Buzz From Gray Box

[Jim Chambers]

I am the new owner of a 2001 Patriot. We just completed our first trip (rally to Sacramento, great fun). We have interior storage for the coach with 50 amp service. Upon arrival home we plugged in the coach. As soon as the coach accepted the AC power we heard a fairly loud buzz from the cargo bay. It alarmed us because we don't recall hearing the noise prior to our trip when the coach was plugged in. The noise is coming from one of two gray metal boxes. They appear identical and one is labeled as a transformer. The noisy one is immediately to the right of the transformer and is unlabeled. Do we have a problem?

[Edward Buker]

I do not think you have a problem. That is the transfer relays and circuit that changes over the power source to the generator a short time after it starts. It has GE relays in it with AC coils that hum loudly and I find it very annoying even up in the coach. A photo of the unit in parts is attached. I found a furniture moving slide that had soft rubber in it, installed under the realys helped quiet the relay hum but it is not a cure. There are other units with DC relays that are much quieter.

later Ed

[Jim Chambers]

Thank you Ed. Interesting that it is involved with the generator. We had not run the generator at all until we used it briefly twice while on the trip. Maybe we woke something up that had been sleeping!

[Edward Buker]

You will get some variation in the hum level depending on how loose the cover is fitting and whether or not some metal in the enclosure decides to resonate or not. Basically the relay coils are vibrating at 60 cycles.

It was a fair amount of work to pull the unit and drill out the rivets, remount the relays with screws and create some isolating rubber mounting system, and then wire it all back into place. I lowered the hum level by maybe 40% but would probably not go through the effort unless I could get to a threshold where I could not hear it in the coach. I was optimistic but this project did not hit a home run, I ended up on second base.

The relays are powered and therefor hum all the time you have the coach AC cord connected to the grid. Those relay coils consume power and hum 24hrs a day. If they had designed it a little differently and wired it a little different they could have used the relays in the "powered" on position only with the generator running. With the generator fired up you would never hear the hum and the relays would be silent while tied to the AC grid. Not sure why they used AC relays, and not sure why they chose to have the relays use the AC coil powered on set of contacts for the AC cord source.

My guess is that there is a circuit designer out there that was forced to go family camping in one of those eary square Winnebagos and had a sewer hose burst that spread everything in the holding tank on his brand new favorite white sneakers.......just my theory as to why we must suffer the hum.

Later Ed