# Generators - Voltage Sources or Current Sources?

by paul2211
Tags: current, generators, sources, voltage
 P: 27 I am actually quite confused on how a generator is supposed to be modeled... To add more power to a bus, we put more generators in parallel at the same voltage (which is the bus voltage). Therefore, I would say generators are current sources as more generator are put in parallel, more current is produced at the bus voltage allowing more P = VI to be produced. However, my more experienced co-workers say generators are voltage sources, but didn't explain why. I would think putting multiple voltage sources in parallel do not increase the power put into the bus, so this model would not work. Can you guys on PF help me clear my confusion? Thanks!
 P: 1,074 Why not both? A generator generates an EMF and it generates the current that flows when under load. It becomes a problem of the meaning of words. The voltage generated is proportional to the rate the magnetic field is cut by the windings. As the electrical load increases, the current increases, until the available power, the mechanical structural strength or the resistive heating of the windings reach a limit. The field is adjusted steadily to control the voltage and therefore the current when under varying loads. So, if the speed of rotation is fixed, the magnetic field strength decides the voltage and the load decides the resultant current. That is the simplest model. When a current flows, a small part of the voltage generated is lost across the internal resistance of the windings. As current increases the field must be increased slightly to generate the same effective output voltage. When multiple generators are supplying the same bus, their individual fields are adjusted to control their current and so their share of the load.
 Sci Advisor P: 3,002 Your question really concerns parallel operation of them. Like a battery, a generator has a volts versus amps curve that is typically not flat. So when they are paralleled they share the load, exactly as do water pumps or batteries in parallel.. Have you tried a search on parallel operation of generators (or pumps , or fans) ? It should take you to the term "droop". It's a very basic concept and i'm sure you already know about it but just haven't quite connected that dot.. In the vacuum tube days when speaking of electronic power supplies we called it "regulation" Learning is largely discovering what you already know. old jim
PF Gold
P: 1,361

## Generators - Voltage Sources or Current Sources?

 Quote by Baluncore Why not both? A generator generates an EMF and it generates the current that flows when under load. It becomes a problem of the meaning of words. The voltage generated is proportional to the rate the magnetic field is cut by the windings. As the electrical load increases, the current increases, until the available power, the mechanical structural strength or the resistive heating of the windings reach a limit. The field is adjusted steadily to control the voltage and therefore the current when under varying loads. So, if the speed of rotation is fixed, the magnetic field strength decides the voltage and the load decides the resultant current. That is the simplest model. When a current flows, a small part of the voltage generated is lost across the internal resistance of the windings. As current increases the field must be increased slightly to generate the same effective output voltage. When multiple generators are supplying the same bus, their individual fields are adjusted to control their current and so their share of the load.
Or more visually:

If you have a 1 volt generator, supplying a 1 ohm load, it will deliver 1 amp.
If you have two 1 volt generators in parallel, they will each supply 1/2 amp.
If you have three, they will each supply 1/3 amp.

P=VI=V2/R=I2R

If V and R are both fixed, then P will be fixed, which means I has to change*.

V is fixed, because that's the way generators are wired.

I've never run across a current regulated generator. Though I imagine they do exist, for very specialized applications.

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* I wonder if this dissuades grammar nazis from entering electrical engineering?
 P: 508 Typically a "V source" and "I source" are models, often ideal, so they can be used for modeling, the more complex the model used, the more accurate the simulation is the real world. As for a generator - since the normal operation for a utility generator is constant voltage over a wide range of currents ( including zero current) - a Voltage model is typically best. Of course the Generator has a characteristic impedance - as well as thermal constraints limiting current at the upper end... Per your inquiry - putting multiple generators on a bus does not increase the power in the real world ether - it increases the power AVAILABLE - but for any given voltage and any given load - the power delivered is constant no matter how many generators you have.
P: 1,074
 Quote by Windadct Of course the Generator has a characteristic impedance -
Well, not quite. But it definitely does have an internal impedance.
If you disable regulation and plot it's voltage against current you will see a gentle slope with a V intercept at the open circuit EMF. If it had a “characteristic” impedance then that relationship would be a straight line passing through the origin of the V~I plot. Any further and I would be accused of being too pedantic.
P: 3,002
 I've never run across a current regulated generator. Though I imagine they do exist, for very specialized applications.
Welders is one application.

OP might want to read up on
"Series and Shunt wound DC machine"
"Compound wound DC machine"
"Cumulative and Differential compound wound DC machine"
"Amplidyne"

they're lots of fun. At least I enjoyed them, in my day.