Output from one winding only will be a single phase sinusoid, full wave rectified, frequency might be 200Hz to 2Khz, not really sure.
Anyhow the field winding is highly inductive, so field current is very likely to be pretty strong dc with "some" ripple.
Could be true for rotor exciter winding. If current does not drop to zero when sine wave from single stator winding drops to zero, that means your full-wave rectifier acts as freewheeling diode. For original 3-phase configuration, the diodes don't provide freewheeling, rather direct current through windings, one of which is trying to push current in same polarity. If inductance of rotor is strong enough it could push current up faster.
Stator coil I think has inductance is much lower. fewer turns of thicker wire.
As for inductance, any transformer or rotating machinery actually has quite small energy stored in that inductance. It is whatever no-load current and the (reduced) inductance it may have at that current. (transformers I've tested operate significantly into saturation on BH curve, carrying 10x or more the current they would at the inductance measured with low current.)
I would think use of alternator would try to decrease current in rotor. Would need to measure voltage/current of exciter vs. load on alternator output.
Fun fact - funny things happen to magnetic field in alternator based on current output. My Honda CB77 had permanent-magnet 3-phase alternator, switchable for how many windings go to DC system depending on whether lights were on. Equalization (electrolysis) in FLA battery was the voltage regulator. You can guess who loses that tug of war. I tried to make a voltage regulator by switching battery connection with transistors. It erased the permanent magnets.
Should be only hundreds of Hz. Engine RPM and pully diameters would tell you. 3600 RPM is 60 Hz.
Power diodes are so slow that about 1 kHz is the limit, beyond that they continue conducting too long in the reverse direction (per my education, haven't tested.) So 400 Hz systems are OK.
I thought this was true for all silicon junction diodes, and Schottky was needed for high frequency, but here are silicon diodes with some nanoseconds reverse recovery time good for SMPS.
It should work fine regulating all three phases, even though power for the field winding now only comes from one phase.
Should be an interesting thing to look at. If field ripple is excessive, a catch diode across the field winding should fix it.
Never did anything with the three small diodes that normally supply the current to the field winding. Power to the field can come from the lower diode full wave bridge, as mentioned in the previous paragraph.
Alternator has 3 positive power diodes, 3 negative power diodes, 3 small positive diodes in the "triode" for exciter winding.
That uses the 3 negative diodes to complete circuit, so if you rearrange windings, they live at different voltage and that circuit wouldn't work.
I think you could transformer-isolate the three windings with secondaries reassembled as delta to feed a 3-phase rectifier for exciter.
If dash board idiot light is needed to wake it up you'd want to feed that a suitable voltage (1/2 of total should get light functioning and avoid backfeeding light)
Output of the three bridge rectifiers connected in series will add together to produce a pretty constant 56v with very low final ripple voltage.
Because as you note it has the ripple of 6 phase not 3 phase, due to individual bridge rectifiers.
Individual phases will each be going from zero to the peak voltage of one phase which will be a lot higher, but the sum of all three at any instant will still be a constant 56v.
The AVERAGE (not rms) of each must therefore be a third of 56v. Diode drops complicate the whole thing, so ignore that, its not really significant.
I disagree. The vector sum must be 56V (the three windings are not in phase). Each will be half of 56V, 28V. When one is pointed straight up, the other two zigzag out and back 28V at 60 degree angle, making 28V in phase with the first. The two form an equilateral triangle open on one leg.
Power for the (rotor) field winding usually comes from three small extra diodes which only need to carry a very few amps, not the six much larger rectifier diodes. The startup energisation that usually comes from the dashboard alternator light can be replaced with a resistor supplying minimal current from the battery. A minor detail. Often there is enough residual magnetism to self start anyway, but its not always reliable.
It does not take very much.
Right - it isn't the 3 diodes powering the dashboard light, it is battery through ignition key powering dashboard. If that is higher than the 3 diodes, current flows through the light into the rotor's exciter winding.
I messed up a couple times when I had bad (shorted) windings in a "rebuilt" alternator I put in a Saab, tried jumpering some things and alternator tried to charge battery through the "triode", burning it out. After having it "rebuilt" multiple times, I told the shop I suspected windings. He watched with a scope while spinning and observed dropouts, shorting between windings. It was fine after getting rewound (not just replacing triode). I also had electrical failures due to corroded crimped grommet in fuse box. I finally went at that with a wire brush and soldered with strong flux, solving the cooling fan failures. But repeated overheating had caused head crack by then.
Residual magnetism - does alternator work if idiot light is removed or burned out?
Some alternators use this field energisation point as the battery voltage reference, and some alternators run a seperate sensing wire right back to the battery positive battery terminal. That is done to sense the voltage right at the battery, not at the alternator, to eliminate errors due to voltage drop in the alternator to battery connection. With luck our 24v alternator may be one of those with the extra sensing wire.
It might be a feature to look for when purchasing a 24v alternator.
Some of mine I think measure from the "triode". When one or two of the positive power diodes fails, battery no longer gets regulated to full charge. I realized they have less heatsink. All that would be required to make it robust would be temperature sensor on that heatsink cutting back output current. But I had this failure in K2500 with built-in regulator, closer to the heatsink so could have been designed that way but wasn't.
External rectifier with big heasink would be the way to add-on protection.
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