Hi folks,
I recently posted a question about how to conect an induction motor as a generator. The attached is the design (Top sxetch) I have settled on for various reasons but now I need some spaciffic help / suggestions.

The output voltage is a function of RPM cap size and load.
Freequency is a function of RPM and load due to the variation in slip with load.

Assuming that the freequency is measured and kept constant by varying RPM I will need a method of regulating output voltage.
The conventional method appaers to be switching caps in and out to create steps and trimming the output by applying a PWM controlled balest.

Given that this isnt particulaly efficiant I wondered about controlling the caps directly as per my second sketch.

I may well be way of base and missing something fundamental ....
I would apriciate your comments and suggestions RE this potential concept and anything else you think may be of interest / use.

(FET driver detail is omitted for clarity)

Thanks
Al

 

If the load remains constant over a range then you could just accept the voltage sag at full load. Example: generator reads 122V open circuit(see ballast idea) and 110V full load. A small amount of ballast resistance will also serve as a voltage regulation mech. for light loads that present variable current draw(small drills or music amps)

It could be placed around a CURRENT sensing relay so the ballast is cut out when the genny is loaded up past 30-50 percent.

Something seems strange about the capacitor switching scheme. Is this taken from something that has worked elsewhere?

Alternately you might try using the current sensing relay idea with a switched capacitance to add an extra cap at full load to raise voltage level, or perhaps cutting out some at low loads to bring down voltage level.

 

I dont realy want to use a ballast at all if I can get away with it, the system must be unloaded until it initially reaches it operating voltage anyway.

I have found several docs that advocate a ballast that is PWM controlled, same as your relay suggestion but offering better regulation, and this could be combined with the ability to switch different caps in and out to cope with disperate loads.

What I need some opinions on is weather the caps themselvs would still create an exitation field if they were PWM conrolled as per my sketch. Right now I dont see why it wouldnt work but then I am no electronic engineer either!

Al