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How a transformer is constructed.

The dead transformer.

The dead transformer.

The joke among electrical folks is it does not work on electrons and all the theory stuff. I actually uses smoke, because when you let the smoke out of it, it stops working.  A few weeks ago I posted this picture of a transformer that “smoked” as soon as power was applied.  I have finally ground out the welds and disassembled it so you can see what the “innards” of one looks like.  As before I still think the fact that it was welded is cheap construction, but it is what it is.  Refer to my post Transformers part 2 for a full fledged rant about the weld.

Top view of the transformer after the I sections of the core were removed.

Top view of the transformer after the I sections of the core were removed.

The reason I wanted to tear into it was to show the cross section area of the core.  The photograph to the right shows a top view of the core after the “I” sections were removed.  Notice the center of the core has a much bigger cross section area.   This is because all the magnetic flux must travel through the center but has two paths on the outside to complete the magnetic circuit.

A single lamination from the core.

A single lamination from the core.

This picture shows one set of lamination pieces removed from the core. Notice the holes in each corner where bolts could be inserted to tightly hold the lamination pack. No excuse to weld, except cheap! (rant rant rant).

 

 

The transformer windings and form.

The transformer windings and form.

The next picture shows the plastic form used and the windings. There are many ways to wind a transformer.  Sometimes winding are stacked on top of each other, but in this one each winding has a separate place on the form.  One winding is in good condition, this is the secondary winding.  The transformer was designed to be a 120 Vac to 24 Vac step down transformer, and the secondary wires are much larger in diameter because the secondary would normally carry a lot more current.

What killed it?  The transformer was installed in a 480Vac circuit so the short obvious answer is too much voltage.   However, if you have read my blog very much you probably know by now I can’t leave a teaching opportunity with a short simple answer.  Going back to the power equations: I2R is the most helpful one when talking about power in an AC circuit.  I = V/Z in AC where Z is the impedance and takes into account both XL and R.   Since the frequency is the same, our first guess would be that Z should be the same.   Because V was increased by 4 over the rated value, I would be 4 times larger also.   This would mean that the power would be 16 times higher and power would create heat and probably this alone would be enough to release the smoke.   But it gets worse.

Magnetization curve of various steels.  (credit at the bottom of the post)

Magnetization curve of various steels. (credit at the bottom of the post)

Materials can only conduct a certain amount of magnetic field before they become saturated.  This is shown on the graph to the left for various materials.  The horizontal axis shows strength of the field being generated in Ampere Turns and the vertical axis shows the amount of flux in the steel. All of the curves have a “knee” where the curve starts to flatten out and more field generated does not increase the magnetic field in the steel by a large amount.   At this point, the inductive reactance value drops in the coil generating the field because the reactance is dependent upon the field being concentrated in the core.  Transformers and chokes are normally designed to operate in the high slope area of the chart.

With four times the designed voltage applied, probably this transformer was well out of the design area of the core.   This means XL decreased. Z would also decreased and current, I, was greater than 4 times the design rating.  This means the power was also much more that 16 times the expected no-load power and the insulation went up in smoke.

This property of saturation of the magnetic field can cause major problems in the power grid from an unlikely source.   In 1989 Quebec Canada and part of the NE USA experienced a power outage.   It was blamed on a large solar flare and charged particles hit the earth’s magnetic field.  This is what causes the northern lights but this  incident was very strong and over the great distances covered by the power lines a current was induced in the ground wires.   This current went through the cores of the large distribution transformers and caused the transformers to saturate during part of the AC cycle. The AC was no-longer a true sine wave because part of the waveform was “clipped”.  This distortion caused some protective relays to start shutting down the system and a major power outage occurred.  Before reading about that I knew sun spots raised havoc on communications, but I had no idea that it could cause power outages.  This is a non-technical article about it.  Solar storms can cause power outages.

Another use for the magnetization curve was an old method to control the speed of machines called a magnetic amplifier.  A small AC voltage was put into a primary of a transformer, but this transformer had a purposely undersized core.   The secondary had a rectifier circuit to turn the AC into DC.   The trick was there were several other windings on the transformer.   These had DC currents flowing through them.   These DC voltages would cause the core to go into and sometimes out of saturation.   This would affect the voltage coming out the the rectifier and ultimately control the speed of the machine.  It was not as precise as machines can be controlled with new technology, but it was very reliable.   Another term used to describe this is a “saturable reactor”.

It is my sincere hope that this post has been interesting and informative to you and that the transformer gave it’s life for the betterment of mankind.

Gary

The source of the magnetization curve is from Wikipedia Commons Area  The file was traced from Charles Steinmetz (1917) Theory and Calculation of Electric Circuits, McGraw-Hill, New York, USA, p.84, fig.42 on Google Books
Author     Charles Proteus Steinmetz — The file is in public domain – Steinmetz died 1923.

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