Regulating the field manually in modern alternators

[Cornelius]

I have a 12V, 70A Bosch alternator ripped out of a Ford that i need to manually regulate the field; a simple rheostat, i'm thinking, to reduce the charging amp.

Now, this is a 'one-wire' alternator, so the regulator are integrated in the brush-house, which are fastened with 2 screws.

My question is:
Can i just solder off the regulator and connedct the rheostat there instead?

The regulator itself looks much like a TO-3 transistor, but with 3 legs; the first are connected to ground/negative, the second are connected to one brush, and the third are connected to the other brush, which also are connected via a spring to the field-diodes (?!) inside the alternator.

Should the rheostat be connected between negative and the one brush? (the one with no connection inside the alternator.)

[BruceM]

I modified an MB alternator to use an external linear regulator of my own design.  The stock unit also has an integrated regulator, and one brush is connected to the alternator body through the regulator body.  The alternator body is grounded. 

You'll likely have to cut off the regulator/connections while keeping the regulator/brush body for the brush holder and ground connection.  You should be able to see the grounded brush connection.  You can use an ohm meter to confirm.

Best Wishes,
BruceM

[Cornelius]

After sitting in my chair, staring at the brush holder/regulator for an hour, i'm pretty sure that it's the negative connection the regulator controls, and the spring that connects to one of the brushes are positive. But i'll go out and measure the spring to see if it's connected to ground. (But that wouldn't make sense to me...)

The reason for this need to control the field, would be the freezing, sunless weather for weeks, and i haven't got any charger to charge my 530Ah solar bank except the solar panels.
So i have this alternator, and a 1Hp 230V electric motor with a pulley which i've mounted together, but the motor are too weak for this alternator when my bank measures 12,1V at rest, so i need to control the field so that the alternator doesn't try to push 70A into my bank.

This is just an emergency charge; i do not dare let my bank sit at that low voltage for a prolonged time. Though, it IS a Rolls 4000-series bank, so they're tough...

Edit:
The rheostat i have, are 100ohm; is that enough?

[mike90045]

The rheostat i have, are 100ohm; is that enough?

Maybe.

It's the watts you dissipate in it, that really count.  If you find you need only 10 ohms at 40 watts, a 120V light bulb (vary wattage to get the resistance you need) can sometimes do the trick!  I've used car headlights and brake lights, to make up high power resistors too.  But i have found that drafting pencil leads (the fat ones that need sharpening) are no good for resistors. And they stink bad.

[Cornelius]

Now, here's some news:

I did connect the rheostat to the brush-holder the way i thought it ought to be, and Behold: It worked!

Adjusting the rheostat to balance output Amp and load on the motor (so that i don't overheat the motor), i get around 13A out of the alternator.

I didn't tell the full story earlier. The motor are actually a 3-phase, 1.5kW 230V motor; 1450rpm that i've connected as a 1-phase with a cap. By doing that, the power are reduced pronouncedly. in addition i have pulleys with a 1:3 ratio giving the alternator a speed of 4350rpm unloaded.
When the alternator now giving 13A, i guess that the motor are running close to half speed, making it rather hot; not burning, but uncomfortable to touch...

I also have a kill-a-watt meter connected to the motor, and it shows around 1400W at the current load.

Must be one of the worst efficiency charger in history...

Btw:
The rheostat gets barely lukewarm to the touch on the windings with currents through... I'm guessing it's set at around 10-12ohm. It has a spec of 100ohm, 1.7A, and the physical size of the porcelain ring, on which the windings are wound around, would be around 6" diameter by 2" depth, and 1/2" thickness.

[BruceM]

Cornelius,
Congrats on getting your emergency charger going.

Auto alternator regulators are now typically switching regulators- the  full 12 volt excitation is pulsed  to the alternator coil with more or less pulse width to regulate the output voltage.

MOSFETs are special power transistors that are very efficient for such switching applications and are used almost exclusively in new designs, though in automotive you still see some Bipolar power transistors used, for their durability.

Using MOSFETs, what's known as "low side switching" or switching the ground rail is simpler and more efficient than switching the "high side" or positive rail.  N-channel mosfets can be used directly for low side switching, which have much lower on resistance than P-channel mosfets, so run cooler.  P-channel mosfets can be used directly to switch the high rail, when power levels are lower and on resistance isn't a big issue.  

In high power, high efficiency applications, N-channel mosfets are used exclusively but special high side driver/isolated power converters are used which generate a modest amount of  isolated voltage above (15-20V)  the high side rail to do the high side gate control with N channel mosfets.

I've never seen low side switching  on an alternator regulator but am not surprised; that's how I'd do it.  On your unit, the battery plus connection ("output", +12) ) is connected to one brush, and the regulator is then switching the alternator body (ground (-)) to the other brush to regulate the excitation current and thus the output at 14.2V or whatever.

[EBI-WPO]

Cornelius,
Congrats on "under fire" engineering. I was going to jump in and try to help, but there were too many fires sprouting up here (at work) and my attention was "diverted". Anyway, hope you get the battery bank charged with your setup, and as long as it works, who cares how inefficient it is? You can always do better when not under the gun. (Just keep an extinguisher handy!)

Terry

[Cornelius]

Thanks all for replies.

But for how long was Adam in paradise?

Not long...
I was just outside for a quick check, and alas; i've let out the magic smoke in the capacitor for the motor... It was literally bubbling! Though, the motor was running happily. I stopped it to disconnect the cap; thinking that the motor might just need it for starting, but the cap was beyond life...
Taking a closer look at the label on the cap, it says 320V, 68uF; plenty i thought... But at the bottom it says: 'Motor Starter Capacitor'...

Now, this cap are from the seventies i think, so i guess old age are to blame. (Or is there a difference on a motor cap and a starter cap?)

I do have a cap from my compressor, but it's 25uF, and apparently not enough to start the larger motor... Oh, well... I'll see if i can get hold on another one tomorrow. I did at least get 20 -30amps into my batteries...

Edit:
I DO have an alternative:
I have a 24V, 50A Forklift battery charger, and my old, almost-dead 12V, 504Ah  battery bank sitting close by. I could hook up the old bank in series with my current bank, and use the 24V charger, but then i guess i'll cook the old bank really good. (It's charged, but heavily sulfated...)

[mobile_bob]

you need a motor "run" cap, not a motor "start" cap

start caps overheat and were never made to stay in circuit for much more than a few seconds at most
while the run caps are made to stay online.

as for low side switching, delco has used that method for what seems like forever
not sure of the other manufactures, but i suspect there are others that use the same

bob g

[mike90045]

A running vehicle, a MSW inverter and a IOTA charger is also a workable combo.   Or one of the many brands of small 1KW inverter/generators, and plug the IOTA directly in.

What was spinning the motor that smoked it's cap ?

[Cornelius]

Thanks Bob; in retrospect, i suspected there was a difference between those caps...

Mike:
It's a 3-phase motor (star connected) that i ran on a single phase with a capacitor to the 3'rd phase (to simulate 3-phase). As Bob pointed out, my cap wasn't designed for that purpose...

And enlighten me: what's a IOTA charger? (Living in Norway; english isn't my native language... ) I do have a 2.5kW generator rusting away in my garage...

[BruceM]

Cornelius- Don't try to use the 24 charger with the old battery bank in series.  The usual rules apply- in series or in parallel, all batteries must be the same size and age, and well matched.

Buying a 12V AC charger seems wise, since you seem to have AC power available.

No help to you directly since it's US, but here $167 buys 55 amps of AC charger.  Surely some solar system distributors might have Iota or something similar in Norway.

  http://store.solar-electric.com/bach1.html

[Cornelius]

My thought is that it's better to cook an almost dead bank (which i got for free) and save a new one (at $2000) from sulfating than letting the new one die young due to too deep discharge. I will ofcourse monitor my newer bank closely during that charge...

I might try instead to unwind half of the secondary winding on the transformer so it delivers 12V instead of 24V, since i have no use for that charger anyway...

[mike90045]

IOTA is a brand of very rugged, medium-high output chargers, that have an internal switching supply, so they are frequency, and somewhat voltage independent.  Good for backup charging, but not for long term, unless you buy the extra "Float" voltage regulator.
http://store.solar-electric.com/bach1.html

I have one I run from a 1KW Honeywell inverter generator

[mobile_bob]

assuming you have an automobile and it is 12volt, maybe you can just get a pair of jumper cables and get the bank back up
into reasonable shape before too much time elapses.

even very good batteries don't like sitting longer than a day in a depleted condition before the normal sulfation becomes hard and crystallizes

you really don't want to have to deal with that in my opinion

get em charged by whatever means possible, as soon as possible would seem prudent to me

bob g