.

Thanks

RE: vacuum contactor transients

Since the vacuum contacts don't allow an arc, you may be experiencing a transient overvoltage caused by a 'restrike' at the vacuum contactors. The voltage spike may be causing pinpoint insulation failures in the motor windings. Possibly a surge arrestor at the motors may reduce the spikes.

RE: vacuum contactor transients

Most Vac Contactor manufacturers recommend some type of surge suppression to reduce the effects of the "restrike" at the contactor pole faces.  In some cases on our 3.3 kV 3 phase motors the inductors (to do the surge limiting)were larger than the vac contactor.
Twenty years ago here o OZ we installed many GEC-EE vac contactors (VC3AD... series I think) which had Vanadium-doped pole faces and appeared not to need surge suppressors.  In all the years they operated we did not see a motor failure attributable to the Vac Contactor and its lack of a surge suppressor.
Cheers

RE: vacuum contactor transients

Most Vac Contactor manufacturers recommend some type of surge suppression to reduce the effects of the "restrike" at the contactor pole faces.  In some cases on our 3.3 kV 3 phase motors the inductors (to do the surge limiting)were larger than the vac contactor.
Twenty years ago here in OZ we installed many GEC-EE vac contactors (VC3AD... series I think) which had Vanadium-doped pole faces and appeared not to need surge suppressors.  In all the years they operated we did not see a motor failure attributable to the Vac Contactor and its lack of a surge suppressor.
Cheers

RE: vacuum contactor transients

(OP)
Just had a meeting with the manufacturers of the shuttle cars and they refuse to believe the contactors are to blame. There are no suppressors on the contactors so I assume they are doped pole faces.
The information I have received point to transients at up to 2.5Mhz and 10-15 times L-L Voltages. What sort of equipment do I need to measure such high freq, high power signals?

 

RE: vacuum contactor transients

Suggestion: One possible start is via oscilloscopes and probes. Some filtering appears to be inevitable.

RE: vacuum contactor transients

Moose2

When you look for transients on a power system you want a Dranetz.  

I've never looked, but I assume a Dranetz will work on DC too.  Maybe some of the EE's here can answer that.

This will get you to their web site:
http://www.dranetz-bmi.com/applications/apps.cfm

You most likely won't want to buy one, but they can be rented all over the US.  For the short term test that you need it shouldn't cost that much.  It's been a couple years, but we rented one in Atlanta to do some testing at a mine.

David  

RE: vacuum contactor transients

(OP)
yeah just checked out a farnell catalogue, HV proves are only AUS$250 for a scopemeter.

Thanks

RE: vacuum contactor transients


As implied, the existing contactors appear grossly oversized.  Has the mining-equipment producer acknowledged or justified the need for that?  Seems like the rating should be based on sound engineering rationale.  Offhand, the control means seems likely of higher cost than the motors.  
  

RE: vacuum contactor transients

Use a Reliable Power Meter(RPM) recorder for the voltage transients. With the proper options, it will show you exactly what's going on, you can even see the dv/dt of the spike. They can be rented or installed by a reputable testing company or consultant.
As far as the current rating of the vacuum contactor; I don't believe smaller ratings are available for vacuum devices, nor do I believe this has anything to do with the problem.
I'll bet if used used a standard contactor, your problem would disappear.

RE: vacuum contactor transients

Moose2

Is the contactor in a permissible area?  If it is finding surge protection may be a problem.

In your meeting did the manufacture offer any ideas about the failures?  Did they tell you any names of other companies that use this type car?

David

RE: vacuum contactor transients

Vacuum contactors do allow for arcing to occur, that is why they can be used for motor applications. The old original vacuum breakers were too good at interrupting the current and hence transient voltages and the need for surge arrestors.

In the case you have there is simply not enough energy flowing in the circuit to vaporize the contactor surface material to form the arc. Therefore the contactor is acting more like a vacuum breaker and will most produce substantial switching transients. It is not unknown for serious motor damage to occur when vacuum contators are used for too lightly loaded applications.

Surge arrestors will help but investigate using a smaller contactor.
At one point there was a white paper that a collection of Toshiba engineers had put together that gave a very complete explanation of vacuum contatcors and breakers. Call Toshiba in Houston and see if they have a copy available if you need more.

RE: vacuum contactor transients

Tmahan mentions a paper that Toshiba wrote.  I'll send an email today and see if I can get a copy.  I would like that for personal reasons.  If I find it I’ll let you know.

You talked about Toshiba and Reliance.  I assume this is a Toshiba contactor and Reliance motor.  Is that correct?

I asked about being in a permissible area.  Is the location such that you can replace with an air contactor?

If you rent a test meter of some type Dranetz, Reliable etc. you may want to check with both Toshiba and Reliance before connecting it.  I’m sure they will accept the results, but they may offer you some recommendations about the setup of the meter that will help resolve the problem for all concerned.      

David

RE: vacuum contactor transients

(OP)
So, as of night shift this is what we now have.
One shuttle car has another, new/rewound motor on it.PLUS we have changed out the vacuum contactors for air break contactors.
The other car has a new/rewound motor but it still has the vacuum contactors.
Now I just have to sit back and wait and see which fails. I can't take in any battery powered test equipment because of the hazardous zone classification, nor is there room within the flameproof enclosures for fitting of semi permanent devices.
d23-David: If you can get a copy of that paper I would greatly appreciate it.
The cars are operating in a methane hazardous environment but all control and power equipment is flameproof so contactor choice is no problem.
QUESTION: Where is a good resource site for finding elec. eng. papers on topics such as this??
I can will be pulling the cars back out of the hazardous zone within a month for testing, when the current mining cycle is complete.

Thanks

RE: vacuum contactor transients

moose2, you can certainly hook up the monitoring equipment at the contactor to catch voltage spikes; it doesn't have to be at the motor.

RE: vacuum contactor transients

(OP)
The contactors are in a flame proof enclosure that is very (read VERY) tight. No room for any monitoring equipment unless you can get a 'scope the size of a matchbox.

RE: vacuum contactor transients

All:

I'm not sure, but would building a ceramic disk capacitor bridge both phase to phase and phase to ground on the load side of the contactor offer a band-aid until better info is available?  This would be a small physical size.  Would there be enough capacitance to offer any help?

David

RE: vacuum contactor transients

(OP)
I was actually thinking about putting a R-C network across the earth reactor to help quench the transients. We are limited to 5A earth fault current, hence the reactor. That is out of the hazardous zone inside the substation so it might be worth a go.
If I get an earth fault trip and THEN the contactor opens, all hell will break loose because of the reactor to earth propagating the transients.
I'd have to check the regulations to see if caps are allowed either in the sub station or in the control compartment.

RE: vacuum contactor transients

Moose2:

I have never been a big fan of MOV's, but you may find some with a high clip volts that you can place on the load side of the contactor.  These may or may not be within your regulations.  

David

RE: vacuum contactor transients

Suggestion: Normally, 5A earth fault current is limited by a high-resistance system grounding scheme. Reactors are used more for a medium resistance/impedance system grounding scheme that permits the ground fault current to be much higher than 5A, e.g. 100A or more.

RE: vacuum contactor transients

It sounds to me that the transition is occuring while the current is still too high, i.e., the motor has not attained the proper speed value! Several questions and/or comments:

1) Can you determine, by current recording, if the current is high when the switchover occurs?

2) Vacuum Breakers introduce a phenomena called current chopping. A very steep voltage wave-front results causing insulation breakdown. Can the rewind shop tell where, on the stator winding, the breakdown is most frequent, i.e., in slot or end-turn?

3) Possible fixes: surge arrestors; MOV's (but remember they can only withstand a finite number of hits); and transition timing adjustment.

RE: vacuum contactor transients

Gentlemen:

You never, Never, EVER apply a vacuum contactor to DC switching. By inherent characteristics, a vacuum contactor only interrupts upon the onset of a zero crossing of the sine wave. If you need to know more about the interruption phenomenon I'll be happy to explain.

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