Ampdog
R.I.P. 23 June 2022
Most of us have read about this some time or another, but I wonder how many had the experience: Assembling a (power) transformer core with non-insolated screws/bolts, and finding that it mattered.
I recently assembled a 35mm stack of E-I laminations transformer and two aluminium end covers, screwed tight with four uninsulated screws and nuts through the ends - and upon switching on found the screw ends burning my fingers after a few minutes. The core got the normal luke-warm after some 20 minuts, while the screw ends quickly reached some 80? C.
Examination lead to a potential difference of some 130mV over the relevant hole ends. Not a very high voltage, but when shorted by a 5mm metal screw .... I wanted to measure the energy dissipated, but could not take the necessary vector amplitude readings. I used fiber insulating washers on both ends of all four relevant screws; all then adopting the normal rising temperature of the core.
I believe part of the problem is the aluminium (shorting) end bells. Aluminium conducts electricity far better than steel - the steel end bells usually used. I have also seen screws/bolts themselves insulated with non-conductive fiber sleeve. Heat shrink tubing should suffice; one uses thin enough screws to accomodate the sleeves.
There then, to folks assembling E-I transformer cores. This was the first such experience in my long years with electronics, still ......
I recently assembled a 35mm stack of E-I laminations transformer and two aluminium end covers, screwed tight with four uninsulated screws and nuts through the ends - and upon switching on found the screw ends burning my fingers after a few minutes. The core got the normal luke-warm after some 20 minuts, while the screw ends quickly reached some 80? C.
Examination lead to a potential difference of some 130mV over the relevant hole ends. Not a very high voltage, but when shorted by a 5mm metal screw .... I wanted to measure the energy dissipated, but could not take the necessary vector amplitude readings. I used fiber insulating washers on both ends of all four relevant screws; all then adopting the normal rising temperature of the core.
I believe part of the problem is the aluminium (shorting) end bells. Aluminium conducts electricity far better than steel - the steel end bells usually used. I have also seen screws/bolts themselves insulated with non-conductive fiber sleeve. Heat shrink tubing should suffice; one uses thin enough screws to accomodate the sleeves.
There then, to folks assembling E-I transformer cores. This was the first such experience in my long years with electronics, still ......
