Any clue why large toro physically buzz's intermittently?

[jacq.]

I have a large 2KVa toroidal that every once in while starts to hum (buzz, vibrate physically) intermittently. Not loud but noticeable. Anyone has an idea what causes such a behavior and how to avoid it?

Jacq.

[Lee Knatta]

In short:
This has two reasons.
One is that the core alters its shape with an alternating magnetic flow through it. I forget the name of this effect.

Second is that magnetic flow and mechanical force always goes together. See a solenoid.
The windings sometimes can vibrate a bit.

Another reason could be DC on the mains. This can make the core to saturate which is bad.

The best measure against the first two causes is to drown the toroid in a vacuum environment with epoxy.

Against the DC you can use a filter but this is not convenient for a 2kVA toroid.

A workaround could be to mount the toroid using rubber mats to acoustically isolate it a bit and dampen the noise.

HTH

[llwhtt]

I have a 10 Amp Variac (essentially a toroid) on my bench and anytime something else on the same circuit is turned on it buzzes. It can be a mixer in the kitchen or the heat gun on my bench, always seems to be a motor that does it.

Craig

[jaycee]

the mixer and heat gun probably use half wave rectification to alter speed. this will put DC on the mains and cause the buzz.

if you have this problem you can build a circuit called a DC trap to cure the problem. Basically this is two electrolytic capacitors say 4700uF 400V back to back bypassed by diodes.

[DF96]

One is that the core alters its shape with an alternating magnetic flow through it. I forget the name of this effect.

Magnetostriction.

[jacq.]

Very illuminating. Thank you all for your inputs.

Jacq.

[smartx21]

Although previously warned not to be used along with that power, attached are same variations for a mains DC filter.

[smartx21]

DIY rubber mats = rubber rings used to hold a car's exhaust + case foot!

[djk]

Note the Crown schematic detail. The big Crown (1160Wx2) uses two bridges in parallel with a soft-start, the smaller Crown (550Wx2) just uses one bridge.

Feel free to add the caps shown in the Bryston schematic detail, Crown doesn't think they are needed.

[h_a]

The Crown version is simply a little less effective as it always cuts off 0.7V around 0 V.

With caps as in the Bryston version, AC always passes (with proper cap size) and only DC is cut off (up to 0.7V).

Just remember using high current bridges, standard heavy duty parts for PSUs are fine.

Hannes

PS: by the way, humm depends also on toroid design itself.