Synchronous generator variable speed and constant excitation 1

[Hanimlat]

Hi All,

The question is regarding an electrically excited synchronous generator.
Consider that this generator is not connected to the grid and thus not locked with the grid frequency.
This generator is spinning at variable speed and gets constant and similar excitation current
meaning that the excitation current does not change with the speed.
Will this in fact mean that this generator is acting as a permanent magnet generator?

 

[waross]

It means that the frequency and the voltage will be changing with the speed.
Without an AVR the voltage regulation under changing loads will be very poor.
Possible as low as 50%.
There will be operating similarities to a PMG.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Hanimlat]

thank you.
I'm wondering if this kind of application makes sense over a PMSG.
would it be possible to use this kind of installation with a regenerative converter like in wind turbines
that usually use an induction machine or a PMSG.
Also, how would you determine what is the optimal excitation current?
intuitively, I would use the current that produces the rated voltage at rated speed (e.g 11kV@1500RPM)

 

[waross]

An AVR (Automatic Voltage Regulator) adjusts the excitation to hold the voltage at the set point.
At constant speed, there may be a 2:1 excitation current ratio between no load and full load.
The PMGs common in the industrial field are typically used to supply generator excitation current and are driven at a constant speed by the generator shaft. They produce a steady voltage into a known load, (the AVR).
Used at varying speed a simple PMG voltage will be proportional to the speed and the voltage regulation will be poor.
PMG vs PMSG?
PMGs are inherently synchronous.
PMSG may be a more recent term used in the wind turbine field.

Interestingly I once had a PMG with a mechanical voltage regulator.
As the speed increased, a centrifugal mechanism moved the permanent magnet out of the electrical center of the stator.
It was a bicycle generator.
It started generating a useful voltage at a lower speed than simple bicycle generators and did not have the same tendency to burn out headlamps on fast downhill runs.
The technique may not be applicable to anything much larger than a bicycle generator but you are free to consider the design.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Hanimlat]

I'm quite familiar with the PMG configuration on the same shaft with the regular synchronous generator from
my previous experience on geothermal power plants.
PMSG and PMG are the same thing. you are correct that PMSG is a recent term used in the wind turbine field.
At the bottom line, the question is if it's possible to replace the PMSG in the wind turbine with a regular
synchronous generator that runs in variable speed with the same excitation at all speeds (coupled to the grid
with an AC/DC/AC converter)?

 

[waross]

Why would you do that?
Add an AVR and possibly get power produced at slower speeds and no over-voltage at higher speeds.
If you have the capability to adjust the excitation to suit the conditions why not do so.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!