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Military MEP and Aircraft Gen-Sets

MEP-005A Noise Reducing Capacitors?


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  #11  
Old 12-08-2018, 06:54:22 PM
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Default Re: MEP-005A Noise Reducing Capacitors?

Quote:
Originally Posted by krc002 View Post
The night she went down, i was running 2/3rds of a 50kw Dayton shop heater to clean out the over the summer slobber/wet stack and for heat in the shop. So mostly resistive load, aside from the small motor inside the heater which would have been inductive.
I don't see how that load would have caused a problem. I'm sticking with my first thought - you're better off without those caps.

If you ever do need to filter noise from those VFDs and the Miller Inverter, it'll take a lot more than a pair of .22 uF caps to ground anyway.
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Old 12-09-2018, 07:31:24 AM
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Default Re: MEP-005A Noise Reducing Capacitors?

I suspect that the design purpose of the capacitors was to help quench the arc across the breaker contacts when switching to open under load. (with the exception of C6 ????)
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Old 12-09-2018, 01:41:51 PM
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Default Re: MEP-005A Noise Reducing Capacitors?

^ could be. Could also be that the design committee had vague knowledge of electrical noise mitigation theory, knew that capacitors sometimes helped, and added these 4 caps to the design spec and called it a day...

I've worked around these old MEP sets enough to be leery of the designs and documentation. I don't think they saw enough test/debug/fix cycles in the design process to get all the bugs worked out.
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Old 12-09-2018, 04:49:13 PM
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Default Re: MEP-005A Noise Reducing Capacitors?

I believe those capacitors did not fail by themselves, some external event may have caused a high voltage transient and punched thru the capacitor's insulation. If these capacitors did not fail during the event what is the next insulator to fail? I think there is potentially a serious problem in your operation.
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Old 12-09-2018, 07:40:12 PM
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Default Re: MEP-005A Noise Reducing Capacitors?

These caps probably didn't just wear out. But it's not obvious it was a high voltage transient. It could have been vibration-induced, or it could have been too much high frequency current from previous runs with the VFD or inverter welder that overheated the dielectric. Hard to know for sure at this point...

If the failure coincided with opening the contactor, then I'd agree for sure...
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Old 12-10-2018, 12:09:30 AM
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Default Re: MEP-005A Noise Reducing Capacitors?

Well, its hard to say whats going on, but i figured id post an update as i was happy about how my afternoon was going until reading that there might be an unseen problem.

Upon arrival at the shop, i removed all four of those capacitors, put everything back together, and fired the unit up and closed the breaker. No problems.

Ran the 50kw heater at 2/3rds for about 2 hours to warm up the shop, and then swapped over to using the welder for about 15 minutes of actual arc on time and also had my 10kw heater running to maintain warmth.

Got hungry for supper and shut down the operation, all in all, i ran for about 4 hours today with no problems.

In the capacitors defense, the unit is currently configured for 480v. the night they went the humidity was high, the iron was cold and the air was warm. im wondering if a couple droplets of condensation laid across the gap between the terminal and the case of the capacitor. Im also not sure that two of them failed at the same time. I would consider there to be a problem if two of them went at the same time, but one of them looked it might have gone out a little while ago when observing the damage.

Neither the 75kw or 100 kw onans that i have, have any sort of capacitors on the output and run all of those loads seemingly without problem. That said i also do not open the main contactor of the shop with any device at more than idle and those units are always disconnected from the shop when i go to shut down. Same with the MEP though.
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