Pick a Number, Any Number Will Do much?” That used to It has since become difficult to assign trates alternator waveforms. The top be an easy question a “one number” standard to all alter- waveform is the three-phase alternat- The question was: ¥ “How much ripple | voltage is too | iy =f to answer when al wall >A ternators were simple and basically the same design. nators. First let’s discuss what ripple volt- age is. Look at Figure 1, which illus- ing current as produced in the stator. The middle one is the same three-phase current after it has been rectified by the FIGURE 1 ALTERNATOR WAVEFORMS 2 PHASE 4.0. GURRCNT AS PRODUGED IN STATOR 2 PHASE A.C. CURRENT RECTIFIED TO FULL WAVE B.C. L RESULTING RIPPLE WAVEFORM | OP ERR RN forum Ow WILT lias Ld rectifier diodes. The diodes basically flip the “reverse” flow current on the bottom of the neutral line to above the line. This lets us use all the current pro- duced in the alternator. Voltage is the height of the wave- form produced by the alternator. No- tice that the height is not consistent. Think of this waveform as having two elements: the “non-varying voltage” with the “varying voltage” stacked on top. This is shown in the bottom diagram. The “varying voltage” is what is commonly called “ripple voltage.” All rectified DC alternators pro- duce ripple voltage. Measuring ripple voltage has historically been used as a way to determine the condition of the alternator. Too much ripple voltage in- dicates an alternator problem. Look at Figure 2. This shows waveforms where there is a defect in the alternator. If you compare the height of the “varying voltage” on these wave- forms to the one in Figure | you can see they are visibly greater. We mea- sure ripple voltage with a voltmeter that has a DC fil- ter, a circuit that blocks the “non- varying voltage” and only passes (and reads) the “varying voltage.” Many test benches and on- the-car testers do this with a “good- bad” scale or “bad diodes” light. Good quality hand JANUARY, 2000 held digital meters usually have a DC filter and read ripple voltage as a spe- cific number value. Now back to the original question, “How much ripple voltage is too much?” As I said that used to be an easy question to answer when alter- nators were simple, basically the same design six-diode rectification and am- perage output. As a rule of thumb any- thing over .50 volt was considered too much ripple. Look at Chart 1 and you’ ll get an idea what this article is all about. The last column on the right shows ripple voltage at maximum alternator amper- age output at 3,000 rpm and 5,000 rpm for several different alternators. They are all good alternators except the last one, which has a known de- fect. Look at the numbers at 5,000 rpm. We go from a low of .433 (be- low the rule of thumb .50) to a high of .928 (85 percent more than .50) for the good alternators. Now look at the last alternator. This is a Delco 1ODN modified for higher output amperage (approxi- mately 100 amps) with one OPEN di- ode, an obvious defect. It only had .903 ripple volts, which is lower than the high of .928 for a good alternator! Both are greater than the rule of thumb of .50 volts. What are some of the things that affect the amount of ripple voltage? Since the alternators measured in Chart 1 were all operated on the same test bench under identically repeated conditions then the answers are: alter- nator design such as the number of rotor poles, rectifier configuration (ad- ditional “wye” tap rectification), rpm, rated alternator amperage output and amperage load. Often overlooked factors are those that are external to the alterna- Page 1 of 2