Alternator Specs

I'm no expert here, but since no one else has responded yet... I suspect the current is the current available to charge a 12V battery. Obviously the exact current will depend on the charge level of the battery.

A short circuit would burn out the diodes and/or the windings.

shorting an alternator "lets the smoke out"

usually rated into a "carbon pile" load which is adjustable, load to 12.0 to 12.4v @ specific rpm.

It has to be a known load, as a short would burn up the alternator.

The common voltage for the Alternator performance is 13.5v. (this is related to the battery)

You can do this increasing the load at each RPM to achieve the 13.5V.

The idea is that 13.5V means max load.

So, you need to have a variable load.

You need also to stabilize the alternator in temperature (usually 15' at each RPM with 13.5v ) before making the measure.

Notice that the performance (amperes @ 13.5V) will decrease with the auto heating.

Automotive alternators always work connected through a rectifier and a regulator to a battery.

Talking of the most common voltage, 12 Volt batteries, the setting of the regulator is 13,5 V dc, required to charge the battery pack.

The field coil of the alternator is always fed al 13,5 V dc. through the regulator at max field allowed intensity, 6 A, for instance

The alternator at idling speed of the internal combustion engine, by which it is driven, very commonly 750 rpm, should be able to meet a demand of, for instance 30 A. This idling speed corresponds very usually to 1500 rpm of the alternator.

At higher rpm of the alternator it is able to give a higher amperage, until it reaches, at 6000 rpm, the maximum output, for instance 90 A, always at 13,5 V dc.

The regulator feeds 6 A the field coil of the alternator the times required per second to produce an output of 13,5 V dc.

At , for instance, 6000 rpm the max. output stabilizes at f.i., 90 A, always at 13,5 V dc because the regulator does no allow more than f.i., 6 A to the field coil.

The rpm- amperage curve shows the maximum regulated intensity output at each speed, at 13,5V dc, and with a max field intensity of f.i., 6A.

If you demand at 1500 rpm more than 30 A, for instance, 40A, the voltage will be less than 13,5 V dc, the alternator will give less than 30A and the remaining amperage will be supplied by the battery, at less than 13,5 V dc, the battery will not charge, and after a certain time, will be discharged, the voltage will drop still more and the system will fall down.

For each intensity or amperage of the curve, 80 A, for i., at 13.5 V dc, you can calculate the resistance of the maximum load by the law of Ohm:

I=E / R, R= E / I.

in our case: R= 13,5 V dc / 80 A = 0,16875 ohms.

The corresponding power output is:

W watts= E. I= 13,5 . 80=1080 watts

These calculations are good enough for the explanation but not exact.

Good luck.

Arturo

GA from me. The OP should be well informed now.

Hello Arturo

Not sure I agree with all of that. On alternators I'm familiar with, the regulator works by in effect putting a variable resistance in the ground side of the field, so while the field supply is always 13.5V, the current varies to maintain regulated voltage. The field current is not controlled to a particular value, for a given output current the field current falls as alternator rpm rises (otherwise the voltage would increase).

The regulator only controls output voltage, the alternator is self-limiting with resect to current (unlike a dynamo where there are separate, usually electromechanical trembling contact regulators for voltage and current (earlier regulators had only one set of contacts but still controlling voltage and current). I've asked the question a few times, including on this forum, why an alternator is self-limiting but a dynamo is not, without a satisfactory reply.

I'm pretty sure the published curve is when connected to a battery of correct nominal voltage, with the voltage regulator bypassed. As there is then no voltage control the volts will rise above 13.5, but not by much in the test duration. It's a bad idea to run it with regulator bypassed at high speed for long as it risks harming the battery.

But an interesting question - what happens if the alternator output is shorted to ground? (before running it up to speed). Does it

a) give zero output, as the field supply is held down to zero volts, hence no excitation.

b) overcurrent/overheat

c) regulate at normal maximum current.

Anybody know?

Codey

Many years ago, I worked for Leece Neville, (don't know if they are still around or not), but there we used an electronic load bank. The typical terminal voltage we used was 14.0 for GM and Chrysler. Ford being a little different asked for 14.4 Volts.

I noticed most of the answers were tied to the battery, but the manufactures allow for voltage drop between the alternator and the battery.

The Current vs Speed curves were to show where the minimum occurred (hopefully below the idling speed of the engine, and the maximum capability. Sizing was done by maximum output and the pully sizing was by current wanted at idle speed..

The newer cars (at least my jeeps) use the computer as part of the regulation loop so it's a bit more sophisticated than when I was a pup.