A BIG DUMMY!

[yachtsmanwilly]

So I was on board the CHEYENNE today running the 30KW 3-71 DC generator to charge the freshly attented to battery bank. We pulled all the connectors for a cleaning on the 110 VDC system with 14 eight volt batteries. I finished up with a 2 hour "mini charge" and ended up with about 105 VDC. Thinking I know it all (haha) I shut the unit down and opened all the knife switches on the DC board. Following this I opened the 200 amp DC main breaker. These two steps are reversed from the book. The main shouldve been opened before the knifes. The main knife made a small arc when I opened it; generator off and no DC equipment running.

I didnt want to open the main with the battery bank possibly back feeding it. The owner seems to think I may have reversed the polarity on the DC generator. We'll know for sure tomorrow with a test run, but honestly, just opening the switch I have my doubts. If it were the opposite and turned on with a load from the BATT bank I could see a problem.

Gonna follow the book for reflashing the unit, but honestly, I hope its not necessary. Any thoughts on this? Theres also a 30 KW Isuzu AC generator which was off and we were on shore power. Whatcha guys with bigger DC units think???

Heres the books say-so on how, but I need to know the why end of it... ws

FLASHING
GENERATOR FIELD
This is done when the generator loses ab冊y to generate or switches polarity
after inadvertent shut down or black out. Wires are located in small box on right
Side of switchboard along with these instructions.
1. Open Generator Circuit Breaker (turn off)
2. Adjust rheostat knob fully clockwise
3. Hook up white wire to white tab on Generator Circuit Breaker
4・ Hook up green wire to green tab in center of rheostat wheel on
back side of switchboard
5. Start the Generator (voltage should read +/- 8V DC
6. Connect Red to Red and Yellow to Yellow at 24v Alarm Batteries
(One to Positive terminal, Other to Negative terminal)
7. Voltage should read + 18V DC- Move Rheostat until Voltage
reads an additional lO VDC (roughly +28V DC)
8. Disconnect ail〃 connections.
9. Adjust Rheostat CCW to 120V DC and load up・

[skycheney]

I'm not sure I understand. If the generator is charging the batteries, how could the polarity be reversed? Were the batteries completely dead? If the polarity of the batteries is correct, then that would seem to show that the generator output is the same, right?

[yachtsmanwilly]

The batteries were at about 85% (theyre old and forklift sized 8VDCs). The theory on this set up is to kill the genny, then open the main breaker, then do some switching on the knife switches. So the main was still closed and I unloaded the board causing a shunt to the generator. In effect, thats how you polarize or reverse polarize the gen end. Ill get some pics of Frankenstein's lab tomorrow. I personally dont think the reverse happened, but I am gonna wait for the owner to test run it. He wants a lesson in repolarizing as well. I guess this is not too unusual. Back in the old car days, I had to polarize generators out of the box. Im not sure if it was a crap shoot or just had to energize it (as in excitation?) to make it put out.

Cheap sluts LOL.... Happy New Year too to everyone!! ws

[skycheney]

Dec 30, 2020 20:18:34 GMT -5 yachtsmanwilly said:

The batteries were at about 85% (theyre old and forklift sized 8VDCs). The theory on this set up is to kill the genny, then open the main breaker, then do some switching on the knife switches. So the main was still closed and I unloaded the board causing a shunt to the generator. In effect, thats how you polarize or reverse polarize the gen end. Ill get some pics of Frankenstein's lab tomorrow. I personally dont think the reverse happened, but I am gonna wait for the owner to test run it. He wants a lesson in repolarizing as well. I guess this is not too unusual. Back in the old car days, I had to polarize generators out of the box. Im not sure if it was a crap shoot or just had to energize it (as in excitation?) to make it put out.

Cheap sluts LOL.... Happy New Year too to everyone!! ws

Ok, so this happened after you ran it and charged the batteries. I see now. Hopefully the polarity is still correct and you don't have to mess with it.

[Avenger]

Probably an obvious question, but have you put a digital meter across anything? That will tell you what pole is where.

[yachtsmanwilly]

Probably an obvious question, but have you put a digital meter across anything? That will tell you what pole is where.

As in below, that is with the unit offline and running??

I am going to hazard a few guesses here: The generator produces a field with permanent magnets? Also the polarity can be tested at the main with it open and the unit running. Theres a voltmeter on the panel when this happened before (poorly marked switches!) and when running offline the VM show a "hard zero", i.e. reading negative of zero by 120 VDC.

The instructions above make it sound fairly common AND not too complicated; We'll see how bad I can fork it up later today LOL... ws

[banned]

I had one of these systems and I'm not understanding. When you say main is that main to power boat or main connecting batteries to generator. Mine (1942 DPC tug) had selectors. So gen directly to boat Gen into batteries and gen off line. So is your concern that you allowed batteries to back feed into generator? I did that one time by failing to switch gen off or to boat before shutting it down. It made noise as if the gen end was acting like an electric motor but it did no harm.



Brian

[yachtsmanwilly]

Quick test waiting on PRIME RIB for dinner! YUMMM... ws

Playing with sizing so you dont have to click on the thumbnail yet not use ALL the allowed storage on this cheap ass forum LOL...Still gotta do it; sorry! At least its bigger on the clicker!

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[yachtsmanwilly]

Careful with that ax Eugene! ws

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