vacuum contactor transients

[moose2]

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.

 

[d23]

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

 

[moose2]

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.

 

[d23]

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

 

[d23]

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

 

[moose2]

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

 

[d23]

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

 

[DougMSOE]

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

 

[jbartos]

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.

 

[moose2]

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

 

[DanDel]

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.

 

[purplepete]

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

 

[purplepete]

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

 

[moose2]

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?

 

[jbartos]

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

 

[d23]

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

 

[moose2]

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

Thanks

 

[busbar]

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.

 

[DanDel]

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.

 

[d23]

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