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Old 12th February 2012, 03:19 AM   #11
AJT is offline AJT  Philippines
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Quote:
Originally Posted by rcbuck View Post
I will try the dip and wipe. I don't need to remove all the wax, just enough to make re-assembly easier.

This is not a transformer designed for high quality. I am making a test transformer for a 70 volt system at 35 watts. Input will be 40 watts at 6.5 ohms. I figured 5 watts of transformer loss. The frequency range will be from 200 Hz to about 5 KHz. I did all the winding calculations based on a low frequency of 200 Hz.

It was a cheap Chinese 60 watt power transformer. As a result, the size of the core is larger than needed for 40 watts of audio with a low frequency of 200 Hz. Once I have it re-assembled I will drive it with a 1000 Hz tone. If I hear the core singing I will heat the transformer in the over at 350 for a couple of hours which should melt enough of the remaining wax to quieten it down.
sounds reasonable......you can also use popsicle sticks, shaped to wedge between the core and the coils if there is vibration...you can even dip it in a polyurethane or shellac varnish if you wish.....
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Old 12th February 2012, 03:35 AM   #12
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String with wire accross a pot and place in the oven and let drip until clean.
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Old 12th February 2012, 07:10 AM   #13
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laminations are about magnetic field so don't worry about perfect electrical contact. Putting them together with tension pushing them towards each other while heating to a temperature sufficient to melt the wax should suffice.

Suggestion - put a few c-clamps on them and heat in an oven past the melting point. If it comes out of the oven with the C-clamps loose, lather, rinse and repeat.
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Old 12th February 2012, 11:20 AM   #14
AndrewT is offline AndrewT  Scotland
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Rusty lamination is the insulation.
It is not there as a result of overheating.

The insulating layer is created during manufacture.
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Old 13th February 2012, 02:01 AM   #15
rcbuck is offline rcbuck  United States
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I managed to get the transformer rewound as an audio transformer. I couldn't hang the laminations because they don't have holes in them. It would have been too difficult without the holes. I finally settled with just reheating the laminations on a flat piece of metal with the torch. This leveled the wax enough so I was able to get all the laminations except for one E piece back into the plastic bobbin.

The transformer performs more or less as designed. Driving it with a 16.2 volt 1000 Hz signal produces 70.7 volts out. At 5 KHz the same drive signal produces 65 volts out. So I have some high frequency roll off. The loss probably is due to not enough inductance in the 6.5 ohm windiing.

My next test will be to find a slightly larger core and re-calculate the winding data and give that a try. This is all being done just to advance my knowledge of transformer design. I am using the information from "Practical Transformer Design Handbook" by Eric Lowdon. The book was published in 1980 and is now out of print. It has a wealth of practical knowledge. No information about SMPS but it does give design info for 400 Hz and 1000 Hz transformers.
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Old 22nd April 2012, 06:46 AM   #16
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It's the oxide on the steel laminations that is primarily responsible for reducing the eddy currents. But you may also use varnish or wax to improve the insulation between the laminations. However, in order to be effective at reducing eddy currents the varnish needs to be applied and cured BEFORE the laminations are put together.

The main reason for adding varnish or wax to an inductor, after it is assembled, is to aid in the insulation of the windings. The varnish also serves to insulate any nicks on the enamel of windings. Varnish or wax also makes for a quieter inductor. Driving a wedge between the coil form and the center leg of the core, tightly bolting the laminations together (With insulating shoulder washers over the bolts to reduce eddy currents.) and vacuum varnishing the inductor, results in the quietest inductor and also one that can withstand the highest voltages.

Welding a bead across the corner of the laminations is to provide a conductive ground path for safety, just in case a winding shorts to a lamination. That way any current and voltage is shunted to ground. People have been electrocuted when they have touched non-welded laminations that were shorted to the windings. The welding does cause a slight increase in eddy currents, however, it is considered a small price to pay for safety.

If you have removed all of the wax from the laminations, at the very least you can dip the transformer in hot varnish. Better still if you can find a vacuum pump and a container to hold the vacuum around the inductor when you dip it in varnish. But be sure you find a suitable varnish.

Scott Novak
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