Alternator Cutoff

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velotec

Alternator Cutoff

Post by velotec » Thu Oct 14, 2010 3:43 pm

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Hi,
I have a 3 wire alternator (10SI, 12SI, CS130 type). I want to shut off the alternator with the (S)ense or (L)amp connections. I don't want a relay on the output terminal.

Anybody have any experience witht his idea?

Thanks,
Tim

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Re: Alternator Cutoff

Post by whitehendrix » Sat Oct 16, 2010 2:47 am

you're looking to isolate field power then. field excitation current should not be so high as to warrant the need of a relay.. tho a small control relay is not always a bad idea.

the field excitation terminal is the L terminal. the S is an output to the battery.

simply add a SPST toggle switch inline with the L terminal from the hot switched side of the ignition. some of those alternators need a little resistance to that lead as well. not straight 12V power. you'll kill the winding and theres typically a 35-350Ω resistance in there somewhere. normally that resistance is sourced from the idiot light.

the lamp (in series) will provide malfunction or stall indication anyway.. the switch will kill the field current to the stator and render it to.. simply be parasitic loss, really..
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Re: Alternator Cutoff

Post by velotec » Sat Oct 16, 2010 3:33 am

whitehendrix wrote:you're looking to isolate field power then. field excitation current should not be so high as to warrant the need of a relay.. tho a small control relay is not always a bad idea.

the field excitation terminal is the L terminal. the S is an output to the battery.

simply add a SPST toggle switch inline with the L terminal from the hot switched side of the ignition. some of those alternators need a little resistance to that lead as well. not straight 12V power. you'll kill the winding and theres typically a 35-350Ω resistance in there somewhere. normally that resistance is sourced from the idiot light.

the lamp (in series) will provide malfunction or stall indication anyway.. the switch will kill the field current to the stator and render it to.. simply be parasitic loss, really..
Hi,
Thanks for your reply.

Any concerns about hurting the alternator?

Have you tried it?

Cheers,
Tim

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Re: Alternator Cutoff

Post by whitehendrix » Sat Oct 16, 2010 8:14 pm

as long as you run an idiot light or a 100Ω resistor in series with power wire running to the "L" terminal, you'll be fine. no need to run both..


did it a few times here and there on older cars i've completely rewired.. as well, field excitation currents must be limited a wee bit as well. cuts down on inrush amperage and limits the current that can flow thru the coil array.
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Re: Alternator Cutoff

Post by In-Tech » Sat Oct 16, 2010 8:47 pm

whitehendrix, this is interesting to me as I've never thought about how an alternator light works and I'm not following you 100 %. Does the alternator light get one wire coming from ign switch with other side of light grounded, then that same ign switch wire continues on to the excite side of the alternator giving a pull down through the filament? That doesn't seem right either or how would the light go out when the system begins charging. :?

Fill in the blanks for me if you will. :)
Heat is energy, energy is horsepower...but you gotta control the heat.
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Re: Alternator Cutoff

Post by whitehendrix » Sun Oct 17, 2010 6:00 pm

absolutely sir.
the light is in series with the field winding . the field winding generates lines of magnetic flux for the rotor to cut thru and consequently induce current flow in the rotor winding..
theres a slight backvoltage generated once things get going that"fights" the outgoing current from the light. this back voltage backically neutralizes the current flowing thru the bulb and extinguishes it's light output.
the light itself is fed on a positive switched terminal off the ignition switch.
once you turn the key, current flows from the battery, thru the switch, light, entire field coil assembly then to the ground, returning to the battery, completing the circuit.

once the alternator starts spinning, the backvoltage, as mentioned, started being generated in addition to an output current to actually charge the battery. the backvoltage is just a counterpart to the current generating process and happens to"work" for killing the light output from the bulb, coincidentally.
if that doesn't quite help, i will gladly break out the MS Paint skills. lol
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Re: Alternator Cutoff

Post by velotec » Sun Oct 17, 2010 6:17 pm

whitehendrix wrote: " theres a slight backvoltage generated once things get going that"fights" the outgoing current from the light. this back voltage backically neutralizes the current flowing thru the bulb and extinguishes it's light output. "
Whitey,
Slightly confused now.

I want to turn the alternator off above 4000 RPM, with an RPM switch for drag racing. Sounds like maybe the back voltage may keep the alternator generating.

Will it go off at 4000 RPM if I disconnect the L wire, without hurting anything?

Thanks,
Tim

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Re: Alternator Cutoff

Post by whitehendrix » Mon Oct 18, 2010 11:38 pm

the backvoltage is generated my saturation of the core with magnetic flux. the residual magnetism in the core and rotor collapses and re-induces power in the excitation winding.
the backvoltage is slight and the field collapses at almost the speed of light.. so, when you pull the switch open you'll see only a second of residual magnetism collapse and induce current. then it's done.
once the field excitation power is pulled there is nothing actually creating a field anymore.


that field is "shared" more or less.. between everything that can become magnetic in general. once the domains align and the cores saturate they will only stay that way as long as the "main field" ( via the excitation current..) remains present. take it away, it's gone instantly. and so is the mechanical load associated with an alternator.

i see what you're doing now. should work out well. you won't hurt anything at all.
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Re: Alternator Cutoff

Post by velotec » Tue Oct 19, 2010 2:06 am

whitehendrix wrote:the backvoltage is generated my saturation of the core with magnetic flux. the residual magnetism in the core and rotor collapses and re-induces power in the excitation winding.
the backvoltage is slight and the field collapses at almost the speed of light.. so, when you pull the switch open you'll see only a second of residual magnetism collapse and induce current. then it's done.
once the field excitation power is pulled there is nothing actually creating a field anymore.


that field is "shared" more or less.. between everything that can become magnetic in general. once the domains align and the cores saturate they will only stay that way as long as the "main field" ( via the excitation current..) remains present. take it away, it's gone instantly. and so is the mechanical load associated with an alternator.

i see what you're doing now. should work out well. you won't hurt anything at all.
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Re: Alternator Cutoff

Post by In-Tech » Tue Oct 19, 2010 3:04 am

Thanks whitehendrix, I'll run a lamp on my next dyno to verify I understand what I think I understand now. :)
Heat is energy, energy is horsepower...but you gotta control the heat.
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Re: Alternator Cutoff

Post by whitehendrix » Tue Oct 19, 2010 11:28 pm

lol no prob guys.
ya it's some weird shit if you really don't understand the mystery behind eddy currents, core saturation and residual magnetism.
all that is what correlates to a backvoltage. then again. all that garbage is something i've been in to for 15 years.
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Re: Alternator Cutoff

Post by cjperformance » Tue Nov 02, 2010 8:28 pm

velotec wrote:
whitehendrix wrote: " theres a slight backvoltage generated once things get going that"fights" the outgoing current from the light. this back voltage backically neutralizes the current flowing thru the bulb and extinguishes it's light output. "
Whitey,
Slightly confused now.

I want to turn the alternator off above 4000 RPM, with an RPM switch for drag racing. Sounds like maybe the back voltage may keep the alternator generating.

Will it go off at 4000 RPM if I disconnect the L wire, without hurting anything?

Thanks,
Tim

Hi, why would you want to turn the alternator off, are you looking to gain et from less hp drag? I have tried it on several occasions back to back and if you run a good (6al or better) ignition system you will gain nothing, the good ignition systems make more hp(better ignition) with more voltage (hence a lot of poeple feeding them with 16V) any hp gain you get from not running the alternator is burned up by not having as much ignition power. In almost every case running the alternator made a 'slight' improvement with never any loss by running it.
In each of my tests I actually removed the alternator belt so there was also no drag from running just the weight of the alternator internals and the belt/pulley friction. And i did it this way because as you suggested, the alternator will just keep charging(the types i used-bosch internal and external reg) above about 1500rpm anyway. BUT there is no doubt a way to stop them from charging if you know a lot about electrics/electronics, which I dont!
After this I just run the alternator slower , what ratio depends on your engines max rpm. Another good way is to run them off of the tailshaft/diff yoke, way less load on the engine, lower alt speeds and less inertia to possibly affect changes in engine rpm as the alternator just builds rpm linear to vehicle speed.
Craig.

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Re: Alternator Cutoff

Post by velotec » Tue Nov 02, 2010 10:57 pm

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Hi Craig,

Thanks for your reply.

I've got three cooling fans, and an electric water pump running all the time. That's a lot of power - about 45 amps total.

I've run a larger alternator pully (Moroso) and picked up a few hundereths, but the alternator would not keep the batteries up with all the start/stop activity in the lanes.

I figure running on the battery for the few seconds during the race should be fine.

Thant's the idea anyhow.

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Re: Alternator Cutoff

Post by cjperformance » Tue Nov 02, 2010 11:15 pm

Have you tried doing a run with just a freshly charged battiry and no alternator belt fitted to see what happens?, even at 45amp draw a fresh battery will do 1 run for you easily.

How much do you underdrive your alternator?

What max rpm does the engine run to?

What type of alternator is it? Brand, charge rate/specs, amps etc
Craig.

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Re: Alternator Cutoff

Post by velotec » Wed Nov 03, 2010 1:43 am

cjperformance wrote:Have you tried doing a run with just a freshly charged battiry and no alternator belt fitted to see what happens?, even at 45amp draw a fresh battery will do 1 run for you easily.

How much do you underdrive your alternator?

What max rpm does the engine run to?

What type of alternator is it? Brand, charge rate/specs, amps etc
I don't underdrive it now. It won't keep up with the larger alternator pulley on it. The stock pulley on it now is about 2 1/2 inches. The Moroso racing pulley is about 3 1/2 inches (from memory).

The engine runs to 6500 RPM max.

The alternator is a 12SI with 1 wire conversion. I had it rebuilt in Sacramento, at the ET finals, and it will put out 105 amps.

I'm losing about 1/2 volt between the alternator and the battery at full load (fans, pump, lights & etc). Not bad.

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