Author Topic: SDA /SDT Generator Manual copy available and question on de-magnetised generator  (Read 785 times)

Alexdick

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I have a manual for an SDA + SDT Generator (see below) if anyone wants a copy I will scan a PDF over to you.
also I have de-magnetised it by shutting down whilst it was still powering my house! Help, how do I re-magnetise it? I am a TOTAL novice so two questions:-
1. Is there an idiot's guide to doing this job myself (photo of generator plate below) or;
2. Can anyone recommend someone in the Berkshire (UK) area to do it for me?
Many thanks
Alex l
« Last Edit: November 12, 2018, 04:58:59 PM by Alexdick »

mike90045

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http://www.dieselduck.info/machine/03%20electricity/flashing_generator.htm

you need to re-flash the generator


https://www.stationaryengineparts.com/st-alternators-instructions-for-operation-and-maintenance-1926-0.html
instructions:



 To stop

1, Remove the load(s) from the output by opening the output switch(es).

2, Stop the prime mover.

Cautions:

Be very careful not to allow short circuits at the output of the generator. This can damage the rectifier diodes in the voltage regulating circuit.

Before stopping the generator, remove all output electrical loading first before turning off the prime mover. If you turn off the prime mover while the generator is under load, the resulting spin-down and gradually reducing electric fields may erase the residual magnetism in the rotor that allows the generator to start self excitation.

If the residual magnetism in the rotor is too weak to start self-excitation, the rotor will need to be re-magnetized.

Problems and Troubleshooting

First, verify that all internal and external connections are correct. Once this has been verified, if there is no voltage output then possible causes are:

Loss of residual magnetism

The rotorís residual magnetism, which enables self-excitation, may be lost over time if the generator is unused for a long time or suddenly if the alternatorís prime mover is turned off while the electrical load is still connected to the generator.

To re-magnetize the rotor, flash connect a 12V storage battery to the field winding terminations F1 and F2 observing the polarity while the generator is rotating. Do not connect the battery in the reverse polarity and only connect the battery momentarily.

ajaffa1

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Hi Alex, Mike is absolutely right sbout how to flash an ST head. Sadly you have a Brush head which is very different. In an ST head the AC windings are stationary and the field is rotated by the diesel motor, the field is energized by feeding it DC voltage via the slip rings.

In a Brush head the field windings are stationary and the AC windings rotate, 240 volt AC is collected from the slip rings.

To complicate things further the generator also acts as a starter motor requiring further dc motor windings and a commutator ring, it also works as a dynamo to provide 12/24 volts to charge the batteries.

I believe that what you need to do is flash the auxiliary field windings. Please see the photo attached and the  rather poor quality circuit diagram.

You will need to remove the cover on the back of the generator to  access the rectifier and rheostat shown in the photo. Flash the field by connecting a pair of jump leads to a 12 volt battery, very briefly (half a second) touch one jump lead to either end of the rheostat shown on the left of the photograph. Doing it this way you do not need to worry about the polarity as the bridge rectifier will only allow current to flow in the correct direction. Please see drawing.

Good luck, let us know how it goes,

Bob

ajaffa1

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Hi Alex, further to my previous post, it is probably safer to disconnect the wire from one end of the rheostat, wouldn`t want to fry the rheostat.

Bob

mike90045

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When I had to flash mine, I used a automotive tail light bulb in series with the battery, to limit amps, just to not burn anything out.  And just one zap is all it takes to refresh the mistake.

I added the ST head instructions, the reiterate the "remove loads" before stopping the engine, it's a universal requirement. 

Alexdick

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Hi Alex, Mike is absolutely right sbout how to flash an ST head. Sadly you have a Brush head which is very different. In an ST head the AC windings are stationary and the field is rotated by the diesel motor, the field is energized by feeding it DC voltage via the slip rings.

In a Brush head the field windings are stationary and the AC windings rotate, 240 volt AC is collected from the slip rings.

To complicate things further the generator also acts as a starter motor requiring further dc motor windings and a commutator ring, it also works as a dynamo to provide 12/24 volts to charge the batteries.

I believe that what you need to do is flash the auxiliary field windings. Please see the photo attached and the  rather poor quality circuit diagram.

You will need to remove the cover on the back of the generator to  access the rectifier and rheostat shown in the photo. Flash the field by connecting a pair of jump leads to a 12 volt battery, very briefly (half a second) touch one jump lead to either end of the rheostat shown on the left of the photograph. Doing it this way you do not need to worry about the polarity as the bridge rectifier will only allow current to flow in the correct direction. Please see drawing.

Good luck, let us know how it goes,

Bob

Bob
Thanks for this. Please see photo below....I assume that is the rheostat I have marked up and just need to remove one wire then touch either end with the jump lead as indicated with the  red and blue circles?!
Again, many thanks for your help
Alex

ajaffa1

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Hey Mike, no criticism of advice offered and you are right about removing all loads before shutting down any generator. Alex doesn`t tell us how he came to be in this situation, but I suspect he ran it out of diesel and it  wouldn`t produce electricity once he restarted it.

Like your idea of a light bulb, very simple and idiot proof, will magnetize the iron core without doing any damage to sensitive items. The residual magnetism required is very minimal as the field voltage will ramp up as it starts to generate, even if it starts from next to nothing.

Lets hope he understands the principals we are trying to explain to him and asks for help if he is confused.
Hope he gets it going without any further problems.

Bob

Bob

Alexdick

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OK I did turn off the generator without switching off the load as I noticed the starting handle was still in it from years ago, was flying around and I was worried it would fly off and hit one of my children.....a few lessons learned!
So, Bob, I assume my photo above in response to yours is ok? I will add the light bulb into the equation too.....I assume i just connect it to the + and - wires from my battery? Or is the light bulb solution separate from the one Bob suggests with the rheostat.
Apologies for the crazy questions but I am a TOTAL novice!
Thank you for your help, I really appreciate it.
Alex

ajaffa1

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Hey Alex, Yes disconnect one wire from the rheostat you have highlighted. connect a battery between those two wires and you should be good to go. Even a small 9 volt torch battery should be OK.

Depending on the age of your equipment and if it has been modified/repaired since manufacture you may find that it has a more modern silicon bridge rectifier or even an automatic voltage regulator.(please see photos) If it is very different from the photos I posted earlier please get back to me.

Bob

Alexdick

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Hey Alex, Yes disconnect one wire from the rheostat you have highlighted. connect a battery between those two wires and you should be good to go. Even a small 9 volt torch battery should be OK.

Depending on the age of your equipment and if it has been modified/repaired since manufacture you may find that it has a more modern silicon bridge rectifier or even an automatic voltage regulator.(please see photos) If it is very different from the photos I posted earlier please get back to me.

Bob

Bob
You are a star! thank you. I will try tonight when I get home. It was all installed in the early 1970s so no, I don't think there are any of the mods you mention!
Alex

ajaffa1

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Hi Alex, there is a very good reason I mentioned modifications, the thing in my original photo, that looks like an old fashioned school radiator, is an old selenium bridge rectifier. They have a life expectancy of around fifty years, yours is due to fail. It will smell of garlic when it does start to fail, please don`t breath this in as it can cause cancer.

It s a very easy fix to replace the selenium rectifier with a more modern silicon rectifier and heat sink, all the parts are available on fleabay for about ten quid. No, I am not trying to sell you something and no, I can`t come and do the work for you. I live in Northern New South Wales , Australia. I live in the middle of nowhere and rely on a old 1970`s Lister/Brush generator for back-up power. If you rely on your generator for primary power you should replace the selenium rectifier before it fails.

Where in Berkshire do you live? I was unlucky enough to go to school in Pangbourne, Berkshire.

I completely understand your concerns regarding starting handles. Many full grown men have been injured by these, children do not belong anywhere near them. If you are hand starting your generator please regularly grease the shaft and ratchet pin so you don`t get hurt.

Bob

Alexdick

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Hi Alex, there is a very good reason I mentioned modifications, the thing in my original photo, that looks like an old fashioned school radiator, is an old selenium bridge rectifier. They have a life expectancy of around fifty years, yours is due to fail. It will smell of garlic when it does start to fail, please don`t breath this in as it can cause cancer.

It s a very easy fix to replace the selenium rectifier with a more modern silicon rectifier and heat sink, all the parts are available on fleabay for about ten quid. No, I am not trying to sell you something and no, I can`t come and do the work for you. I live in Northern New South Wales , Australia. I live in the middle of nowhere and rely on a old 1970`s Lister/Brush generator for back-up power. If you rely on your generator for primary power you should replace the selenium rectifier before it fails.

Where in Berkshire do you live? I was unlucky enough to go to school in Pangbourne, Berkshire.

I completely understand your concerns regarding starting handles. Many full grown men have been injured by these, children do not belong anywhere near them. If you are hand starting your generator please regularly grease the shaft and ratchet pin so you don`t get hurt.

Bob

Bob

It is a small world! I live about two miles away from Pangbourne!! We know quote a few people there as St Andrews (the school our children were add) is a local feeder for it!
I had better replace the rectifier and no doubt I will need advice on what/how at some stage! I will take some photos this evening. I only use the generator foe backup power if there is a power cut which, thankfully, is a fairly rare occurrence.
Thanks again
Alex

ajaffa1

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Alex, good luck and get back to me if it all goes to sh1t. Quarter past ten at night here so going to bed. will check the forum in the morning.

Bob

Alexdick

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Hi Bob
No joy sadly. I removed the wire off the rheostat and sparked it with the main 12v battery (could I have fried it??) reattached the wire and fires it up. There is obviously a tiny charge getting to the house as the light on the heating control comes on dimly in the house but thatís about it! Maybe I fried the rheostat?!
Alex

ajaffa1

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Hey Alex. Did you flash the auxiliary windings or the rheostat?  When you started it you say there is a dim light that comes on on the heating system, is this Generator a start-o-matic system that will self start when there is a load present? SOM systems use a 12 or 24 volt sensing circuit to trigger start up, this could explain the light on the heating controller. SOM generators have a control panel that looks something like the photo.

If it is not a SOM and you are getting a dim light on your heating controller then the generator is still producing electricity from residual magnetism, in which case I suspect the selenium rectifier is on the blink and not energizing the field windings.

Bob