mook Johnson said:
I've seen those static 3 phase generators and was wondering how they
work.
I would look at using a triple half bridge and convert the single phase
AC to DC and PWM that it through a 1:1:1 transformer.
Is there a simpler solution than that? Some fancy transformer wiring and
AC capacitors or something?
I made a 3 phase converter from an inductor, a capacitor, and some power
resistors. Basically, you use RC and LC values to generate 60 degrees phase
shift lagging and leading. There are other tricks you can do with
transformers. Once you have a phase angle, you can generate any phase you
want.
V1 --------------------+--------------+
C1 | | L1
+--|(----+ +---UUU---+
| |
| ----- ----- |
A O--+-| R1 |--------+------| R2 |---+--O C
----- | -----
|
V2 ----------------------------+--O B
I used something like 70 uF for C1, 100 mH for L1, and 26 ohms for R1 and
R2. I think XC1 = XL1 = R1 = R2. You actually need less than 220 VAC for V1
to V2, so I used a variable autotransformer (variac). I produced 208 VAC
from 120 VAC single phase. I had a big inductor and some motor capacitors,
and 2000 Watt heaters for R1 and R2, so my rig is capable of about 2000
watts, but you have to be able to adjust the resistance to match the load.
For running a three phase motor, it is not too critical, but it does run
smoother when the phases are well balanced.
It is a good project for a simulator like Tina or Spice. I also tried
adding a center tapped autotransformer across A, V1, and C (or perhaps A,
B, and C). If you greatly increase the resistance, the voltage from A to C
approaches infinity! Of course, the capacitor or inductor will break down
first, but I got some good sparks when I tried it. The simulator just
showed some huge voltage build-up when I ran a transient analysis.
Solid state PWM VF drives are really the best way to go, but a static
converter works well for some purposes.
Paul