Brushless Exciter and Field Current

[GTstartup]

I have a gap in my knowledge about the interaction of brushless exciters and generator field. I have tried to find some text on line and elsewhere to show me the theory about how field current is generated when an exciter is brushless.

Now I know that the AVR field current is applied to the exciter stator and, due to the rotation of the exciter rotor, transformer action results in an AC current being induced on the exciter field. This in turn is rectifed by rotating diodes and sent to the generator rotor.

But.

The actual exciter current AND voltage is much lower than the generator field current and voltage so this extra power comes from the prime mover.

Can somebody explain the physics of how this is happening?

Primarily,

How does increasing the exciter current by say 10% result in a generator field current increase of 10%? Lets say the exciter current is 20A and the field current 1500A. If I increase the exciter current the field current must increase also. This implies a change in load angle of the generator.

Why does such a small increase in exciter current result in an increase of shaft power? What is the interaction between the increase of exciter current and field current?

Hope you generator gurus can help.

 

[LionelHutz]

In the most basic sense, the DC field current input just creates a fixed magnetic field. That is all it does. If you applied the field current to a stopped machine you would create the magnet field but you would not get any output on the AC leads.

The output power comes from the prime mover moving the rotor windings past the fixed magnetic field or from the prime mover moving the magnetic field past the fixed stator windings. The alternator can be build with the AC coils moving or the DC field moving. In the basic sense, a coil of wire is passed through a magnetic field which induces a voltage in the coil.

If you try to resist this voltage by connecting a load to the coil, then the coil will resist passing through the field or be harder to push through the magnetic field. This resistance to passing through the magnetic field is how the prime mover gets loaded. The prime mover creates or outputs enough power to force the coil to travel through the magnetic field and produce the voltage even when there is a load attached.

So, to address how the output current can increase much more than the field current again.
10% more field current = 10% stronger magnetic field. Now, move the coild past the 10% stronger field at the same speed and you get 10% more output voltage. Connect this 10% higher output voltage to a fixed load and you get 10% more output current. It's not always the same percentage all the way though the system in the real world but let's stick to simple theory for now.

It is the prime mover forcing the coils through the magnetic field that creates the output power so the output power comes from the prime mover. Same exact theory applies to the rotating exciter as to the main alternator. A large machine just has 2 stages, the pilot field current increasing 10% increases the rotating exciter output 10% which forces 10% more current to the main altnerator field which then causes the main alternator to output 10% more voltage which then forces 10% more power to flow into the load.

If you still don't understand this then you really do need a textbook as already suggested. This thread really can't go much further.

Guys, quit trying to discuss how regulators work to maintain the voltage. The basic question is - how does the alternator use the small pilot field current to produce a larger output power?

 

[LionelHutz]

I just went back and read your posts again. I think you are missing the fact that the "exciter" is not a transformer. It is a generator and works exactly like the main unit but just on a much smaller scale. It is also built opposite of the main generator - the stator has the field windings and the rotor has the AC windings.

It uses the pilot field and a little prime mover power to generate a larger output which is recitifed and fed to the main field. The main generator then uses this field and the rest of the prime mover power to generate an even larger output which is fed to the AC power grid.

 

[GTstartup]

Ok as you say, lets start over.

Let me ask just this simple question. It might help to focus my point.

How is exciter output power more than its input power?

Ignore any influence of the AVR due to change in load, voltage etc assume it's at a steady state

For definition of terms

Exciter input = VA DC from AVR (Assume it's constant in this case)

Exciter output = VA DC on the field after the diodes

 

[GTstartup]

Ahah,

I posted before reading Lionels post.

Now I get it. Thanks for pointing out where I was going wrong. The exciter is a generator, driven by the prime mover since it's on the same shaft. The power from the AVR is the "field current" for the exciter, the output of the exciter is the stator output to a fixed (more or less) resistance of the main field.

I am sure that it's been stated more than one above, I just didn't get it. So Warcross et al, ignore my last posts. The penny has dropped.

Thanks!!

 

[LionelHutz]

Just as an FYI, there are synchronous motors built that basically have a rotating 3-phase transformer - well the primary is fixed and the secondary is rotating. The output is fed to rotating rectifiers which feed the motor field. And these don't generate more output power than the input power.

They are built so that the field will remain constant even if the speed of the motor changes so the motor can be used with a VFD and still allow brushless operation. If the field current is constant then the ouput torque of the motor can be constant at all speeds.

The exciter as discussed above would lower the field current as the motor slowed down so it is not nearly as suitable for that application - on a variable torque load it might work.

I guess the same system could be used on a generator and it would be nothing more than straight transformer action but I can think of no advantage to using it in that application.