The capacitor I was referring to was on 3600 RPM units.
From what is see regarding these capacitors, it looks like they are being used in a resonant circuit and must be the more expensive AC rated capacitor.
The following is some stuff I found with a few minutes on google regarding generators and capacitors:
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My assumption is that, if it is resonant at 60 Hz, similarly to a voltage regulating transformer, the residual magnetism in the rotor will set up a 60 Hz weak pulsating field when the unit comes up to speed.
That will "pump" the resonator, inducing a current. The resonator will have a larger current in it, due to resonant energy storage, than you would expect just from the weak residual field.
That current must then induce a current in the rotating field windings, which pumps the resonator harder, inducing more in the rotating windings. That cycle continues until the resonator 'clips" by local saturation (assuming it is like a Sola as gar suggests).
The rotating field also induces voltage in the output windings. Since the induced field current is dependent on the resonator, the limiting of resonator energy also regulates the output.
I would expect the output windings are coupled magnetically to the resonator, which provides the feedback of "volts per turn" necessary to regulate the output voltage.
At full voltage, the "clipping" of the resonator limits the induced voltage in the field, and so the output voltage.
It sounds like a perpetual motion machine, until you realize that the input is the mechanical energy of turning the rotor. That rotates the field and pumps the resonator.
Because the resonator is somewhat decoupled from the output, it isn't so dependent on the output load as say a shunt DC generator. Those won't "build up" with too heavy a load on them because the load steals too much current and the field does not get enough current to have positive feedback and build up.
That all means there will be a "best value" for the cap. Any other value will give lesser performance.
I said the frequency is set by RPM, and that would be so.... but per my thoughts above, the wrong value cap would tend to make the whole system work best at a different frequency....
So too big would cause the best frequency to drop, and too small causes it to go up, by the square root of the cap value error.
Thus, too big a cap, and it could be a 50, or 55 Hz generator, with max output at a lower RPM and lower frequency than 60.
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08-01-2006, 10:29 PM #14 jmead
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Well said J Tiers, I think the judges'll buy that. By your description it seems like for another dollar or two for a coil they could have made a really accurate resonating circuit but that would be less profit I suppose.
I should've studied harder in school.
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08-02-2006, 11:14 AM #15 jim rozen
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There *is* a coil there. It's the stator
winding. Hence my suggestion to just measure
that inductance with an inexpensive L meter.
Also you don't want that ckt to have too high
a Q. Because the speed on the motor varies,
it should probably be less then 5 or so.
Jim
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08-02-2006, 01:03 PM #16 440roadrunner
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Gar is absolutely correct. That would be a "resonating" cap and is undoubtedly used for voltage regulation.
The value is going to be critical--I have no idea how to calculate, other than trial and error
It needs to be an AC cap, such as a run cap
I doubt that it has anything to do with excitation
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08-02-2006, 01:03 PM #17 JST
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That works, but so will setting it for resonance (largest voltage) using a small transformer and resistor as a source.
Or trying values and checking for full output at correct RPM/ frequency. An electric clock with a sweep second hand will quickly ( a couple minutes) show if frequency is far enough out to be a problem.
The "Q" will be already set, since it is strongly dependent on the cap value. The cap in turn can have only one value to resonate at the correct frequency with the existing coil.
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08-02-2006, 01:15 PM #18 chuckey
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1, Apologies for my first post, what I said is a load of b*****ks, should have looked at the circuit closer. Another thought, I have heard that cheap generators tend to produce a wave form that is a series of steps, rather then a sinewave. Could be a tuned circuit to notch out the third harmonic of 60 hz in an attempt to improve the waveform. According to my maths 24Mfd will tune with .3H at 60 hz, so it will take .3/9 = .033 H to tune to 180hz. To measure the inductance without a bridge, get a bell transformer and a known resistor,say 10k ohms, connect the reistor in series with the bell transformer secondary and the sub coil. Energise the transformer. Measure the voltage across the subcoil (= x ) and the resistor (=y). Providing y is much bigger the x, the impedance of the coil is x/y times 10k ohms. The inductance (at 60Hz) is the impedance / 2 x 3.14 x 60. If the voltage measured at x is too small for your meter, use a 100 ohm resistor.
Frank
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08-02-2006, 07:43 PM #19 gnorbury
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What you have there is a "mase" generator. The capacitor is there to provide feedback/resonate the auxiliary windings and in turn induce a magnetic field in the rotor windings, and hence usable power in the stator windings.
The following comments are taken from "The Marine Electrical & Electronics Bible" by John C. Payne:
1) While rotating, the residual magnetism and permanent magnets induce a voltage into the auxiliary windings for excitation. This voltage is fed to the capacitor, which generates capacitive current in the circuit.
2) The capacitive current creates a magnetic field, which is rectified by the diode, supplying a DC current to the induction winding. This generates a rotating magnetic field for generation of output.
Varing the size of the capacitor may give you some measure of control over voltage regulation, especially loaded vs unloaded.
YMMV
Graham
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Anyhow, 3600 RPM units DO have a capacitor.
I am sure inverter units also have capacitors in them but for different reasons, they may (or may not) be of the construction that can withstand cold temperature.
There are capacitors that can withstand extreme temperatures, but in general they are more expensive, and they are usually made in small values. A power circuit like the circuit in a 3600 RPM generator would most likely require a large value capacitor and therefore it probably would not be of the construction that is made for extreme temperatures.
Also be aware if this capacitor is rated for AC voltage, you can not replace it with a lower cost DC only capacitor if it goes bad.
