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Strange way for a MWO to fail

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Nik

Sat Sept 02 2006, 02:21AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I have a 1200w that I picked up off the side of the road, the power cored was cut off so I spliced in a new one (safely) and pluged it in. There was a single beep (telling me that it was getting power). When I put in a time and pressed start I heard the MOT relay click but the MOT didn't hum, the fan didn't start and the light didn't some on. There were no other faults (no poped breakers or strange noises).

So, I opened the case and stared tracing the 120v wireing to find the problem. What I found was really damn confusing. One of the wires connecting to the MOT/fan/light relay had the insulation melt onto 2 other wires. The wire going to the relay that melted was the line 120v, the 2 wires it melted to BOTH went to ground. Also the wire had burned itself off the connection fo the relay.

I stripped the offending (120v) wire and soldered it onto the relay terminal in hopes that that would fix the problem. It did _but_ when running one of the components makes a buzzing noise, much like a very small arc from a mot. I (wearing goggles for fear of something arcing and me getting the flash of 120 to ground) looked over the inside of the oven but could not see the source of the noise.

Now what confuses the hell out of me is how the line was in contact with 2 ground wires and not trip my breaker. And how can the line melt to 2 ground wires?

I have photos of the wires as I found them.

...

Sat Sept 02 2006, 03:01AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

My best guess is that somehow the mo was not grounded (either by improper house wiring or a broken ground wire in the mo) and somehow the mot managed to float it's primary at a few kw relative to the ground so you got some arking, then it eventually managed to get enough contact with one of them to explode the conniption to the really.

As to why it didn't trip the breaker, either the mo wasn't grounded properly (you did splice on a cord...), or (if the wire was on the switched side of the relay) just wasn't in contact with mains, or it is possible that the ark created when they first touched was violent enough to spread out the wires so that were not in contact anymore...

Nik

Sat Sept 02 2006, 03:44AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

When I found the fault I had to pull the 3 wires apart because the insulation had fused on all 3 wires although that doesn't mean the wires were touching (I guess). I will check all the ground connections with my meter tomorro to see if there is an issue some where.

Hazmatt_(The Underdog)

Sat Sept 02 2006, 07:10AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I was watching a program on the TWA 800 crash and the outside investigator concluded that the wiring was the culprit. The design had HV leads running along with the LV fuel tank leads in the bundle. Add aging/cracking insulation to that mix and you have an accident.

Looks like the same case here in your MO. From the insulating plastic piece going to the relay, I would believe that the relay leads are at pretty high voltage, maybe more so then the insulation class of the wire >.<.

Nik

Sat Sept 02 2006, 07:57AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

The relay is switching 120v to the mot and such but that is still something to think about. The 1 wires that were stuck to the 120v line were low voltage lines one running to ground the other to a switch then to ground. If the primary did some how get raised to HV then the insulation would not hold up at all (it was 300v or 600v I can't remember). If there was a small hv spark from the 120v to the gound wires that would probably explain the charring on the connector and insulation.

Update: I used my meter to check that all the ground conenctions worked (they did) and found no other problems. I put some water in the over and turned it on and the water boiled. Aside from that strange buzzing it seems that there is nothing wrong.

Steve Conner

Mon Sept 04 2006, 02:48PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It looks like the blade terminal on the relay was making a loose contact and getting very hot. The heat probably travelled along the wire connected to the terminal and melted it into the other two wires. The wires must not have been making any electrical contact with each other inside that burnt mess.

That's quite a likely explanation, since pulling the terminal off and replacing it with a solder joint would fix the contact and that would explain why it started working again. Also, the terminal looks pretty badly burnt as you'd expect if it was heating and arcing.

I've seen some MOs that make a very arc-like buzzing sound but work fine. I get the feeling it might be a sharp burr of metal or a piece of crud inside the waveguide, causing a tiny bit of corona or something. The E-field is pretty high in there, and I think that's why the magnetron antenna is rounded, to stop it breaking out.

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