I'm rewinding a small power transformer to use as an audio transformer. I have removed all the laminations. The transformer had been dipped in wax when it was made.
Does anyone know of a way to remove the wax from the laminations? This will help in being able to put all the laminations back without having to force and possibly damage the last few.
I placed all the laminations on a sheet of metal and heated them with a propane torch hoping to burn the wax away. That wasn't very successful as it just seemed to level the wax but not burn it away. The fact that wax is now smooth on the laminations may be sufficient to allow easy re-assembly. But removing it if possible would be better.
Does anyone know of a way to remove the wax from the laminations? This will help in being able to put all the laminations back without having to force and possibly damage the last few.
I placed all the laminations on a sheet of metal and heated them with a propane torch hoping to burn the wax away. That wasn't very successful as it just seemed to level the wax but not burn it away. The fact that wax is now smooth on the laminations may be sufficient to allow easy re-assembly. But removing it if possible would be better.
Sounds like you have a Hammond transformer and that would be more like glue and wax together.
Wax is not meant to insulate one lamination from the other. Insulating laminations would not stop eddy currents it would cause them. The old TV transmitter plate voltage transformers would have a nice long bead welded along the top of the laminations to keep such eddy currents from forming if corrosion set in.
I'd suggest not bothering to try to get an EI core power transformer to become an audio transformer unless you really want the experience. Cheap toroidal core power transformers actually have good audio bandwidth performance and would be easy to drop in. Just observe the ratio between input and output. I once listened to a Matrix amp which had been built using only about $120 worth of parts and it used cheap toroidal core power transformers for output transformers. It sounded much better than one could have imagined it would.
Wax is not meant to insulate one lamination from the other. Insulating laminations would not stop eddy currents it would cause them. The old TV transmitter plate voltage transformers would have a nice long bead welded along the top of the laminations to keep such eddy currents from forming if corrosion set in.
I'd suggest not bothering to try to get an EI core power transformer to become an audio transformer unless you really want the experience. Cheap toroidal core power transformers actually have good audio bandwidth performance and would be easy to drop in. Just observe the ratio between input and output. I once listened to a Matrix amp which had been built using only about $120 worth of parts and it used cheap toroidal core power transformers for output transformers. It sounded much better than one could have imagined it would.
Taken from Wikipedia.
Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies use laminated or similar cores.
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.
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.
Hi Nigel, Sounds like both you and me and Wikipedia are not 100% accurate but that's no surprise. It's all interesting. I've seen power transformers with rusty laminations that run very hot because of eddy currents compared to new ones of the same type that run cool. I've never seen anyone put anything between laminations and I expect that that would result in heat from eddy currents and lack of efficiency.
regards
regards
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.....
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.
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.
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.
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.
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
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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