DIY dc generators discussion !

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DIY dc generators discussion !

Postby jaahn » Tue Mar 14, 2017 9:33 am

Hi :)
On another thread here there was some discussion about building a DC generator out of an alternator. For higher voltages than 12 V, and suitable alternators to use. A topic that might interest some people who wish to DIY and get their hands dirty someone said :roll:
I did a bit of testing quite some years ago and did go on to build several 'generators' for some friends who were off the grid. If any one was interested we could have some discussion here to spread the information. A generator for a 2500AH battery bank is no toy :lol: and in the days before cheap solar panels it was used regularly as the main charging using home made biodiesel.
Cheere Jaahn
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Re: DIY dc generators discussion !

Postby Gordon-Loomberah » Tue Mar 14, 2017 12:04 pm

I rewound a 12V car alternator years ago to use on a wind turbine I built to charge a 24V battery. It worked ok until a lightning strike blew some diodes. I then mounted some diodes externally, but upgraded to a larger alternator on a larger turbine, so it didn't get any more use.

I've got a spare ex US military 220A 27/28/29V unit if you need one ;) The 29V setting is for arctic, and the 27V is for the tropics.
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Re: DIY dc generators discussion !

Postby jaahn » Wed Mar 15, 2017 9:12 am

Hi :D
That's some serious amp genny Gordon. Bullet proof perhaps ;)
To kick off the discussion here are a couple of things I have gleaned. A 12V car alternator has no problem achieving a higher voltage to charge 24V batteries and possibly even higher. If you disconnect the regulator and drive it at a suitable speed EG 6000+ RPM it can output well into the hundred + volts with 12v excitation.

People have used them in the past to produce 110V AC and they do OK. But using older style alternators. High cycle AC though. :lol:

HOWEVER all modern alternators EG from about 2000 or so depending on the manufacturer will possibly have Zenner Diodes fitted as the main power rectifier diodes. These will zenner and conduct at about 20volts so you will burn them out if you run at a higher voltage. This is not generally known and has been introduced to protect the system from surges. I was told that some manufacturers drove new cars to storage without batteries using this to regulate the voltage. Not sure of that fact ??

However the older 12V alternators can run at 24v charging voltages quite reliably without problems. I have no experience of higher voltages but they will certainly get up to the voltages OK. The current is self limited in these alternators. The power required to drive the alternators is real and is more than calculated so the process is less than 50% efficient. :idea:
A 5HP petrol can struggle to do 50A at 30V unless geared right. The excitation will need to be switched off to get started and enable the motor to get up to speed. The exicitation is the next installment.
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Re: DIY dc generators discussion !

Postby melmik » Wed Mar 15, 2017 12:07 pm

You are the type of guys my wife needs to work out what I should do with the 'potentially useful' stuff in my shed. Like a beautiful 240v motor from a dead Bosch washing machine. Spins at ridiculous speeds and very stable and quiet.
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Re: DIY dc generators discussion !

Postby Gordon-Loomberah » Wed Mar 15, 2017 12:07 pm

jaahn wrote:People have used them in the past to produce 110V AC and they do OK.


What did they do with the unregulated variable voltage and frequency (varies with load) 3phase AC output???

However the older 12V alternators can run at 24v charging voltages quite reliably without problems.


Running too high a voltage through the field coil in the rotor causes an unstable output, so if you put a suitable resistor in series with the field winding, to keep the voltage near the designed for 12-15V, it will perform much better. Coiled nichrome wire works well, as it has to dissipate a fair amount of heat.
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Re: DIY dc generators discussion !

Postby melmik » Wed Mar 15, 2017 1:20 pm

Probably getting ahead too far here. With the regulator removed and at lower than car standard or designed minimum rpm (basically before it reaches its self sustaining speed) would the current be unregulated (dependent on battery state and any coil resistances?) And therefore potentially damaging? Or would a resistor between the battery and the field coil solve that?

Edit: sorry Gordon, got distracted before saw your last post. Answers my question. Interestingly I've been using nichrome wire for a completely different purpose. Pottery. Use it to hang beads in a kiln. Wire takes the kiln heat and beads can be completely glazed.
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Re: DIY dc generators discussion !

Postby Gordon-Loomberah » Wed Mar 15, 2017 1:44 pm

With the regulator removed, output is directly related to alternator RPM, which will be zero until the revs are high enough to equal battery voltage. In bulk charging it wont matter much, but once you are into the absorb stage (constant voltage), you could do some damage if it goes too high.

Adding a resistor in the field winding circuit will control output, make it a variable one if you want some control. An alligator clip that can be attached along a section of nichrome works well- just increase or decrease the resistance in series with the field winding until you acheive the desired output. Old style regulators acheived regulation with a mechanical relay arrangement that operated a bit like PWM- you adjusted the spring pressure on an arm to set the votlage, and it buzzed away. New electronic ones use some sort of jiggery pokery to achieve the same result ;)
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Re: DIY dc generators discussion !

Postby melmik » Wed Mar 15, 2017 2:06 pm

That variable resistor is exactly the rheostat in an electronics kit i had 40 years ago. Good things dont change much.
You'd need a switch in there as well, to stop battery drain to the field windings when there is no action from say the wind or water wheels.
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Re: DIY dc generators discussion !

Postby Gordon-Loomberah » Wed Mar 15, 2017 2:34 pm

melmik wrote:You'd need a switch in there as well, to stop battery drain to the field windings when there is no action from say the wind or water wheels.


Only if you are supplying power to the field winding from the battery. Take power it from the rectified alternator output and it wont drain the battery :) If there is enough residual magnetism in the rotor it will self-excite, but if not, just dab full battery voltage onto the field winding to get it going... that's how I did it for many years with a 24V truck alternator outputting 40V to an MPPT controller to charge a 24V battery. Much better to do it like that than forget to turn off the switch and flatten the battery!
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Re: DIY dc generators discussion !

Postby jaahn » Fri Mar 17, 2017 11:19 am

Hi :)
Some discussion ensued !!
Looking at my old notes which are a bit faded and my writing has always been bad :cry:
Using an old Bosch 12V 60A alternator with 12V excitation these voltages were achieved;
engine RPM/voltage; 1200/56v, 1500/65v, 2000/90v, 2300/100v, 3000/140v, 3600/170v
The alternator was driven at about 2x the engine RPM. No load. NOTE dangerous voltages produced !!!

Using normal 12v Bosch alternators the excitation can be supplied by 3 internal diodes provided for that purpose, there is a brass finger which makes contact with the regulator for this supply. I generally used an older style brush holder off the even older external regulator alternators which was a simple brush holder and connections. Then the regulator did not get in the way. Just manual control of the exciter current.

To control the exicitation I made up a simple board with quite a few 5 W resistors and plug in sockets. Not much cost at DS at the time but probably more than a length of nicrome wire :roll: . As I was designing for someone else to use I tried to make it easy. However I found that he just left it at full voltage and after quite a long time the rotor winding failed in 24V truck alternator. So continuous running full voltage exicitation at full output power is not recommended :evil: However by that time it was superceeded by another bigger generator. :roll:

For the big battery bank we got a 24V Bosch 50 A truck alternator, powered by a 8 HP B&S motor which ran OK at governed speed of around 3600RPM and the alternator put out up to about 60 A continuous for many hours into the 2500AHr batteries. The excitation resistance was fixed minimum ~8 ohm + ~8+ variable. Pulley size was 8" to 5" pulleys at ~1.6/1 over driven. Manual control is OK as the big battery bank voltage moves up slowly at 50/60A some a check every hour is OK.

Next generator was a DC welder of 250A. Hear how he burnt that out :twisted: !
Jaahn
PS jaahn wrote: People have used them in the past to produce 110V AC and they do OK.
What did they do with the unregulated variable voltage and frequency (varies with load) 3phase AC output???

This was out in the islands 20 years ago and I believe they used it for lights and heating which is ok on that raw power !
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