question about synchronous generators

[JohnCU]

the prime mover is moved by steam or a motor or some other mechanical force and is supplied with a DC field which spins and creates the 3-phase voltage. but, if you need power to make power, how does one start? chicken or the egg?

 

[bobsmith1492]

The power has to come from an external source, which comes from another external source, which comes from... somewhere. That gets back to the problem of the origin of the universe; where did all the energy in the universe initially come from? On the grand scale of things, that's where science hits its limits, and is now stuck trying to work its way around the implication that there is a source outside of the universe itself; the glaring conclusion is "God," but scientists can't take that for an answer to anything because of the implications.

So anyway, you need an external source of energy, which for us on the earth, is basically the sun (which drives weather patterns which leads to wind/water movement for power, growing plants for energy storage to burn for energy, etc.)

 

[spikespiegal]

Entropy

 

[BrownTown]

wow @ bobsmith1492, someone asks a question about how a motor works and you get it into a speech about God. Maybe it would be easier to just say the power comes from burning coal/oil/natural gas, or nuclear breakdown, or wind etc...

IF you want to keep going then be my guest, but thats as far as the technical discussion goes...

 

[bobsmith1492]

He asked a philosophical question, I gave a philosophical answer.

 

[futuristicmonkey]

Originally posted by: JohnCU
the prime mover is moved by steam or a motor or some other mechanical force and is supplied with a DC field which spins and creates the 3-phase voltage. but, if you need power to make power, how does one start? chicken or the egg?

electrochemical source

Research alternators

Edit: And how was this a philosophical question?

 

[CycloWizard]

The general power cycle involves some input energy source (coal, gas, oil, nuclear, wind, water, solar, whatever). This energy is used to heat/boil the water or some other heat transfer agent, which is then passed through a turbine. Thus, chemical potential energy (coal, gas, oil), nuclear energies (nuclear, solar), or kinetic energy (wind, water) are transformed to thermal energy. The turbine expands the steam; thus, the steam does work on the turbine. This work is transformed in to electrical energy via a dynamo. Then, the heat transfer fluid is cooled/condensed and the cycle is repeated.

There are other power cycles, but this is the classical one that is probably used in most power plants, albeit the real cycle is much more complex to improve efficiency.

 

[JohnCU]

i was just wondering where the DC field that excites the generator in order to get the 3 phase electricity going comes from.

or is it a permanent magnet used to generate the 3 phase?

 

[CycloWizard]

Originally posted by: JohnCU
i was just wondering where the DC field that excites the generator in order to get the 3 phase electricity going comes from.

or is it a permanent magnet used to generate the 3 phase?

PowerEngineer could probably answer this for you, but I'm really not that familiar with the electrical portions of the cycle.

 

[BrownTown]

Heh, I'm an intern for power transmission right now, so i can get you every part of the system AFTER the electricity is produced .

To my understanding though generators use permenant magents rotating to create the magnetic flux which creats the electrical current. I'm sure you could google it.

 

[Mark R]

The main generator uses an electromagnet rotor.

The power for this comes from an 'exciter' which is a seperate generator mounted on the same shaft. This makes voltage regulation easier - the voltage can be controlled by changing the current on the main generator rotor. However, you can use a less powerful voltage regulator to change the current on the exciter field winding (this will change the exciter output, which will in turn change the generator output).

If the generator needs to 'self-excite' then some permanent magnets can be included in the exciter, otherwise an alternative power source will be needed to bring the exciter on line. Once the generator is running, the exciter will take power from the main generator.

In very large generators, the exciter will often have its own exciter.

 

[JohnCU]

Originally posted by: Mark R
The main generator uses an electromagnet rotor.

The power for this comes from an 'exciter' which is a seperate generator mounted on the same shaft. This makes voltage regulation easier - the voltage can be controlled by changing the current on the main generator rotor. However, you can use a less powerful voltage regulator to change the current on the exciter field winding (this will change the exciter output, which will in turn change the generator output).

If the generator needs to 'self-excite' then some permanent magnets can be included in the exciter, otherwise an alternative power source will be needed to bring the exciter on line. Once the generator is running, the exciter will take power from the main generator.

In very large generators, the exciter will often have its own exciter.

ahh thanks!

 

[PowerEngineer]

CycloWizard and Mark R are correct (as usual) I'll add a few comments...

The DC field for synchronous generators is produced by running current through a winding (made up of copper bars) in the rotor. The current is produced by the so-called excitation system, which draws its power from the output of the generator (or where ever else the other power plant loads are fed from). The amount of current produced by the excitation system is controlled by a voltage regulator. As the name implies, the voltage regulator tries to adjust excitation to maintain acceptable voltage at the terminals of the generator or at some nearby substation bus.

This brings me around to what might be a misconception on the part of the OP. The electrical power output of the generator is not produced by establishing the DC field; it is produced by moving that field across the windings in the generator's stator (the windings surrounding the rotor). Any movement is opposed by currents induced in the stator winding. The "prime mover" provides the energy required to spin the rotor against the opposing forces of induced current in the stator.

Consider the simple demonstration of Faraday's Law using a permanent magnet and a loop of wire. The magnet does not provide the energy needed to produce the current in the loop, it's you pulling it back and forth that does.

Because the stator coils have inductance, there is some voltage drop as current increases that does slightly change the amount of current needed in the rotor winding. Generator power output, however, is a direct result of the so called phase angle between the rotor excitation and the stator's voltage from the induced currents. The harder you spin the shaft, the bigger the angle, the more power is produced.

Many generators do use what amount to smaller generators on the same shaft to produce the excitation current. Newer units often use DC rectifiers off the plant's AC power supply to do it; these have the advantage of responding faster to voltage fluctuations (but are also more vulnerable to these fluctuations).

 

[JohnCU]

Originally posted by: PowerEngineer
CycloWizard and Mark R are correct (as usual) I'll add a few comments...

The DC field for synchronous generators is produced by running current through a winding (made up of copper bars) in the rotor. The current is produced by the so-called excitation system, which draws its power from the output of the generator (or where ever else the other power plant loads are fed from). The amount of current produced by the excitation system is controlled by a voltage regulator. As the name implies, the voltage regulator tries to adjust excitation to maintain acceptable voltage at the terminals of the generator or at some nearby substation bus.

This brings me around to what might be a misconception on the part of the OP. The electrical power output of the generator is not produced by establishing the DC field; it is produced by moving that field across the windings in the generator's stator (the windings surrounding the rotor). Any movement is opposed by currents induced in the stator winding. The "prime mover" provides the energy required to spin the rotor against the opposing forces of induced current in the stator.

Consider the simple demonstration of Faraday's Law using a permanent magnet and a loop of wire. The magnet does not provide the energy needed to produce the current in the loop, it's you pulling it back and forth that does.

Because the stator coils have inductance, there is some voltage drop as current increases that does slightly change the amount of current needed in the rotor winding. Generator power output, however, is a direct result of the so called phase angle between the rotor excitation and the stator's voltage from the induced currents. The harder you spin the shaft, the bigger the angle, the more power is produced.

Many generators do use what amount to smaller generators on the same shaft to produce the excitation current. Newer units often use DC rectifiers off the plant's AC power supply to do it; these have the advantage of responding faster to voltage fluctuations (but are also more vulnerable to these fluctuations).

yeah, i knew the DC field needs to be moving to induce the voltages, my original thread was poorly written.

i work at a nuke power plant and we have 2 huge generators, the main plant generators, and it just got me thinking that, well, if you need a DC field to generate the AC voltages/currents, what would they use to make the DC field. thanks.

 

[huberm]

often a smaller generator is used to produce the DC. An exciter is similiar to an alternator and is rectified to produce the DC.

 

[BrownTown]

what nuke plant do you work at?

 

[brandonbull]

Some generators have a small amount of permenant magnitism in their excitor. when the excitor spins it starts to generate a small voltage in the main windings which then starts to feed the excitor with a stronger current. rinse and repeat until the generator reaches the set voltage. some generators will have a brush rigging wich acts as a mechanical rectifier to supply a dc voltage to the excitation windings and some use non-mechanical means such as transistors to create a dc voltage. small generator sets have magnets.

 

[OSX]

So that's what flashing the field is?

 

[PowerEngineer]

Generators don't usually use permanent magnets, but some (particularly the rotating exciters for big generators) do rely on residual magnetism (imparted to the rotor by running for hours with the magnetism induced by the exciter windings) to provide the initial voltage in the winding that can then be rectified and feed back into the rotor winding to build up the excitation again.

If the unit is kept out of service for a long time (maybe for days), that residual magnetism may decay away to the point that voltage doesn't build up. When this is the case, you "flash the field" with a short jolt of current to get that initial voltage induced so that the electromagneticly induced field begins to build up.

 

[harrkev]

lotsa hamsters, lotsa wheels. Done.