Alternator amperage draw when engine is off

[Flyguytki]

I started the big fan up front for the first time this weekend and was running through a bunch of checks to make sure everything was operating like it should. Something that caught my eye was that when the engine was NOT running and the Alternator field switch was moved to on, my Vertical Power would show that the Alternator was drawing 4.4 amps. I am not an Electrical Engineer so I do not know the specifics on how or why this would happen but my question is Should it be happening? Just from how I see it doenst make sense on why it would draw so many amps when it is not doing anything, unless it is powering the Voltage Regulator.

Probably a dumb question but I figured I would ask before I fried something.

-david

[Brantel]

Yes this is normal.

What you are seeing is the field current flowing thru the rotor's windings. This is what turns it into an electromagnet.

[Mike S]

Sounds normal.

When you turn on the field switch, you are feeding power to the field coils and energizing them to create a magnetic field, when the engine turns the guts of the alternator in the is magnetic field, the result is that the alternator is putting out a lot more power than you are feeding into it. You are literally turning the rotational energy into electrical energy.

[Flyguytki]

Originally Posted by Mike S

Sounds normal.

When you turn on the field switch, you are feeding power to the field coils and energizing them to create a magnetic field, when the engine turns the guts of the alternator in the is magnetic field, the result is that the alternator is putting out a lot more power than you are feeding into it. You are literally turning the rotational energy into electrical energy.

Perfect Answer, Thanks alot for the help, It all seemed normal but I figured I would ask.

Thanks again!!!

-david

[BobTurner]

All above is correct. One more little thing, the alternator output is regulated by adjusting the field current. With the engine off but alternator on the regulator senses "need more output" (because there is none!) and so it increases the field current right up to its max.