Car Alternator Output: Voltage Decrease in Heavy Loads

[thkking]

Hi.
I have what seems like an easy question, but I'm having trouble answering it myself.

As say a standard 3-phase car alternator has a heavy load placed on it, the voltage regulator would increase the current into the windings creating a greater magnetic field and thus increasing alternator current output.
But why would voltage decrease through the car as the load became heavy?
It would seem that the voltage would increase (or at least maintain it's maximum regulated level) as alternator current output increased, until the maximum regulated alternator output was reached, and at that point would still be maximum.
Thanks for any info.

 

[berkeman]

It is because any voltage source has an associated output impedance. As the load becomes heavier (lower resistance), the increased current drawn by the load generates a larger voltage drop across the source impedance of the power source (in your case the alternator & voltage regulator), so the output voltage seen at the power supply terminals droops.

The beefier the power supply, the lower the output impedance. And when there is a voltage regulator in series with the power source, the regulator will have a fairly low output impedance (and hold the output voltage pretty steady), up until the point where the regulator goes into current limit (to protect itself from overheating), or until the input voltage to the regulator from the power source drops below the minimum input voltage of the regulator.

 

[thkking]

That makes total sense.
That’s what I was thinking, but it seemed too easy, lol, and my linear thinking was that as the alternator’s nominal output is going to be increased (as you said “beefier”) the impedance is increased…but then again it wouldn’t make sense to purchase a high output alternator if the impedance was increased with design.
Thank you.