How do I calculate the output current of an alternator

[torbor]

Do alternators obey Ohm’s law? The alternator output voltage is proportional to the rotor excitation current. When reducing rotor excitation, the output voltage drops and the load resistance stays constant. Can why still use ohm law to determine the output current.

Example alternator 480 v / 4 ohm current output is 120 a if why now reduce rotor excitation output voltage drops tu 220 v / 4 ohm output current is 55 a can why calculete like this.

Another example is car alternator have 900 w of pawer tu give that pawer with 13.7 v output current is 65 a and load resistance is 0.21077 this all calculation are based on ohm low.

Can why calculate like this of course every equipment must support rated current

also i understand principle of impedance can why calculate a show i above examples

if why can apply ohm's law tu alternator haw why can determinant output current

i am really confused about this because i am always thought that resistance determine the current same people say they alternator does not follow ohm law ok but haw then calculate current output and load itself

 

[CWatters]

You are correct. You are applying ohm's law to the load resistor.

 

[jim hardy]

Have you studied anything about AC circuits ? Do you know the difference between resistance and inductive reactance ?

There's a concept called "Synchronous Impedance" that you should look up.

Impedance of an alternator is mostly inductive not resistive .

So your Ohm's Law must take into account the phase angles.

To determine synchronous impedance
1..Lock excitation at some value, probably rated for normal operating condition
then
2. With no load connected measure circuit voltage. That's V_internal
3. Short circuit the output and measure how much current flows.

Divide result of EDIT result of 2 by result of 3, volts by amps..
You'll get the "Synchronous Impedance" for that alternator.
That many ohms of inductive reactance in series with V_internal is the equivalent circuit of your alternator.
You can use that to predict alternator behavior if you know what is its excitation.

Remain aware that in automotive applications the voltage regulator adjusts excitation to keep output voltage constant.
So the alternator will appear to not follow Ohm's law .
That is probably what "same people" are telling you, is my guess.

What do you think ?

old jim

 

[cnh1995]

same people say they alternator does not follow ohm law ok but

Who are these people by the way? Your professor(s)? I remember one of my junior college professors saying 'transformer is an exception to Ohm's law.'