To the electrically minded

I have a quick electrical question to the EE's out there who know more than me. I have a kz750n-1 spectre with spotty charging. I'm looking to test the stator rotor output. No load I get about 33 VAC output at idle between any two stator leads.

So, in order to isolate my regulator/rectifier from the system, I bought 6 heavy duty diodes from Radioshack and assembled them into a 3 phrase full-rectifier arrangement.

What I need to check is my math. So 33VAC floating phase to phase is acutally 33/sqrt(3) line voltage, so the stator at idle would be outputting approx 20 VAC line. The phases ripple between 88.6% and 100% of this value on the DC output so I should maintain a minimum 18 VDC out of the rectifier bridge I put together.

Since two silcon diodes are always being used to rectify the stators AC output, there will be a .7*2 V drop across the diodes, so I should see a rippling 16-18VDC out of the rectifier bridge at idle.

For any EE out there, is this correct?? More generally, if the measured phase to phase voltage is X VAC at a particular engine speed, then I should measure at least (((X/1.73)*.866) - 1.4) volts out of the full 3-phase bridge rectifier?

Measuring yellow wire to yellow wire, alternator unconnected, I get 21 VAC RMS at idle (1500rpm). That is 60v peak-to-peak on an oscilloscope.

At 3000rpm I get 45 VAC RMS, 64v p-to-p.

At 6000rpm I get 71 VAC RMS, 100v p-to-p.

Of course, it's not very meaningful without a load.

Here are some details on the waveforms:
www.geocities.com/loudgpz/GPZAltntrWaves...rnatorWaveforms.html

I find on larger diodes with larger current, the diode drop is closer to 1 volt per diode.

Post edited by: loudhvx, at: 2007/05/22 12:40

So the multimeter reading at idle between two stator wires is already the VAC RMS, not peak to peak?

If it is RMS, that is the DC equivilent, yes? So after rectification, with 1 volt per diode, I should measure ~ 25 VDC at the rectifier output?

If I use the headlight, 55W blub as a load on the DC side, what should i expect to measure across the rectifier leads in parallel with the blub?

I'm fairly well electrically minds, but this one is testing my knowledge. I just want ot make sure I am getting sufficient output from my stator.

My test equipment includes a DMM Voltmeter, and a length of wire i've calibrated to .001 Ohms to use as a current shunt if need be.

Mockup of setup

DC meters measure average DC voltage.
Good AC meters measure AC RMS voltage.
RMS is not the same as DC average.
Cheap AC meters measure 1.1 times the AC rectified average.

3-phase, rectified, output measured on a DC meter would be about 95% of the peak.

Peak is half of peak-to-peak.

AC RMS voltage (for a sinewave) is .707 times the peak.

Ignoring the diode drops, if the yellow to yellow AC reading is 21vac, you have 60v peak-to-peak, or 30v peak, then the expected DC average at the output of the rectifier (with no load) would be about 28.5v (measured on a DC meter).

The diodes drops with no load will be smaller than 1v, so lets say .6 or .7 for each. The DC meter would then read about 27v. This is at 1500 RPM or so.

With a load, you can make no prediction based on theory alone on the voltage as in your drawing. It depends on the alternator's behavior.

But we know it should easily power the 55w bulb so a reading of 12 to 18v would be expected based on experience. Even higher wouldn't be a surprise. Be careful not to blow the bulb, it happens quickly without a regulator.



Also, as a side note, you cannot use a cheap AC meter to measure AC RMS voltage for non-sinusoidal waveforms. Also, measuring the AC side of the rectifier when the regulator is active (or even if just a battery is connected) is almost meaningless.

Post edited by: loudhvx, at: 2007/05/22 13:58

A good test would be to use two 55w lights in parallel. If you can light them and get 14 volts at 3000~4000 RPM, you know the alternator is good.

If the bulbs blow out then you have a really good alternator, and $10 worth of blown bulbs.

I like your thinking, by the way. You should definitely consider electrical engineering.

Post edited by: loudhvx, at: 2007/05/22 13:44

Physics and EE always were a great hobby for me, even a as a kid.

However, I got sucked into the "pure" side of mathematical logic.

Thanks for the 2 blub idea, that sounds just up my alley. Definitely seems like a legimate way to isolate the load ability without trying to find a way of measureing flux density!!

And I thought Norwegian was a reach.

Patton wrote:

And I thought Norwegian was a reach.

yeah these guys are so far above my head they should be getting a call from nasa ! :blink:

wireman wrote:

Patton wrote:

And I thought Norwegian was a reach.

yeah these guys are so far above my head they should be getting a call from nasa ! :blink:

I was right there till they got to...

AC RMS voltage for non-sinusoidal waveforms

...I think there is a sine wave in there somewhere. :whistle:
:laugh:

I thought "sinusoidal" meant you have a sneeze in the works,,, :laugh:

Everything seems good to me!

It'd be interesting to see it on a scope and what happens to the waveforms once you have the regulator running.

I bet you can pick up one of those humongous CRT Oscilloscopes for real cheap!