In "Motor/Generator Analysis Redux" I wrote, but nobody replied to:

wrote:
On 10 Jun 2005 15:00:48 -0700, wrote:

wrote in sci.electronics.design and
rec.crafts.metalworking:
I have ordered an LCR meter and a less expensive cap sub box that
should be rated 200 VDC instead of 50 VDC.


Doug


The LCR meter has arrived from Hong Kong.


The inductance of the motor coils in parallel is 112 microhenries.


The resonant capacitance is (confirm?) 62 microfarads.


There are 36 poles on this split-capacitor motor.


The synchronous speed would be (confirm?) 400 rpm.


The motor runs at 225 (rated) rpm.


The motor runs with 175/400 = 44% slip.


I need to run the motor at 400+44% = 576 rpm.


I have to do that because of the impedance protection, right?


So the efficiency will indeed be low.


At 90 pedal rpm, with my existing cog, I will need a


x * 90 / 8 = 576;
x = 576 * 8 / 90 = 51 tooth cog, which is just what I have.


However, if I splice two motors together at 225 rpm, I will have to
recompute.


I recall that damping reduces the apparent frequency of an impulse
driven resonant system, and wonder if the substantial resistance of
this impedance protected motor will reduce the continuously driven
resonant frequency, or whether my recollection only applies to impulse
driven resonant systems.


Yours,


Doug Goncz


I've picked up this thread late so I've probably missed important
bits. However the following comments may be useful.


If I've understood the post correctly you are aiming to use an
impedance protected 36 pole motor as a self excited induction
generator.


Self excited induction generators rely on the tiny residual
pattern of magnetisation of the rotor being reinforced by the current
flowing in the near resonant stator winding circuit. It has to be
operating close to resonance for the current build up to be large
enough to reinforce the rotor field pattern. It has to be on the
capacitative side of resonance to permit the phase angle of the stator
current to reinforce the rotor field pattern.


It is a positive feedback regenerative system and on a large
efficient motor the output can build up to far beyond its rated motor
power until limited by magnetic saturation. This effect is sometimes
used for regenerative braking of single and three phase motors and can
result in a spectacularly short stopping time.


With a care and control of speed, self excited induction
generator systems are possible but they're pretty touchy devices.
If you're unlucky with the the rotor iron they may not retain enough
initial magnetism to enable the output to build up (manufacturers
strive to reduce this because it degrades the efficiency when used as
a motor) Also it must use a reasonably efficient motor for the
magnetic feedback to exceed the system losses.


Efficiency is your major problem. An impedance protected motor
means a motor with deliberately large leakage inductance so that the
impedance of this inductance limits the current that flows when the
motor is stalled or overloaded. With limited stalled current the
starting torque (already poor because it is a capacitor run machine)
has to be boosted by the use of a high resistance rotor and this
results in your observed very high slip speed. Even if there were no
other losses of any kind the motor efficiency could not be any better
than the % synchronous speed - 56%. With other losses taken into
account the motor efficiency is probably no better than 40%.


With the uH to mH correction your sums are OK but this level
of efficiency is too low for a succesful induction generator.


Jim



Many thanks to Jim and other contributors to the thread
"Motor/Generator Analysis".

I have put a lot of money and time into this, and I want to give it my
best shot, but I don't want to whip a dead horse, so to say.


Frankly, I don't understand magnetics. At least not as I understand
resonance. I'm an amateur musician; I understand resonance and know a
little about phase shifts near the peak. I do understand that because
the slope of the curve is negative on the high-frequency (capacitative)

side of resonance, loading of the generator, within limits, will result

in additional power to meet the load.


But B x I makes my head spin. I'm fine in 3 dimensions. So I get some
of it. And I get that in the cylindrical coordinate system, B and I can

be locally orthogonal, and can vary in time, with phase shifts, while
being wrapped into a connected topology. I just don't feel that the way

I feel resonances. It's not intuitive.


Would replacing the rotor "windings" with copper wire or bus bar
(easy), and rewinding the stator with bigger wire (hard) have any
chance at all of working together by lowering the leakage inductance
and rotor resistance to allow resonance?


That's my best question; is there any hope at all?


This is a one-off demo, not a production prototype!


Yours,


Doug Goncz
Replikon Research
Falls Church, VA 22044-0394