[The Bogg]

I know some of you guys here are "electrical" guys and hope someone might be able to help.


My equitech 7.5q balanced power unit has a 40amp 240volt input. It works great. Lately between 9 and 11pm there is a loud buzz from it which can be heard throughout the room. I don't have anything on a timer in the house or any other device which operates specifically at those times.


I called the power company and they sent someone to analyze the power from the transformer supplying my house. I asked them to check for dc, harmonics, etc... I was told my power was fine and that the 3 legs of the supply were in balance. Of course they came during the day, not at night when it acts up. They did offer to send someone out between 9 and 11pm so I'm waiting for that. I might call Equitech and ask them for advice.


Any of you guys have thoughts/suggestions about what might be going on and how to resolve the issue? I did disconnect the coaxial cable line and it made no difference.


thanks.

 

[Curt Palme]

It sounds like a combination of temperature and humidity are causing the transformer laminations to buzz. (assuming there's a big transformer within the unit)

 

[1kevinm]

If you have a voltmeter, check the input voltage when it is buzzing versus when it is not buzzing. Changes in voltage will cause a transformer to buzz.

 

[The Bogg]

the input voltage is 244 volts with or without the buzzing.


not sure that temp or humidity are the issue, because it's at a specific time of the evening leading me to think it's something from the mains supply.

 

[The Bogg]

c'mon, where's MG and the rest of those electrically knowledgeable guys? This must be more of an interesting challenge than trying to figure out if biwiring will cause frequency dependent phase shifts that would irritate a bat.

 

[Michael Grant]

Sorry, but I really don't know. Not a power guy. I look forward to hearing what you find out.

 

[fastl]

A residual DC component on your powerline can cause transformer buzz, according to the folks at Furman. In their premium power conditioning product line, they utilize Plitron toroidal transformers that employ some sort of proprietary winding/core technique for minimizing that type of buzz.


I used to experience a similar problem with my Monster AVS2000 line voltage stabilizer, late at night, typically after 11 PM. However, that unit made so much mechanical noise that drove me nuts, so I finally replaced it with a Furman AR-15 Series II. The Furman is pretty much silent all of the time.

 

[The Bogg]

I had asked them to check for dc on the line. They're going to send another team out between 9 and 11pm one night and hopefully can get to the bottom of this. I'll keep you all posted in case something like this happens to someone else.

 

[The Bogg]

Just a little update:


I had the electrician in...didn't find anything wrong in the breaker box, tightened a few grounds etc.... He tightened the ground on the water pipe.


One of the problems I had noticed recently was that the Equitech would trip my breaker the first few times before I could get it to stay on (coming from an "off" breaker position, for example if the power went out). I just assumed it was the inrush current tripping the breaker although it seemed a little odd that it would trip a 40 amp 240volt breaker even with all circuits off. The electrician measured only 2 amps draw when the breaker would trip. He suspected a "dead short". Bottom line, he figured the problem was with the Equitech unit and not with the power coming into the house or the breaker box.


I got to thinking that the problem may have started when I had been repositioning the unit. I had checked many times to make sure the connection into the Equitech was snug (it's a California style 50amp 250volt twist-lock connector). The 6 gauge wire is very thick and unwieldy and was pressed against the back wall. I think that perhaps it was causing a little bit of traction on the connection because when I pulled the Equitech out a couple of inches and then reconnected it with as much force as I could get on it then waited till 9:20pm (when it always started buzzing/humming) I was pleasantly surprised that there was no humming or buzzing. THANK GOD.


I could've bet that it was a powerline issue related to power quality or something since it came on at the same time each night. The only thing I see is that the voltage goes up 1 or 2 volts at 9:20pm for whatever reason. How does the saying go? "Throw out the impossible, whatever is left, no matter how improbable, must be considered as possible" or something like that. Well I'm glad I decided to recheck the Equitech itself.

 

[fastl]

Well I'm glad you tracked down the problem. Maybe the loose connections were what was vibrating. As far as breaker tripping goes, remember that an unenergized (large) autotransformer can typically draw hundreds of amps during initial power-up, while the core is magnetizing. Variacs are known to blow fuses quite readily for this very reason.


As to your 1-2 volt rise at 9:20, it could be the "tap changer" in your local substation stepping the line voltage upward.

 

[The Bogg]

My understanding was that the type of transformer used in the Equitech draws very little inrush current - consistent with the 2amps measured by the electrician. The breaker was tripped because it may have "thought" there was a "dead short". I am a bit surprised that the 50amp 250volt connector doesn't have a more solid connection with a "click" when locked in place. Unless I'm still not doing it properly! In any case it is working and not making noise so I'm a happy camper.

 

[fastl]

The magnetizing current drawn by a kVA sized isolation transformer or autoformer easily can run into the hundreds of amps. The only way that you can achieve good voltage regulation in a transformer is to minimize winding resistance. When you energize the beast, the AC line sees this low DC resistance in series with a small amount of winding inductance until the core magnetizes and stabilizes. Some AC powered products use either thermistors or soft start series circuitry to reduce the start-up surge, but I'll bet that it is a lot higher than 2 amperes!

 

[The Bogg]

2 amperes was what was measured by my electrician. I'm not sure if the Fluke meter would pick up a short-duration peak greater than 40amps for a short period of time but then again that shouldn't trip the breaker anyway. I believe there's something on Equitech's website about why their transformer has low inrush current, I'll take a look for it.

 

[fastl]

Still not clear as to what was causing your problem. The wiring must have been partially shorted to trip the breaker. Anyway, the magnetizing current is for a couple of cycles, so a Fluke probably wouldn't register the peak value.