Why are most electrical sources, voltage sources?

sighman

On wikipedia:

http://en.wikipedia.org/wiki/Current_source

Under the heading, "Current and voltage source comparison"

It says that most sources of electrical energy are best modeled as voltage sources, that is supplying constant voltage. But why do most sources provide constant voltage as opposed to constant current (i.e. current source) ?

vk6kro

An ideal voltage source produces a constant voltage regardless of the current drawn from it.

If all the devices in a car need 12 volts, for example, then they need 12 volts regardless of what other devices are drawing current.

This is the same for devices in the house. They need the mains voltage to stay the same. Otherwise, the lights would dim every time the refrigerator turned on.

Constant current sources have their uses, but these are pretty specialized.

If you charge a capacitor with a constant current, the voltage across the capacitor increases linearly instead of exponentially.
If you send a constant current through a resistor, then the voltage across the resistor depends on the size of the resistor.

Current transformers used to measure current in mains power sources are current sources and must have a load on them or they will produce very high voltages across their secondaries.

A truly constant current source could be a dangerous thing. A 1 amp current source would produce a million volts across a 1 megohm resistor.
Fortunately, most current sources are just a voltage source with something in series to regulate the current, so they can't generate more voltage than the voltage source can supply.

KingNothing

In theory, many circuits have what is called a "dual" which would have current sources rather than voltage.

In practice, the most obvious reason is that you would need to always have something connected to the source. With voltage sources, you can pull out all the wires and it maintains voltage - with a current source, pulling out all the wires would be analogous to shorting a voltage source.

cabraham

Electrical sources are commonly CVS, constant voltage source, by design, not nature. The ac generators that provide power to our homes & businesses could just as easily be configured for CCS, constant current source operation. CVS works better.

With CVS, the insulation losses are computed by V^2*G, where G is the conductance, or parallel resistance across the conductors. This is very small in comparison to the conduction losses, computed as I^2*R. R is the conductor resistance.

With CVS, the generator is always outputting full voltage, but current varies w/ loading. This keeps losses lower than would be if CCS were used. With CCS, full current is always present, & voltage varies per loading. Losses are much greater w/ CCS mode of operation.

In addition, the power system uses transformers to step up to a very high voltage for transmission, then step down xfmrs are used for local distribution.

With batteries, a similar case exists. Batteries could be designed for CCS operation, but losses are too high, regulation is not as good as CVS. Primary cells like NiMH, Li ion, alkaline, CZn, NiCd, etc., work best in CVS mode. Nuclear batteries, aka "nucells", OTOH, work best in CCS mode. Search under "nucell" for details.

A car alternator can be a CCS or a CVS, likewise for the power plant generator. For CCS mode, spin with constant torque. As loading changes, so does the speed. Torque is related to current, & speed to voltage. Constant torque produces CCS, & voltage/speed vary w/ load.

Spinning the generator/alternator at constant speed produces CVS mode. In addition, constant frequency is obtained due to constant speed. On the power grid, the freq is very precise making synchronous motors run at steady speed.

Did I help?

Claude

sighman

I see, that makes sense, thanks