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Author Topic: SDA /SDT Generator Manual copy available and question on de-magnetised generator  (Read 14779 times)

BruceM

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There are also adjustable wire wound power resistors in various wattages, see Digikey or Mouser.

jonnymcc

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I saw those Bruce, look like exactly what would be required, didn't find an exact match for resistsnce and power, though i need to confirm requirements first....

BruceM

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A rated wattage above your needs is not a problem.  Once you get Bob's info on values, if you need help, let me know.  You can always put resistance in parallel or series to get the values you need, either in resistance or wattage.

Alexdick

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Hi Alex, sorry about the confusion about when you wanted to make a start on this revamp, thought you were still waiting on parts.

Your fist job is to remove the selenium bridge rectifier and old rheostat from the plastic cover. Clean up the cover and fabricate a couple of L brackets to mount your heat sink. Then fit your new rectifier onto the heat sink. I found self taping screws worked very well into the aluminium heat sink. (see photo)

Second job is to disconnect the rheostat from the old selenium rectifier, give it a bit of a clean. If you are as obsessive as me you can give it a coat of high temperature spray paint (these get hot, exhaust pipe paint is ideal). Next is to refit it to the plastic cover and reconnect the wiring. You will need to fit new lugs to accommodate the larger screws in the new rectifier, use solder or quality crimp fittings (not the cheap sh1t they sell in auto centers), heat shrink is optional but I think it makes for a safer/more professional finish. The two wires connect to the side of the rectifier marked with a squiggle/sine wave symbol, the polarity does not matter for the rheostat. (see photo)

Third job is to remove the front plastic cover and clean up the larger rheostat found within, when you are done it should look like the photo.

Bob

Bob

I have finally done the job! see photos below. What next? I guess i need to put the other wires on but what goes where?! Thanks for your help with this and a belated Happy Christmas and a very Happy New Year!
Regards

Alex

ajaffa1

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Hi Alex, happy new year to you and yours, welcome back. Congratulations on the quality of you work, very professional.

You should now have four blue wires hanging out of your generator. Two of them have black heat shrink on them. These two are the AC side of things and are connected to the rectifier using the same two lugs that your rheostat is connected to. (sine wave marks on rectifier) The polarity does not matter as these are alternating current.

Once those two are connected, you should now have two blue wires left one will have a red heat shrink on it, this is the positive and needs to be connected to the lug on the rectifier marked +.

The last wire should have a white heat shrink on it, this is the neutral and should be connected to the rectifier lug marked -.

You will probably need to fit new lug terminals to the wires coming out of the generator, as the screws in the new rectified are larger than on the original. Do these one at a time and replace the heat shrink to match the original color coding as you go, you do not want to get them muddled up.

Have fun,

Bob

listeroil

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To Jonnymac

On the charging problem, I've found the problem, the resistor is damaged. Any ideas on repair/replacement? Could i re wind it with resistor wire?

I have rewound a few of these resistors in the past and buy the wire from these people  https://wires.co.uk   This is the wire that I used  Bare Constantan.

50g 0.71mm Copper Nickel Wire
Ref: CN0710-050
Price: £5.44 Exc VAT (£6.53 Inc 20% VAT)
50 gram reel of 0.71mm 22 SWG Bare Copper Nickel Wire (Copper Nickel = Constantan = Eureka)
Data: approx: 1.24 ohms per metre, 50g = approx 14 metres

This was correct for the resistor that I needed yours will probably be different. What I did is count the number of turns. Measured the diameter of the resistor calculate the circumference the multiply that by the number of turns to give the length of the wire needed.  In my case it worked out to 8 meters of wire needed for 115 turns to give me the 12 ohms required.  Also you need to measure the diameter of the actual resistance wire.

Nickel Chrome resistance wire is also available but would have only filled about half the resistor. But It might have been used in your case.  This why you need to do a bit of maths so you don't get the wrong wire.

50g 0.71mm Nickel Chrome Wire
Ref: NC0710-050
Price: £6.75 Exc VAT (£8.10 Inc 20% VAT)
0.710mm 22 SWG Bare Nickel Chrome Wire (also known as Nichrome Wire / Hot Wire Cutter / Foam Cutter / Heater Wire / Element Wire)
Data: approx 2.7 ohms per metre, 50 gram = approx 15 metre

Mick

BruceM

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Nice to learn about Constantin alloy as a resistance wire, listeroil, thanks!
Might be very useful where very high currents demand lots of area of lower resistance heating wire.

ajaffa1

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Hi Listeroil, perhaps you can explain to me how these are wound as tight as they are without any insulation. Did they acid etch them or coat them with something to to stop them from shorting between the windings?

I have found that high temperature paints work for restoration but am curious about the original insulation.

Bob

Alexdick

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Hi Alex, happy new year to you and yours, welcome back. Congratulations on the quality of you work, very professional.

You should now have four blue wires hanging out of your generator. Two of them have black heat shrink on them. These two are the AC side of things and are connected to the rectifier using the same two lugs that your rheostat is connected to. (sine wave marks on rectifier) The polarity does not matter as these are alternating current.

Once those two are connected, you should now have two blue wires left one will have a red heat shrink on it, this is the positive and needs to be connected to the lug on the rectifier marked +.



The last wire should have a white heat shrink on it, this is the neutral and should be connected to the rectifier lug marked -.

You will probably need to fit new lug terminals to the wires coming out of the generator, as the screws in the new rectified are larger than on the original. Do these one at a time and replace the heat shrink to match the original color coding as you go, you do not want to get them muddled up.

Have fun,

Bob


Bob

Many thanks for this. I have had a DISASTER!!! I went and cut the ends off without marking them before I put the new lug terminals on.... impossible to believe I know but I have done the unthinkable. The lesson for me is not to do something like this on New Yearís Day after a big night out!!! So I am now totally screwed....
Alex

Alexdick

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Bob
further to my last....i have had a closer look and see two of the wires run to the shiny things in the photo below. Are these the ones that go to the sine screw on the rectifier? if so that accounts for two of the wires and just leaves the other two to work out.....
I have looked at the larger rheostat on the other side and see the copper band is broken. may be that caused the issue? can i just replace it with the one i took off the smaller rheostat that i cleaned up yesterday....it didn't have a wire off it so i assumed it wasn't needed....pics below.
thanks
Alex

ajaffa1

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Ok Alex, don`t panic. One wire on the AC side of things goes to the slip rings (shiny brass things in your photo) the other AC wire goes to the neutral in your top box, you should be able to identify them with your ohm meter. These two wires connect to the two Lugs marked with a sine wave, the same lugs that your rheostat are connected to, polarity does not matter.

Of the other two wires one should be about four inches longer than the other, The longer one is the neutral and the shorter one the positive.

The Copper band can be replaced with the unused one off the other rheostat. This rheostat feeds the main field windings and would certainly explain your lack of power generation, well spotted.

Hope you have recovered from the excesses of new years eve

Bob

Alexdick

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Bob
Thanks for this! Iíll get the copper band done immediately...... it has slipped down  so where should I put it on the rheostat?
Regarding the wires.... you mention checking the ohm readings but I donít know what I am looking for. I have the four wires and an ohm meter.... what next? Sorry for being hopeless!

Thanks again
Alex

Alexdick

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Actually I have worked it out now!!! Iíll revert ASAP!

ajaffa1

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Hi Alex, The main field rheostat ( the one in your recent photo) allows you to adjust the output voltage from your generator. A low resistance will push it up to 240 volts, while a higher resistance will drop the output to around 210 volts. If you can`t see where it was originally positioned stick it halfway and we can adjust it later.

Checking wires out with an ohm meter isn`t rocket science, I worked out how to do it and I`m as dumb as a brick. What you need to do is turn your meter to the ohms range, then touch the two probes together, the screen should show zero ohms. What that proves is that the two probes are connected to one another. Now if you put a piece of wire between those two probes it will also show 0 ohms resistance, proving that one probe is connected to the other probe via that wire.

 So if you put one probe on the neutral terminal in your top box, one of your four wires will give a zero reading with the other probe. That wire is one of the AC wires and goes to one of the sine wave marked terminals. Now do the same by sticking one probe onto one of the shiny brass slip rings, use the other probe to identify which of the remaining 3 wires is connected to it. This is the other AC wire and should be connected to the other lug marked with a sine wave.

I explained the other two wires earlier but if you have any doubts I am happy to explain further.

Don`t feel intimidated or scared of this task, so far you are doing very well. It`s better and a lot cheaper to not know and ask, than to think you know and learn the hard way that you don`t!

Have fun,

Bob

Alexdick

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Hi Bob

OK all done. I fired it up and checked the ohm reading on the red/black terminals on the top box and it says -1 .......!! can't seem to get any voltage readings anywhere either.

alex