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Old 09-09-2020, 15:32   #1
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Budget alternator derating? how can I do it?

Can any of you electrical gurus out there tell me how to derate my 55 amp Bosch alternator so it only puts out say 10 amps?
Why i want to do this is we have a 7hp diesel which is overpropped & we cant run the alternator when the battery is low & drive the hull at the same time.
Ok I know I can get a programmable smart regulator but I'm after a budget solution. My electrical knowledge is very limited but I thought I had a brainwave & put a dimmer ( potentiometer ) in the field wire to reduce the current & therefore the field strength. The voltage regulator is on the back of the alternator & has no D+ wire & the field wire is fed from a fed from a relay in the instrument panel.
This didn't seem to work, turning the dimmer knob didnt change the output & the amps showing on the B+ wire were jumping between 0 & 12 according to the clampmeter. I thought turning the dimmer down would stop the output completely but nothing changed going up or down.
I had tested the dimmer on a 12v bulb & it worked fine.
It's rated for 8 amps but it cost $3 frm China so 4 amps mite be closer to the truth.
Battery was at VR control point (14.8V ) so i turned fridge on which has about 6 amp draw when i was trying this trick out.
Of course the way I wired it could be the problem but....
If someone can explain why it wouldn't work that would be nice too.
But I also need a budget way of reducing the alternator output hence power robbing from engine if anyone can help.
We have a switch in the field wire to cut the alternator out but we want some charging if possible.
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Old 09-09-2020, 15:40   #2
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Re: Budget alternator derating? how can I do it?

There used to be a device that was just a big rheostat to control your alternator. The problem with them is you could pound too much in your battery even though it was overheated. I think it was sold by Cruising Equipment Company or some name like that.
I guess it just controlled the voltage sensing to the Alternator...?
Seems you could put a switch in to allow "manual control" with the rheostat (no regulator at all), or switch back to the normal regulator.
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Old 09-09-2020, 16:43   #3
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Re: Budget alternator derating? how can I do it?

Budget way is probably to just change it to a smaller altenator
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Old 09-09-2020, 19:40   #4
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Re: Budget alternator derating? how can I do it?

The real budget way is to use a bike spoke in the line to the battery, acts like a resistor and raises the back voltage the alternator senses. If your alternator has a voltage sense line to the batteries the bike spoke won't work and you need to raise the voltage in the sense line. Placing a voltage in series in the line, say a 1.5 volt battery, might do the trick.
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Old 09-09-2020, 20:04   #5
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Re: Budget alternator derating? how can I do it?

https://www.240turbo.com/AdjustableV....html#external
$50. You pull the regulated brush set off and replace with just a brush set.
Voltage adjustable and hence current or you could put a rheostat on the field wire.

I turn my motor off when I want full power docking etc and it kills the external reg power. (Mark grasser ext reg)

Sorry, didn't read all your post I have a back up brush set if you need one and I can replace from Dave. Also have got a MC614 that I'm not going to use for awhile.
With belt manager de rater. Spendy tho.

Df should be from voltage regulator, not instrument panel and the other brush to ground. Then reversed for external as they are generally "P" type.

AFAIK, which ain't far owing to not flash memory.
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Old 09-09-2020, 20:25   #6
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Re: Budget alternator derating? how can I do it?

I use a transpo540HD external adjustable voltage regulator.
~30$ on fleabay. Not all versions of of this model is adjustable. Look for screws on the cover, in addition to having 'adjustable' in the description.

I got under the potting holding the trim pot to the circuit board and snipped the 3 legs and soldered wires to two of them, run to a remote 2K ohm bourns 10 turn potentiometer.

I can lower the target voltage to have the alternator put out as little amperage as I want. a few tenths of a volt higher than battery voltage and only one or three amps will flow into battery...

I generally do the opposite until the K type thermocouple on my alternator casing gets above 160f, then I back it off to try and keep it in that temp range,or until the battery bank's acceptance cannot overheat it anymore.

I've added a heatsink to its underside of the TP540HD, adjacent to the heat producing transistor and attached a 60mm computer fan, as it will get to 165f itself in about 15 minutes, when fully fielded. Now it's rare that it exceeds 116f.

You can't just put a potentiometer on the field wire, they generally only handle 2 watts max whereas a fully fielded it might be 30 to perhaps 50 watts.

There are other adjustable voltage automotive voltage regulators one can use as well, though I do not know their part numbers offhand. The transpo540HD is for a 80's Ford. Pretty simple to wire up. I can elaborate if desired.
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Old 09-09-2020, 21:13   #7
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by sailorboy1 View Post
Budget way is probably to just change it to a smaller altenator
Ah yes forgot to explain maybe that i want the full output if we need to charge at anchor, just want to reduce hp draw when we are motoring.
Haven't heard of a 10amp alternator which is about what I suspect we need to derate to.
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Old 09-09-2020, 21:17   #8
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by lateral View Post
https://www.240turbo.com/AdjustableV....html#external
$50. You pull the regulated brush set off and replace with just a brush set.
Voltage adjustable and hence current or you could put a rheostat on the field wire.

I turn my motor off when I want full power docking etc and it kills the external reg power. (Mark grasser ext reg)

Sorry, didn't read all your post I have a back up brush set if you need one and I can replace from Dave. Also have got a MC614 that I'm not going to use for awhile.
With belt manager de rater. Spendy tho.

Df should be from voltage regulator, not instrument panel and the other brush to ground. Then reversed for external as they are generally "P" type.

AFAIK, which ain't far owing to not flash memory.
Thanks for that info.
Looks like that would be the bees knees. I wonder what electrickery he's got inside his sealed unit?
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Old 09-09-2020, 21:59   #9
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by Sternwake View Post
I use a transpo540HD external adjustable voltage regulator.
~30$ on fleabay. Not all versions of of this model is adjustable. Look for screws on the cover, in addition to having 'adjustable' in the description.

I got under the potting holding the trim pot to the circuit board and snipped the 3 legs and soldered wires to two of them, run to a remote 2K ohm bourns 10 turn potentiometer.

I can lower the target voltage to have the alternator put out as little amperage as I want. a few tenths of a volt higher than battery voltage and only one or three amps will flow into battery...

I generally do the opposite until the K type thermocouple on my alternator casing gets above 160f, then I back it off to try and keep it in that temp range,or until the battery bank's acceptance cannot overheat it anymore.

I've added a heatsink to its underside of the TP540HD, adjacent to the heat producing transistor and attached a 60mm computer fan, as it will get to 165f itself in about 15 minutes, when fully fielded. Now it's rare that it exceeds 116f.

You can't just put a potentiometer on the field wire, they generally only handle 2 watts max whereas a fully fielded it might be 30 to perhaps 50 watts.

There are other adjustable voltage automotive voltage regulators one can use as well, though I do not know their part numbers offhand. The transpo540HD is for a 80's Ford. Pretty simple to wire up. I can elaborate if desired.
Thanks for this info,
Yes i see there are a few adjustable automotive regulators now that you've pointed it out.
The dimmer I bought for the field wire says its rated for 8 amps at 12v. I didnt burn it out either when I tried it. I think the field wire is 3 amps IIRC
but it didnt work as I wished as I described in OP.
What you have described is what i want to achieve & will wait & see what other ideas pop up before I consult you on wiring method.
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Old 09-09-2020, 22:02   #10
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by Cheechako View Post
There used to be a device that was just a big rheostat to control your alternator. The problem with them is you could pound too much in your battery even though it was overheated. I think it was sold by Cruising Equipment Company or some name like that.
I guess it just controlled the voltage sensing to the Alternator...?
Seems you could put a switch in to allow "manual control" with the rheostat (no regulator at all), or switch back to the normal regulator.
Ok interesting idea thanks.
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Old 09-09-2020, 22:09   #11
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by RaymondR View Post
The real budget way is to use a bike spoke in the line to the battery, acts like a resistor and raises the back voltage the alternator senses. If your alternator has a voltage sense line to the batteries the bike spoke won't work and you need to raise the voltage in the sense line. Placing a voltage in series in the line, say a 1.5 volt battery, might do the trick.
Thanks for the ideas.
I would like something thats easily switchable. Present setup has no battery sense wire. Only 2 wires on alternator, field wire & B+.
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Old 09-09-2020, 23:04   #12
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Re: Budget alternator derating? how can I do it?

How does the alt currently charge everything? It should not be putting out more then 10a while charging the engine battery.

Just disconnect the charge to the house bank when you need power. Ie if you have an acr. Disconnect the ground wire via switch. If you have a 1-2-all switch move it from all to eng batt.

This is easier the screwing around with a fake regulator
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Old 10-09-2020, 01:25   #13
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by Compass790 View Post
Thanks for the ideas.
I would like something thats easily switchable. Present setup has no battery sense wire. Only 2 wires on alternator, field wire & B+.
Where does the field wire go to - the key switch or the charge light? I will assume it is going to the charge light in which case you are SOOL as your alternator is presently internally sensed and internally excited.

However if it goes to battery +ve via the key switch then likely the alternator is externally excited which means you can disable the alternator by putting a switch in this wire.

Rather than derate the alternator to 10 A, you will be derating it to 0 A but it's cheap .

The problem you have trying to derate an alternator to a set current is that you have no control over the output current, you only have control over the output voltage. Even if you control the output to say 12.5V, as soon as you turn on some loads that say draws a total of 20A, the regulator will crank up the field current until rotor field is strong enough for the stator to supply 20A at 12.5V.

It is called a voltage regulator not a current regulator. The name is the give away.
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Old 10-09-2020, 02:45   #14
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by Compass790 View Post
........ the field wire is fed from a fed from a relay in the instrument panel.
................
We have a switch in the field wire to cut the alternator out but we want some charging if possible.
Opps, missed these two important bits of information - sorry!
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Old 10-09-2020, 05:18   #15
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Re: Budget alternator derating? how can I do it?

Quote:
Originally Posted by Compass790 View Post
Thanks for the ideas.
I would like something thats easily switchable. Present setup has no battery sense wire. Only 2 wires on alternator, field wire & B+.
The bike spoke acts as a variable resistor. If you increase the resistance on the B+ wire the voltage seen by the regulator will increase and it will wind back the excitation and lower the output amps. I do it when I need to charge the house batteries using the engine alternator so that I don't burn out the alternator.

A SS bike spoke will cost you about 50 cents. Cut the B+ wire, strip about 1 1/2" of insulation and wind each one around the bike spoke. You then slide the wires back and forward along the spoke until you get the amps where you want them. Be careful as the spoke will get very hot at high amps.

Once you have got the right output amps you can wire a switch in parallel to short out the bike spoke resister to get full amps from the alternator.

These things ain't rocket science.
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