Variac "Buzz"

[AMPREPAIR]

I have a very nice General Type 5 Variac I have been using for many years. It still works great! However, the windings began buzzing without a load. It has never been stressed with heavy loads. I gently probed the Variac's windings (with an insulated tool) to see if pressure at various points on the windings would make it quiet again; sadly, this had no effect. Fortunately, it still works perfectly. Any thoughts?

[Chuck H]

Mechanical noise isn't unusual with transformers. The only real fix would be to "pot" the offending coil of winding. Do check to see that there is no voltage on the chassis or anywhere voltage shouldn't be though because excessive current can have this affect on otherwise well behaved windings. If there is no voltage where there shouldn't be and the unit operates with the same range it always has, I'd bet that the unit is fine. As long the noise isn't too offensive.

[Steve Conner]

Maybe you suddenly got DC on your mains? I once heard a story where somebody bought a bunch of energy saving bulb holders that contained diodes, so the bulb would only get half cycles and run dimmer. He installed them in his porch lights and lo and behold, the toroidal transformers in his hi-fi gear began to buzz.

[km6xz]

Is it buzzing when the sliding contact is on the "0" volts level? If it is not, your slider contact is worn. Notice how it is designed to contact only one, two or a series of several exposed flatened windings. The material of the slide contact can be bronze...rare... or more commonly carbon graphite. The most common are carbon because it allows for making contact with more than one turn at a time will moving so there is a smooth transition(the bronze brushes require raised spacers between windings so one and only one turn can be in contact at a time) between voltage steps. Making contact with 2 or more turns at a time forms a shorted turn that can generate a high circulating current in that loop. Carbon has enough resistance to limit the peak current when that happens. It also is soft so conforms to slight mating contact imperfections without arcing. Hum can result when 2 turns are shorted by the contact brush in resting position, instead of only when moving. That happens when the tapered brush material, carbon/graphite mushrooms out to be wider as it wears down.
If the hum is there when the contact brush position is in the zero position, think of what would make a fixed inductor to hum. Losses and leakage. You would expect it to hum a little from turns becoming looser with age when current is flowing through it but when there isn't supposed to be much current flow as when there is no load. In that case look for leakage paths possibly to the frame and returns. Check the condition of the insulation for the wiper's slip ring connection. If all seems ok, take it to an industrial transformer rewinder to pressure soak it to secure the windings again. It is cheap and should return it to normal sound. To do that you need to disassemble the unit to get the core out. Replace the brushes anyway, after this time the surely need it. A repair shop wears out brushes a lot faster than industrial applications where it is seldom turned.
There are really inexpensive Chinese and Russian made voltage metered variacs that would fit so consider replacing the transformer in yours if your case has a current meter.....only current metered variacs are really useful in a repair shop in my opinion. If yours is the M5 series that did not have a current meter consider replacing it with one that does would be a good step up in functionality.
Back home I have a number of lab type voltage and current metered variacs I bought new for salvage value because their outer shipping boxes got wet. Inside, the inner box and plastic sealed bags were perfect so I saved $1200 each on 11 of them. Unfortunately they are for 120 mains only so I can't use them here but I keep them in storage in case I ever move back to the US. Good metered variacs are very productive on a bench and earn their keep easily.

[soundguruman]

I have a very nice General Type 5 Variac I have been using for many years. It still works great! However, the windings began buzzing without a load. It has never been stressed with heavy loads. I gently probed the Variac's windings (with an insulated tool) to see if pressure at various points on the windings would make it quiet again; sadly, this had no effect. Fortunately, it still works perfectly. Any thoughts?

I thinks your commutator brush is worn out.

[AMPREPAIR]

Odd thing happened! I turned on the Variac this morning to see if still "buzzed" without a load when set to "0" or no volts. It was quite! No mechanical noise! Regardless where the value of the Variac was set (0 thru 140 VAC).

For what it's worth, I have another device on my bench (an Astron 12 Amp Linear Power Supply) that will "buzz" when ever my Brothers laser printer is printing. The Buzzing will continue numerous times after the print cycle is complete for about 10 minutes. However, the printer was not powered (unplugged) when my Variac was buzzing - and the power supply was quiet! In any case, I will power down all other devices on the same branch circuit next time the Variac makes noise to see if I can find the culprit.

I fully agree; a variac should be metered. I build a really nice "AC Power Supply" that has two meters; AC Volts/AC Amps. I find myself watching the AC Amp meter every time I first power-up an amplifier I am repairing. Meters are VERY handy!

For what ever reason, the Variac is quiet today. As mentioned, there may be something external causing the buzzing sound. Fortunately, this new discovery reduces the chance for loose windings. Should I find the problem (Variac or external device), I will post my discovery on this "thread".

[Tom Phillips]

Maybe you suddenly got DC on your mains? I once heard a story where somebody bought a bunch of energy saving bulb holders that contained diodes, so the bulb would only get half cycles and run dimmer. He installed them in his porch lights and lo and behold, the toroidal transformers in his hi-fi gear began to buzz.

I have seen this happen too. I have a light in my shop that utilizes a diode to implement a hi/low switch. If the low setting is used then the toroidal transformer in the bench power conditioner buzzes.

[AMPREPAIR]

Mystery solved! My Variac will create a mechanical buzzing sound whenever I set the room humidifier (manufactured by Duracraft) to the "LOW" position. When the humidifier is set "HIGH", the buzzing in my Variac goes away. Perhaps one day I will open the humidifier to verify it uses the diode trick for LOW power. Thanks to all for the terrific comments!

[Jazz P Bass]

I can tell when the front room space heater turns on.
As well as my son's overhead light dimmer.

[AMPREPAIR]

Hi Jazz P Bass.
Do you know if your space heater and your son's overhead lights are using a diode as a power reducing device?

[Jazz P Bass]

Don't know about the space heater.
It is a fairly new one.
Electronic control.

Now the dimmer is probably a triac.