Transfo lamination bolts super hot at 20 min and no-load (solved)

[Cliff Matthews]

I have 2 Hammond 500va  transfo's (100vct, but now split as 2 x 50v) which had some thick fiber washers under the bolt-head on one end. These are from 1982 and originally marked 115v pri, but these days mains is 122v. After splitting out the center-tap on one, all checked OK (voltage, inter winding isolation, capacitance under 400pF, etc..) but the lamination's were a bit buzzy without the screws and end-caps fastened. 

After re-assembly with stronger steel washers to keep down the buzzing, I decided to let the first one idle on the bench connected to the Variac at 115v but it was still quite buzzy. After 20 minutes, I checked and all seemed fine but noticed localized heat on the end caps and probing further, burnt my finger on the head of a lamination bolt. I'm not aware of what happens with stray flux acting on these bolts, but what is "standard procedure" with these bolts and fiber washers? I've never seen (felt..) this before..

* The Eventual plan is to run 4 RD-tech type modules from the split windings.

[jaycee]

Insulate the length of the bolt with some heat shrink sleeving

[Andy Watson]

You have created a short turn through the bolts and laminations. You need to break the loop - isolate the nut and bolt head with fibre/insulating washers. You may need to insulate the shafts of the bolts too.

[Zero999]

As mentioned above: you've created a shorted turn through the bolts and end covers. There's a reason why they used fibre washers! Another thing you could try is nylon washers, which might be more ridged than the fibre ones.

[ArthurDent]

If you google information on the conventional "E" and "I" type of transformer you will find what others have said about making sure that the bolts are insulated from the core. Using a paper tube like a drinking straw can be used for the length of the bolts and solid fiber washers should be used on the ends. Nylon or any material that can distort under pressure isn't a good material for the washers. Don't ignore insulating the shafts of the bolts because you can still have shorts from the shaft of the bolts to the edge of the holes of the laminations.

The problem is that any hole through the laminations will act similar to the window for the windings and any wire or steel bolt passing through these holes will produce voltage. The uninsolated bolts will not only get hot but you will be losing power through these shorted turns. Here's what a couple of articles say:

"When a transformer using E-I laminations is bolted together, it is important that the bolts are insulated from the core. If not, this would allow large eddy currents to circulate through the end laminations and the bolts, reducing performance dramatically."

"Core bolts were always needed to be effectively insulated where they passed through the core limbs and yokes in order to prevent current flow in the hole (of the core bolt) and additionally it was insulated from the core to prevent shorting out individual laminations at their edges."

[Cliff Matthews]

I tried the heat shrink on one bolt but it wouldn't fit, so I got some Teflon plumbers tape and it works just fine!
Thanks everyone! As John Candy and the Bobsledding Jamaican's used to say, "Cool Runnin" 

[amyk]

You are lucky you didn't melt anything. A single turn is going to have a ridiculous amount of current flowing through it at a very low voltage --- probably only limited by the resistance of the loop itself. I wouldn't be surprised if those bolts were passing 100A+.

[Cliff Matthews]

You are lucky you didn't melt anything. A single turn is going to have a ridiculous amount of current flowing through it at a very low voltage --- probably only limited by the resistance of the loop itself. I wouldn't be surprised if those bolts were passing 100A+.

True. It's possible only a few lamination's were touching the bolt since the 5 amp breaker on the Variac didn't trip? I've had this 15 pound beast idling on the bench all evening at 122v - it's quiet and still only mildly warm. I bought both for only $25 

[TERRA Operative]

I would go for Kapton tape over the Teflon tape, the Teflon tape is pretty soft and weak and may wear through in a relatively short time with the vibration of the transformer (even a tiny bit of vibration could eventually wear through).

[Cliff Matthews]

I would go for Kapton tape over the Teflon tape, the Teflon tape is pretty soft and weak and may wear through in a relatively short time with the vibration of the transformer (even a tiny bit of vibration could eventually wear through).

I'll buy Kapton tape soon but Hammond (a notable mfg.), only thought it necessary to isolate one end. They could have been in error, so I triple wrapped the full length of all bolts and re-installed the fiber washer. It's run all night without further rise in temp.

Today I'll rewire the other unit, idle it for 30-minutes and if all goes fine, both can drive 300 watt resistive heater loads for an hour with the both 50v windings on each in parallel. That'll warm-up my shop (I need it, today we have -17C outside..)