Generators - Voltage Sources or Current Sources?


by paul2211
Tags: current, generators, sources, voltage
paul2211
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#1
Aug21-13, 07:09 AM
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!
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Baluncore
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#2
Aug21-13, 08:32 AM
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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.
jim hardy
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#3
Aug21-13, 09:05 AM
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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

OmCheeto
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#4
Aug21-13, 09:32 AM
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Generators - Voltage Sources or Current Sources?


Quote Quote by Baluncore View Post
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.

---------------------
* I wonder if this dissuades grammar nazis from entering electrical engineering?
Windadct
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#5
Aug21-13, 11:00 AM
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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.
Baluncore
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#6
Aug21-13, 11:18 AM
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Quote 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.
jim hardy
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#7
Aug21-13, 11:48 AM
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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.
cabraham
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#8
Aug21-13, 04:28 PM
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A generator can be regulated for either - constant voltage or constant current. CV is preferred so utility power generators use that, as well as car alternators. To regulate voltage, the speed must be held constant as well as mag field strength. But when load current occurs, the mag field surrounding the stator winding tends to oppose the mag field from the rotor (field winding). A generator regulated for CV operation has a series inductive reactance on paar with load resistance.

Example, if load resistance R is 100%, or 1.0 per unit ohm, open circuit voltage is 1.0 pu volt, the inductive or synchronous reactance XS, is between 50% and 250%, 0.50 to 2.50 pu ohm. Let's take 1.0 pu ohm for an average value of XS. If Voc (open circuit voltage) is 1.0 volts per unit (V pu), with load resistance of 1.0 ohm pu, we must add the j1.0 ohm pu of reactance to the load. The total impedance is 1.0 + j1.0 = sqrt(2) ohms, with a 45 degree angle, or 1.414 ohm pu magnitude.

The current would then be 0.7071 amp pu, not 1.0 A pu. The terminal voltage at the load is 0.7071 Vpu. If you wish to maintain 1.0 V pu at the terminals, then the field current must be changed to compensate for XS. As load current increases, field current is increased so as to increase field strength to make up for voltage dropped across inductive reactance.

Of course the stator windings have resistance as well, but the voltage drop due to this resistance is but a few percent. The drop due to inductive reactance can be 29% as above, 40%, 50%, or so since inductive reactance is quite high.

A good CVS (constant voltage source) ideally has a very low internal impedance, low resistance and low inductive reactance. But generators, including car units) have a lot of turns wrapped around high permeability ferromagnetic material to get the strong magnetic field, as well as a small air gap between stator and rotor. This means that inductance is very large.

A CVS w/ a large series reactance will not maintain a steady terminal voltage as current changes, so regulation is needed. In addition, a car unit has speed changes as motor speed changes. So the regulator must correct for this as well.

A CCS (constant current source) can be achieved as well. Instead of constant speed, we use constant torque to get constant current. Again, winding resistance, inductance, and for long transmission lines, capacitance, can affect regulation. So regulation is needed with a CCS as well.

CVS is used by power companies for several reasons, as well as in cars. I won't elaborate in order to stay focused on original question. CVS or CCS can be had with genrators, but CVS is intentionally realized for practical reasons.


Claude


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