Slightly OT: Smoking Transformers

demmons said:

I ran into an interesting situation:

We received a machine that had three single phase transformers mounted next to each other on the floor of the panel with a metal stud coming up through the middle of each. A steel flat bar with a hole for each stud held all three transformers down.

The transformers were wired together to go from 220 Delta to 380 Wye. Upon appying power we began to see smoke within seconds. The transformers were not hot but the studs holding them down were.

There were no shorts on the transformers, the transformers were wired correctly, they were isolated from the metal by rubber, and upon removing the clamp bar, the problem went away.

My coworker suggested magnetism was causing the heat. Any ideas?

I have attached a sketch of the layout if I have been unclear.

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leitmotif said:

I went to a transformer manufacturer here in Seattle and asked why they do not short out thru the frame. THe answer was "they cant".

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Eddy current losses occur whenever the core material is electrically conductive. Most ferromagnetic materials contain iron: a metal that has fairly low resistivity (roughly 10-7 Ω m). The problem is intuitively obvious if you consider that the magnetic field is contained within a 'circuit' or loop formed by the periphery of the core in the same way as it is contained within a turn on the windings. Around that periphery a current will be induced in the same way as it is in an ordinary turn which is shorted at its ends.
What is needed, then, is some method of increasing the resistance of the core to current flow without inhibiting the flow of magnetic flux. In mains transformers this is achieved by alloying the iron with about 3% of silicon. This lifts the resistivity to 4.5×10-7 Ω m. Depending upon the amount of silicon this material is called 'transformer iron', 'electrical iron' or 'armature iron'. The alternative name 'silicon steel' is a misnomer because steel is iron alloyed with carbon; and carbon does no good in a transformer core. The silicon does, though, increase the mechanical hardness of iron in the same way as carbon - try sawing up a transformer core and you'll discover this quickly.

In any resistive circuit the power is proportional to the square of the applied voltage. The induced voltage is itself proportional to f×B and so the eddy losses are proportional to f2B2. The flux is also related to the size of the loop. Figure PLM shows how the idea of lamination is used to reduce the power losses caused by eddy currents in mains transformers. The same principle applies to motors and generators too. Using a solid iron core (as in cross-section B) results in a large circulating current. So, instead, the core is made up of a stack of thin (~0.5 millimetre) sheets (cross section C). Here I have shown only four laminations but there will normally be many more. The lines of magnetic flux can still run around the core within the plane of the laminations. The situation for the eddy currents is different. The surface of each sheet carries an insulating oxide layer formed during heat treatment. This prevents current from circulating from one lamination across to its neighbors.

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The transformers are toroidal.

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