Alternator Amps ?

[Grizzly]

Alternator Amps

I am with those people who suggest another electrical feed. Lets face it the car companies are not going to use a feed heavier than is need to keep them out of trouble. You start adding high draw equipment (fans, stereo) and you're just asking for trouble. I have always done it for stereos and aux lighting.

Car stereo books are a good place to look for wire sizes and feed systems.

And for safety's sake don't forget to fuse that new feed right at the battery!

 

[MaineShark]

Tom: Close. An alternator does produce electric power. Or, more precisely, it converts mechanical power (hp) into electric power (watts). Just as mechanical power is made up of torque and rpms, electrical power is made up of volts and amps (watts = volts x amps).

You're right that the voltage is regulated, so the change in rpms sees a change in amps, not volts. Which is what we want, in a car.

And increased electrical load will also equate to increased mechanical load. There is no free power. If you draw a watt of power on the electric side, you need to extract a watt of power from the mechanical side. Actually, you need to extract more, since the alternator is not 100% efficient.

Bear that in mind, when adding electrical systems. Something like electric fans instead of a mechanical one is not bad, since you are increasing the electric draw, but decreasing the mechanical draw. But a large stereo or something incrased the draw, without helping out anywhere else. So, the car will have less available horsepower.

TANSTAAFL

Joe

 

[TomOB]

A change in RPM will not change the amperage. The RPM only affects voltage, and due to the regulator, the voltage should stay close to 14. Spinning an electric motor will produce a voltage with or without a circuit or load. I assume the only reason why this wouldnt be true on an alternator woudl be the regulators.
The amperage will depend on the resistance of the car's circuit. Turn on the stereo and although more things are using power, the equivalent resistance of the entire system must drop, so the amperage will go up while the torque int he coils goes up, etc etc. Sit at idle and turn on the headlights, the idle might drop because the alternator is putting a stronger load on the engine.
What may actually be a concern if some are considering putting a 150amp alternator in is the draw on the engine at that kind of current.
150x12-14 (depends what figure you want) 1800-2100w
If you are idling at 750rpm, the 2100w draw will be almost 20lb-ft. Now i dont assume you'd be idling with all that equipment on, but it is a concern....When we add some little device and it adds 5lb-ft we are happy, so a draw at idle of 20lb-ft shoudl be considered.
Notes:
1)draw will probably be higher due to inefficiencies in the alternator
2)Torque will go down as rpm's increase, but the HP loss will be 3+ all around.

 

[rrhayden]

My 2 Cents

You can safely add a larger alt to your vehicle. The alt only supplies as much power as your accessories requires, and charges the battery. The only wire on the vehicle that " could " be an issue, is the charging wire from the alt to the battery. This could cary more power that it can handle due to draw from your new circuits. With regard to added circuits, connect them seperatly through a lead from the battery and a new fuse and your fine.

 

[clar2001]

Tks for keeping it simple for me, and tks to everyone for your input.............Happy Vetting

 

[MaineShark]

A change in RPM will not change the amperage. The RPM only affects voltage, and due to the regulator, the voltage should stay close to 14. Spinning an electric motor will produce a voltage with or without a circuit or load. I assume the only reason why this wouldnt be true on an alternator woudl be the regulators.

I'm afraid I have to disagree. Clamp an ammeter on one, sometime. Idle the car. Then have someone push in the gas pedal. At idle, the alternator doesn't produce much current at all. That's why you need to be careful with accessories, and measure the actual output of the alternator, not the manufacturer's "140 Amp" (or whatever) spec. That number is for some higher rpm. At idle, there is much less.

The amperage will depend on the resistance of the car's circuit. Turn on the stereo and although more things are using power, the equivalent resistance of the entire system must drop, so the amperage will go up while the torque int he coils goes up, etc etc. Sit at idle and turn on the headlights, the idle might drop because the alternator is putting a stronger load on the engine.

You seem to have it correct, here. The current is only produced to power a load. If there is no load on the alternator, the drag against the engine will be fairly small. It's when you start attaching loads that the current needs to go up, so the drag becomes greater. But the maximum current that will be produced is limited by rpm. At low rpms, no matter how much load you attach, you will only produce a certain number of amps. Raise the rpms, and that number goes up.

What may actually be a concern if some are considering putting a 150amp alternator in is the draw on the engine at that kind of current.
150x12-14 (depends what figure you want) 1800-2100w
If you are idling at 750rpm, the 2100w draw will be almost 20lb-ft. Now i dont assume you'd be idling with all that equipment on, but it is a concern....When we add some little device and it adds 5lb-ft we are happy, so a draw at idle of 20lb-ft shoudl be considered.

At idle, the alternator won't produce the full 150A. It won't get up there until you get the rpm's up.

With regard to added circuits, connect them seperatly through a lead from the battery and a new fuse and your fine.

Again, I'd stress that connecting this way will increase the load on the charging wire. The power that is being drawn at the battery terminal has to be replaced, and it will travel in through the alternator. If you want to replace the charging wire, you'll be fine. But if you don't, this kind of connection can over-amp that wire.

Connecting to the main buss in the engine compartment is the correct way to connect an accessory. That's where all the car's electrical systems draw from. That's where the car's system is designed to see electrical loads.

Joe

 

[TomOB]

I think we're going to go back and forth here, as long as no one gets angry, why not.

The beauty of electric motors/generators is the reversability. If only i could crank an engine backwards, feeding it some emissions and water and carbon dioxide, and get gas and air!

Anyway.....electric motors will supply a torque dependent on the current and reach a speed dependent on the voltage. An electric (dc for now) generator like they used to have in cars will generate a voltage based on speed and amperage based on torque.
The magnetic coils in the alternator, the windings, create a magnetic field. I believe it's faraday's law that says that -dB/dT = V, where B is the magnetic field measured in teslas, t is seconds and V is volts. Translation: the negative instantaneous change in magnetic field will equal an induced voltage. If you imagine a magnetic field coming up from the floor, and a loop held n your hand, parrallel to the floor, as you turn it, the amount of "floor" you can see through the loop will change as the angle changes. This is why the voltage graph for a generator will look liek a sine wave.
Anyway, voltage is defined as simply a potential difference...a circuit need not exist and a current need not be induced. However, hook the generator up to a circuit and the voltage will create a current. As the current rises, the amperage in the loop goes up. the flow of electrons will create a secondary magnetic field which i believe is in opposition to the first, creating the load we would see.
Because of this, if i do spin the generator faster, the value of -dB/dT will be of a higher magnitude, creating more voltage, and thereby creating more current. However, the alternator would then be creating 4x the voltage at 3,000 rpm than it does at idle, whihc isnt a pleasant situation for the car. The regulator will reduce or increase the voltage according to the situation. I was wrong earlier, i thnk, about it turning on and off. Rather, it changes the value of the magnetic field in the windings.
This is all DC stuff, but most should apply to AC (slightly different as i believe the current and voltage are out of phase).
If indeed the amperage increases with RPM, it is one of two things: 1)the alternator was unable to produce 14v at idle 2) there is a management system in effect that does not allow the alternator to run at 14v or does not place as large a load on it.
RPMs can only affect voltage, which has nothing* to do with the torque load on the engine.
The idea that more RPM is needed for more Amps goes against casual observation. Ill go back to the headlight example....turn on the headlights and the tach will show less rpm. If more were needed, the rpms would need to go up by the previous logic.
I've tried with with headlights and stereos.

And just to be a picky little one, the drag on the engine does not increase so the alternator can supply more power to it's load, its the decreases restance and accompanying rise in current that creates a greater drag.
tomas

 

[Mike Carter]

Wow, there's a lotta good stuff here, I think it's good to see technical discussion. Really technical, I'm listening and learning.

Thanks
Mike

 

[geekinavette]

Joe,

Take your battery out and see how long the car runs...

Mike

It will run as long as you want it to (if it doesn't you have a problem!). The alternator MUST be able to provide enough current at the proper voltage level to supply all of the electronic systems...including the battery, which is "seen" by the alternator as a resistive load (albeit a very small one when fully charged).

Tom is right also, theoretically the alternator is a constant voltage/variable current supply. An increase in RPM produces an increase in internal AC voltage, NOT an increase in current. The rectifier converts that to a DC voltage and the regulator keeps the output at a (theoretical) constant voltage level.

Bill

 

[TomOB]

i think it should work the other way for a certain amount of time. (taking out the alternator and relying on the battery)

Could the increase in amperage at higher rpm be due to say the spark plugs needing to fire more? I really don't know the exactness of the ignition system. What i do know (if i was taught right) is that the spark doesn't come from the current in a conventional way....the coil will 'charge up' that is, it will have a magnetic field created in it, then when the circuit is broken, the magnetic field collapses, which goes back to the dB/dT thing, except it's collapsing very rapidly, creating immense voltages, enough to arc across the plug gap. I guess that a higher RPM will create more sparks, each one being less powerful than at a lower rpm, but the sum of the sparks should be higher at a higher rpm.
Then again i've heard that the current numbers are very very very small, so i don't know if there'd be a significant difference.

EDIT
Bill, just read your profile, i think you're the man we've been lookin for, who better than an EE to answer these questions? Do you do work in automotive systems too?

 

[Mike Carter]

I think if we mix in ignition systems we went from apples to oranges. Everybody knows E=IR right? The voltage is based upon the resistance of the wire and the collapse of the magnetic field from the coil. I think that's different than the draw of accessories and the storage and recharge capability of a battery and alternator although related. Good thread and technical. Enjoying.

Mike

 

[TomOB]

I was trying to think of a reason why the amperage would rise with higher rpms. I thought of the ignition system.V = IR is only true for a stabilized system. In a coil, as in the ignition system, flicking a switch doesnt mean the current immediately flows. As the current increases, so does the magnetic field, inducing a voltage in the other direction, slowing the rise of current until it becomes asymptotic to the V/R line.
I'm glad you're enjoying it. Since i didn't get a car until january and NYC space and the fact that im a college student has forced me to become a theoretical mechanic. oh well.
tomas

 

[geekinavette]

Tom, as a matter of fact I have been working mostly with automotive systems recently (the ONLY work I've been doing recently...this economy is hurting me!). I made a board to adapt later ECMs into a CFI car: http://misterbill.homeip.net I'm also working on a FLASH-based replacement for the EPROM chip, for people that like to do their own tuning. That one's just about done.

That's an interesting theory about the ignition, but I think Mike hit the nail on the head, the current is probably negligible. Remember the current formula, I=V/R...what's the measureable resistance between a distributor output and ground....infinite (due to the spark plug gap). He is also correct in that the collapsing field of the transformer secondary windings generates the very high voltage for the spark plugs.

 

[TomOB]

Hey! i said it first!

then when the circuit is broken, the magnetic field collapses, which goes back to the dB/dT thing, except it's collapsing very rapidly, creating immense voltages,

Then again i've heard that the current numbers are very very very small

im just kidding!


Bill, I am a business student at upenn but for a while i was considering a dual degree in MechE so i could build cars later in life. It turned out to be a choice between one or the other. I actually am planning a ME independent study into tuning car engines, I need to figure out how to make an electric valve!

 

[geekinavette]

I think that what's coming into play here is the fact that theory and reality are never the same thing. Theoretically, an increase in RPM should NOT have an effect on current output. Would I be surprised to see a small increase if I stuck a current meter in series with the alternator output?...absolutely not. There could be a hundred different explanations...one of which may very well be ignition-related. You may just be dealing with the fact that at idle the alternator is at the lower limit of it's operating range, and just simply isn't operating at it's maximum efficiency. Raise the RPM just a tad and that changes. If only we lived in a perfectly linear world.


Tom, electric valves aren't terribly difficult. Picture a carb or TBI throttle plate...stick a stepper motor on the shaft and you've got an electric valve. An EFI IAC system is also an example of an electric (plunger-type) valve. Kinda crude, but you can picture it. What kind of media does your valve need to flow? Shoot me an Email if you like.

Bill

 

[TomOB]

Oh no no no! i know the electric "open and close or somewhere inbetween" valves are very simple. I'm talking about the "open and close 3,000 a minute ones!"

 

[Jack]

Typical 12v alt has 14.9vdc output. Typical alt has about 85% efficiency. Alts respond to load; when system requires less than alt's max output ... the net hp used will be correspondingly less. 50 amp max alt uses about 1.15 net hp at max load ... 140 amp max alt uses about 3.22 net hp at max load. Belt drives use additional hp ... serpentine being more efficient than v-drive.

BTW ... during lifetime of average CACC member (probably sooner) ... 12 volt systems will become as passe as did 6 volt. Electric brakes ... then electric steering ARE coming ... regardless IC, elec, hybrid, hydrogen etc. Much more efficient to operate new generation components/systems with hi volt-low amp. Don't be surprised to see 60 volt (or more) systems ... Manufacturers & suppliers are presently working toward a new standard. No worry ... we'll still find batteries etc for our old toys.
JACK:gap

 

[TomOB]

I hear 42v is coming in a few years.

 

[Jack]

I hear 42v is coming in a few years.

It may be 42 ... or more. The standard has yet to be established.
JACK:gap

 

[MaineShark]

Theory aside, I think the best bet is to attach a ammeter to the system, and find out.

Joe