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.

 

[tmahan]

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.

 

[d23]

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

 

[moose2]

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

 

[DanDel]

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.

 

[moose2]

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.

 

[d23]

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

 

[moose2]

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.

 

[d23]

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

 

[jbartos]

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.

 

[Shortstub]

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.

 

[swgrmfg]

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.