Does anyone know how much horsepower an alternator uses? Howabout an AC unit?

If you put more draw on the current, does the horsepower to run the alternator go up? If you cut down on the current you were using, would the amount of horsepower used decreased?

Someone told me that if you used more current, that you'd use more horsepower. Is this true?

Its absolutely true that it takes more HP to turn the alternator under heavy demand.
As far as how much do the math. 1HP = @750 watts. Figure 14 volts and maybe 80 amps thats 1130 watts, add maybe 20% for frictional loss and you're getting close to about 2 HP.

Ok...now, why?

What is there in an alternator that makes it harder to turn, when under heavy current draw?

It's Magic....




No, it's magnetism....spinning the charged (magnetized) rotor within the charged stator is what creates the output from the alternator...The higher the output the stronger the magnetic force being applied...

Ok, now I'm really not understanding.

An alternator works by spinning wires inside of magnets, and that's how it generates power. Supposedly, the shaft is supposed to be "frictionless", right? The only part which touches is the bearings, because there are no brushes. There is no contact between the magnets and the wires, so where does the resistance come from? Assuming the motion is constant (constant RPM) then why would it be harder to turn when the draw is 40 amps as opposed to 20 amps? The magnets don't change...the field resistance doesn't change...the number of loops of wire doesn't change...so where does the additional loss of horsepower come in?

I understand that it takes more HP when the alt is under heavy load but I am awed when an individuals math conversion skills are higher than mine(which actually includes almost everyone) . when an alt kicks on or if I full field one I can hear the eng slow down.

Lost horsepower from automotive alternators comes from several places, some of them insignificcant, but they all add up, and are manifested mostly as heat

First, there is some loss in the belts and pulleys. A V belt has much more loss, fer instance, than a serpentine system

There is SOME loss just in the rotor current. The power needed to operate the field, while small, IS a loss

So you have

Losses due to friction in the bearings

Losses due to friction in the pulleys/ belts



electrical losses--no machine is 100% efficient. THIS IS WHY alternators run so hot--the heat you feel is not only radiated heat from the engine, but the alternator itself gets hot from internal electrical as well as mechanical losses. Some of these are caused by

Losses due to rotor current--while rotor current is small, it IS a loss


rectifier diodes

Losses in the copper windings

Magnetic losses in the core known as hysteresis

Alternators are not as efficient at some RPM's. I can't quote a "curve" for this, but all AC devices suffer from this, even your stereo.

I'm sure someone with an accurate engine dyno could built a chart showing how these add up

Thank you. Thank you all very much, that was a really good explanation.

I was reading on this board a few months ago, a thread on HHO. I have recently been looking into HHO, and I couldn't understand how the losses suffered in horsepower would offset any gains made by feeding straight oxygen and hydrogen into the airflow.

I mean, I can see how on newer engines it'd be a wash, because the computer would be monitoring the entire process would simply lean or richen the mix as necessary...but I was curious to know what would happen on my '72, which has no computer.

I'm still curious. Anyone want to add their thoughts on this?

I pulled the alt and PS belt on my 12.90 coronet once just for shots and grins,,, only thing it did was make it hard to steer. Still ran 12.90 at 103. Put both back on for the rest of the day.

What a mess. If you don't believe it takes power to turn an alternator shaft, you must not think a motor shaft can develop power. One is the inverse of the other.

There is a really simple way to think of an alternator and that is, it is a power converter. It is a black box that converts kinetic energy into electrical energy. It cannot make more energy than it takes in, actually because we live in an imperfect world it will always put out less electrical energy than the kinetic energy it takes in, the residual losses show up as heat.

It is not magic and there are rules. One horsepower equals 746 watts of electricity. One watt equals one volt times one amp.

So if your "12 volt" system is actually generating 14 volts, then a one amp current will make a power output of 14 watts. Dividing 746 by 14 we get 53.3 amps. So if your system is DRAWING 53.3 amps then the alternator is outputting one horsepower's worrth of electrical energy. Stands to reason that the alternator would have to input more than one horsepower to account for the losses. Let's assume with all the inefficiencies it takes 1.2 horsepower input to produce 1.0 horsepower (746 watts or 53.3 amps @ 14 volts)output. We can calculate the efficiency as (1.0 over 1.2) or 0.8333 or 83.33% efficient.

This is probably not far from reality, I'd say a safe guess would be around 75% overall efficiency. So if your system is drawing 100 amps at 14 volts with 75% overall efficiency it will take exactly 2.50 horsepower from the crankshaft.

Now there is another component to acceleration and that is rotating inertia. An engine has only so much power and the power it takes to accelerate the rotating pieces is not available to accelerate the entire mass of the car. Every little bit hurts, so besides accelerating the alternator in a straight line and carrying the load of the current draw, the engine has to accelerate the rotating part of the alternator along with all the spinning things in and on the car. (yes, and when the car is at steady-state speed no power is lost accelerating all of the spinning things.)

R.

To chime in on the AC compressor side of things,... I had read that our old AC compressors take about 3 horsepower to operate, and that a modern sanden compressor is about 9 times more efficient, which should mean it takes about 1/3 of a horsepower.

I don't have any supporting math.

Tav

Dogdays, you are right, I bow to your wisdom.

I am impressed with your math, especially after I had already read this on Wikipedia:

"Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges; at part load, efficiency is between 50-62% depending on the size of alternator, and varies with alternator speed. In comparison, very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, achieve an efficiency of around only 60%. Larger permanent magnet alternators can achieve much higher efficiency."

Your rough estimate is not very far off from theirs.


(Grunt.)

But I just can't help it. I'm going to have to set it up and give it a shot myself. Don't get me wrong, I admit freely that you all are right, I'm just the type that has to prove it to myself, even after I have all the info and the knowledge of my superiors to draw upon.

On the other hand...I'm not nearly in any sort of all-fired hurry like I was a few months ago.