Toroid mounting getting hot ...

[deduikertjes]

Hi there,

I've this strange problem with mounting my toroidial transformers form my class A amp. The mounting gets really hot:

I've two identical 160VA 18V toroids wired in series to give me postive and negative supply rails.

These toroids are mounted with metal bolts trough their centers. Now comes the funny (and worrying) part:

When I clamp down the toroids by means of 1 piece of metal (be it steel or aluminium) with two holes trough which the bolts go then the metal gets very hot very quickly.

When I use 1 piece of metal per bolt (so each toroid has its own piece of metal clamping it down), then these stay nice and cool.

The worrying part is of course that these bolts screw into my heatsinks which are HUGE. So in my tests of an hour or so these don't get hot.

I don't sleep well on this. Something is going on which I don't understand and it seems to be rather powerfull. The speed with which the one piece of metal get's really hot worries me.

Can anybody explain to me what's going on and if I do need to worry.

Thanks, MArco

 

[es44]

You don't want to make a short circuit like that. When you clamp the toroids down that way, you make one winding, similar to the real windings on the toroids.
That means one BIG short , as that winding has no ends, and is therefor seen from the primary side as a short circiut, drawing all the current i can, only limited by the resistance in your bolts and the clamp.

Best regards
Ebbe

 

[Elvee]

You don't want to make a short circuit like that. When you clamp the toroids down that way, you make one winding, similar to the real windings on the toroids.
That means one BIG short , as that winding has no ends, and is therefor seen from the primary side as a short circiut, drawing all the current i can, only limited by the resistance in your bolts and the clamp.

Best refards
Ebbe

Correct analysis.
I can add that in your case, a solution does exist: you simply have to reverse the primary of one of the transformer, and the problem will disappear: the voltages induced in the bolts will have opposite phases and cancel each other.
LV

 

[es44]

Correct analysis.
I can add that in your case, a solution does exist: you simply have to reverse the primary of one of the transformer, and the problem will disappear: the voltages induced in the bolts will have opposite phases and cancel each other.
LV

Also correct

Or maybe use some non-conductive material for the clamp, a piece of plastic instead of metal.

Simple solutions to something that could have had serious consequenses. At least for the toroid.

 

[cviller]

I have two toroids I need to mount, so I would like to understand this problem correctly in order to avoid it. I have drawn very rough sketch on how I understand deduikertjes connected his and how I would connect mine - is the drawing correct?

I won't be using any metalbar on top of the toroids - so they will only be connected on the bottom plate.

 

[es44]

You are exactly right cviller. you will have no short with the way that you plan to mount yours.

All the best
Ebbe

 

[cviller]

Cool, thanks!

And I'll be able to melt metal between the bolts to impress friends!

 

[es44]

Yep, you don't need tubes to get that glowing in the dark, just big fuses. hehe.

 

[deduikertjes]

Well,

Thanks for the quick and informative answers. I do understand now how to clamp down those toroids but not yet completely what is happening.

You don't want to make a short circuit like that. When you clamp the toroids down that way, you make one winding, similar to the real windings on the toroids.

Do I understand correctly that a tension (voltage) is created in the bolts by the electro-magnetic field in the toroids. Although the clamp is a non-magnetic material the electric coupling of the two bolts results in the bolts and the clamps working as a sort of transformer winding with very little resistance (am I lucky that these bolts were very dirty ...).

Connecting the bolts on one side only does not break the electrical connection but does prevent the assemblage working as a transformer winding.

MArco

 

[es44]

Do I understand correctly that a tension (voltage) is created in the bolts by the electro-magnetic field in the toroids. Although the clamp is a non-magnetic material the electric coupling of the two bolts results in the bolts and the clamps working as a sort of transformer winding with very little resistance (am I lucky that these bolts were very dirty ...).


MArco

You are kind of right....The clamps don't have to be magnetic, as long as it's electrically conductive, then it will act as a single winding seen from the transformers point of wiev.
If yo use non-electrically conductive material, there will be no problems, Elvee's solution is also useful, and cviller's as well. Just no short circuits

best regards
Ebbe

 

[AndrewT]

Hi,
the one turn that comes from the common clamp behaves just like one wire turn wound around the transformer core.

The voltage from that one turn could be in the range 0.3V to 0.7V depending on the number of turns on the two primaries. from the two tranformers the voltage can be doubled.

The resistance around the shorted turn could be around 10mohms.

The current that flows around your accidental circuit is I=V/R and the power is V*V/R.

Doing some numbers on these guesstimates gives about 100Aac and 100W. Not yet enough to burn out the primary but certainly enough to cause extra heating in the primary winding, in the magnetic core and certainly the heat you felt in the shorted turn.