Pick a Number, Any Number Will Do Part Il By Juan C. Grube Last month I dis- — cussed how much § 4 ripple voltage is too | gy. much. Chart | (the —_—— y same as last month) , A showed that ripple a voltage was all over the place. Some alternators were less than a half volt while others were nearly one volt. I listed several factors that affect the amount of ripple voltage. One of those factors was rectifier configuration. By that I mean alternators that have a “Wye” stator with the neutral tap recti- fied. Most people call that an eight- diode alternator. Why would an alternator that has the “wye” tap rectified have greater ripple voltage than one which does not? First glance would lead us to think that the more points rectified, the “smoother” the result. Look again at Chart 1. I’ve marked the alternators with “wye” tap rectification with an arrow. There is a technical explanation (that’s a little complicated for me). It includes words like “the third harmonic is superimposed on the sinusoldal fun- damental wave at three times the fre- quency of the latter.” Try this explanation. Since all three phases are tied together at the “wye” the frequency (of change from posi- tive to negative) is three times that of the individual phases. Look at the waveform at the top right of Figure 1. Note that the peaks and valleys of the “wye”’ waveform do not coincide with the regular three-phase waveforms. One of the things we learned in grade school math is that if we add the posi- tive numbers we must subtract the negative numbers, too. When we add the positive pulse of the “wye” wave- form we get more output. But we also have to subtract the negative wave- form. What results is an odd shaped waveform with a narrower width so that the point of overlap is actually lower, resulting in higher ripple volt- age. The other diagrams on Figure | explain this better than my words. Study them a little. Why would we want a method of rectification that produces more ripple voltage? Because it produces more total amperage output by as much as 15 percent. That’s 15 percent more amps with no more copper or iron, just two more diodes. Look at Chart | at the two FORD EXT. alternators near the bottom of the list. The first one is a standard every- day round-back, externally regulated Ford alternator with the normal six di- odes. The one below is the same al- ternator with two additional diodes (one positive, one negative) connected to the “STA” (WYE) terminal. Note that the amperage output went from 55 amps to 60 amps at the same rpm. No changes had to be made to the rotor or stator to get 10 percent more amps output. Literally every alternator manufac- turer in the world except Delco and Prestolite are building alternators with eight diodes. This means everything we used as a guide for ripple voltage is no longer reliable. The bottom line ques- tion is still the same as last month: What should you do? The answer is still the same as last month. First, wherever possible, mea- sure stator voltage. You only need to measure at one phase connection to be confident. Ifit is half of the output FEBRUARY, 2000 voltage (at any rpm and amperage out- put) you have an alternator that has the right stator-rectifier balance! Then use an oscilloscope. If the pattern is consistent in height you have correct stator-rectifier balance. If the pattern is not consistent in height and the deviation repeats every third phase you have a stator-rectifier defect. That’s my opinion, “exchange” yours. Juan Grube is president of JIMCO, Inc., Raymore, MO. Page 1 of 1