On Sat, 25 Jun 2005 12:38:05 +0100, Tony Williams
wrote:

In article ,
Rich Grise wrote:

I just had a thought - I'm not that conversant with induction
alternators, but I've heard the term "slip" - which, driving
the shaft mechanically, faster than the synchronous speed, will
generate power - now, my quesion is, if, say, you're on a
windmill, the amount of slip will increase as the RPM increases,
right? Is there some kind of formula or graph - I'd think that
if the "slip" gets up to, like, 90 or 120 degrees, that your
generator efficiency would go back down somewhere, or am I
letting the drugs interfere with my common sense?

Speed= 0 Speed= Sync Speed= 2xSync
Slip = 1 Slip = 0 Slip = -1
| | |
| |
+Torque | |
| _ \|/ |
| / \ | |
| / \ | |
|_____/ \ | |
|____________\|_____________|
| \ ______
| |\ /
| | \ /
| | \_/
-Torque

/|\ /|\ /|\
|---Motoring--|--Generating-|

Plot a normal Torque-Speed curve from 0 to Sync Speed.

If the shaft is then driven faster than Sync then the
Torque-Speed is a mirror image of the first quadrant.

That applies to grid-connected induction machines where grid frequency
and number of poles in the machine determine the sync speed.

In self excited induction generators which are not grid connected then
sync speed is not constant, it will vary with shaft speed.

Induction machines (motors or generators) can only operate with a
lagging power factor (required to produce rotor field), typically
about 0.8 at full load. The connected "load" must therefore have a
leading power factor to match. In the case of grid connection
generators and distributed power factor correction capacitors provide
the required leading power factor. In the case of a self excited
induction generator enough capacitance must be connected to insure the
required leading power factor.

The required capacitance varies depending on shaft speed and
electrical load. There are large commercial windmill generators out
there which use proprietary controllers to optimize load power factor
for maximum output power at any wind speed within range, rectify the
output to a DC bus and invert to grid frequency. (There was an
article on wind power in Mechanical Engineering magazine a few years
ago which discussed these but I can't find it now).

While the mfgr's are not disclosing any details, I am fairly sure they
are using a PFC type rectifier with the current control set point not
exactly tracking voltage but leading it by an adjustable amount.
Should be reasonable to adapt this strategy to a small battery
charging windmill induction generator, at least for someone with a
solid understanding of PFC methods. I would be inclined to start with
a DSP implementation of a PFC controller like the one that TI provides
sample code for as part of a UPS reference design, but other
approaches are possible.

For the hobbiest who wants to experiment with induction generators
without learning DSP PFC methods, simply connecting varying values of
power factor correction capacitors should provide a means of adjusting
output with changing speed, at least with a resistive load. Not sure
how well this will work with a three phase diode rectifier battery
charger, but it should work well if a PFC battery charger is used.

Sorry if the above is redundant, I missed most of this thread.

Glen