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Author Topic: SDA /SDT Generator Manual copy available and question on de-magnetised generator  (Read 776 times)

ajaffa1

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Hey Alex 0.07 on a 2000 ohm scale equates to 140 ohms, not ideal but not too surprising considering the age of the selenium rectifier in the circuit. The good news is that we have a circuit, which suggests that the AC winding, carbon brushes, bridge rectifier and the auxiliary winding are intact.

The next thing I would like for you to do is to flash it again. This time I would like for you to connect a wire from the positive battery terminal to the same red wire in the connector block, just touch it very briefly and let me know if you get a spark

Bob

Alexdick

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No spark positive to positive

ajaffa1

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Ok , let us assume that some current flowed through the circuit, 12 volts through a resistance of 140 ohms equates to only 0.085 amps so wouldn`t produce much of a spark.

In the hope that this will have provided enough residual magnetism please start the generator and set your multi meter to Volts AC. Very carefully check the voltage between the red and black wires in the connector block. (if this is live you are looking at 240 volts and a possible 30 Amps, do not touch anything but the plastic handles on the multi meter probes!)

Bob

Alexdick

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It say 11.6 !!!

ajaffa1

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Wow, I wasn`t expecting that. I have to conclude that the problem lies with the bridge rectifier having died. I suspect that you should also replace the charging circuit diode. I am very concerned that the voltage you are showing is very similar to to the battery voltage and you could have some sort of a leak between the AC and the DC side of things. I will have to take a whole load more resistance tests in the morning and ask you to compare your readings with mine.

I guess that when all else is eliminated what remains must be the truth (Sherlock Holmes)

Bob

Alexdick

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Thanks Bob .... sounds complicated! I really appreciate your help with this. Thanks.

ajaffa1

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No worries, I`ll try to post resistance figures tomorrow.

Bob

Alexdick

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Bob Iím thinking now might be the time for me to go shopping and buy a new set of things that need or could do with replacing anyway. Itís 45 years old so I think it deserves the expenditure.... I just need to find a good supplier and some advice on what to do. Good plan? Alex

oldgoat

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Looked up bridge rectifier on your local ebay 1000piv 50 amp cheap as chips

Alexdick

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Itís a family heirloom so I am keen  todomit and replace whatever is needed. I guess I just need to know exactly what to buy!

oldgoat

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At 2pounds 60 p that is the one you need

ajaffa1

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Hi Alex, 240 Volt AC is a sine wave it swings from +380 volt to -380 volt. Get a Bridge rectifier that is good for 1000 volt. I personally prefer ones with screw terminals, something like this would be ideal https://www.ebay.co.uk/itm/1600V-Single-Phase-Diode-Bridge-Rectifier-Power-Module-60-100-150-200-300-400A-G/142636237023?hash=item2135c840df:m:mHdv3EQvIAK-AnA4EtBmkeQ:rk:10:pf:0

You will also need a heat sink, a piece of 6 mm x 50 mm x 150 mm aluminium would be ideal.

The Selenium diode in the top box can be replaced with a modern silicon diode, something like this should be fine https://www.ebay.co.uk/itm/1N5408-Diode-Silicon-Rectifier-Bridge-Diode-3A-1000V/201950776806?_trkparms=aid%3D555017%26algo%3DPL.CASSINI%26ao%3D1%26asc%3D20160706105120%26meid%3D2e29568a7e32460eb9b9b0afffde8b2c%26pid%3D100508

You will need to be very careful to mark the wires connected to the old bridge rectifier to ensure polarity is right when fitting the replacement. Take lots of photos as you go. You may find that the lugs on the old wiring are the wrong size to fit the screws on the new rectifier. You will probably need to solder on some new lugs. Yes I use solder, I do not trust crimp connectors on any wiring supposed to handle 240 Volt at 30 Amps

When you mount your new bridge rectifier it should look something like the photo

Bob

Alexdick

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Hi Alex, 240 Volt AC is a sine wave it swings from +380 volt to -380 volt. Get a Bridge rectifier that is good for 1000 volt. I personally prefer ones with screw terminals, something like this would be ideal https://www.ebay.co.uk/itm/1600V-Single-Phase-Diode-Bridge-Rectifier-Power-Module-60-100-150-200-300-400A-G/142636237023?hash=item2135c840df:m:mHdv3EQvIAK-AnA4EtBmkeQ:rk:10:pf:0

You will also need a heat sink, a piece of 6 mm x 50 mm x 150 mm aluminium would be ideal.

The Selenium diode in the top box can be replaced with a modern silicon diode, something like this should be fine https://www.ebay.co.uk/itm/1N5408-Diode-Silicon-Rectifier-Bridge-Diode-3A-1000V/201950776806?_trkparms=aid%3D555017%26algo%3DPL.CASSINI%26ao%3D1%26asc%3D20160706105120%26meid%3D2e29568a7e32460eb9b9b0afffde8b2c%26pid%3D100508

You will need to be very careful to mark the wires connected to the old bridge rectifier to ensure polarity is right when fitting the replacement. Take lots of photos as you go. You may find that the lugs on the old wiring are the wrong size to fit the screws on the new rectifier. You will probably need to solder on some new lugs. Yes I use solder, I do not trust crimp connectors on any wiring supposed to handle 240 Volt at 30 Amps

When you mount your new bridge rectifier it should look something like the photo

Bob

Thanks Bob, I really appreciate this. I will order the parts tomorrow. You mention ensuring polarity is right which I understand but what I don't know is which wires are positive and which are negative on the existing  rectifier? also, which is the selenium diode in the top box? there are so many things there i don't know what's what.....
the journey continues!
thanks
alex

ajaffa1

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Hi Alex. If you look very closely at the photo you originally posted. there are six wires connected to the old selenium rectifier. The two larger wires nearest to us in the photo are AC, one goes to the slip rings and one to the neutral in the connector block. Also connected to the same lugs are two smaller wires which go to the rheostat. Polarity on these smaller wires does not matter.

The two other large wires, furthest from us in the photo, are the auxiliary winding wires. One has a red end the other is white. The red one should be positive but we will check later by comparing it to mine. We will also be checking the resistance across all these windings and comparing them to mine.

The selenium diode in the top box looks exactly the same as the one in the side cover except that it has only four vanes on it. Something like the photo attached.This should only have two wires attached to it. white and yellow. The white wire goes to the positive on your battery. The yellow goes to the charging circuit. We will replace this with a silicon diode using a new terminal block similar to the main connector block in your top box.

You will also need some new crimp connectors, as you can see from the photo, they come in different sizes for different wire thickness and to fit different terminal screw size. These will be soldered so do not worry about buying an expensive crimping tool. I can not tell you which lugs you will need as I do not know what size your new bridge rectifier screw terminals will be. You should also try to get some heat shrink, also shown in the photo, you will need red and black in around 6mm diameter. You local electrical retailer should have all of these items in stock for very little money.

We should also check the condition of the carbon brushes on the commutator ring  and slip rings while we are about it.

Bob.

BruceM

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Like Bob, I once preferred soldered connection.  Alas, the technical consensus today is that for many applications crimp connections are superior.

Solder does insure good connection, but also causes embrittling of copper wires since copper gets hardened by slow cooling, and softened or annealed by fast cooling.  It's the opposite of steel and iron.  So good quality crimping (with the proper crimp tool) is often a better choice, especially when vibration is present and the individual wire strands are not so large as to assure non-breakage with continued mechanical stress.  With a soldered connection, there is a big mechanical stress riser right after the edge of the terminal, where the wires are no longer supported by the terminal.  With vibration stress, this is where the wire/strands will break off over time as the heat hardened copper succumbs to stress cracking.

This is the reason you will find virtually no soldered connections in the automotive engine compartment. They crimp to reduce costly in-warranty troubleshooting and repairs.

It is strongly counter-intuitive to think that a crimp is better, but were there is vibration, it really is...just because of the very odd tempering characteristic of copper.