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Topic: Alternator Ripple Output (Read 1970 times) previous topic - next topic

Alternator Ripple Output

mattg's thread about driving an alternator with his exercise bike got me thinking about alternators.
The alternator is three phase with full wave rectification. I see no filter capacitors in the circuit, so the output must be a rippling DC.
There should be six sine wave peaks per RPM of the alt.

Leaving the battery out of the picture for a moment, how far down the slope of the sine wave does it drop before it meets the next sine wave coming up?

I found some info on the Internet that says the ripple will be 4.2% of the output voltage.
The frequency of the ripple will be 6 times the RPM of the alternator and the alternator RPM is what?, 3 or 4 times the RPM of the engine.

If the average DC output is 14.5 volts I expect the peaks must be higher.

Has anyone ever put a scope across the battery with the alternator running to see what it looks like?
 

Alternator Ripple Output

Reply #1 –
The battery would act as a filter and ideal voltage supply resulting in a straight line across the scope. Very little ripple would be observed even on a  small volt/div setting. The only exception would be if a varying high current load were placed on the battery. Battery voltage would dip with load. With the battery (load) removed a measurable dc and ac rms voltage could be obtained. Car alternators are rectified three phase. This is why the old school trick of removing the battery terminal to verify alternator operation is not recommended. Excessive ripple and potential for increased voltage in parallel with the electronics would result.

Alternator Ripple Output

Reply #2 –
We still have weight issue. Unless  What of you engineer's now how?
Old Grey Cat to this.88 Cat, 5.0 HO, CW mounts, mass air, CI custom cam, afr165's, Tmoss worked cobra intake, BBK shorty's,off road h pipe, magnaflow ex. T-5,spec stage 2 clutch, 8.8 373 TC trac loc, che ajustables with bullits on the rear. 11" brakes up front. +

Alternator Ripple Output

Reply #3 –
There is some ripple - you only need to install a cheap stereo to hear it through the speakers. The battery smoothes a lot of it out, but a lead acid battery cannot charge or discharge fast enough. A big ass capacitor between the alternator's output and ground would clean it up a bunch. Those noise eliminator things for cheap stereos are actually nothing more than capacitors.
2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

1988 5.0 Thunderbird :birdsmily: SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣

Alternator Ripple Output

Reply #4 –
There may be slight measurable ripple but I would think the cheap stereo would have to be relativley high current draw to load down a good battery significantly. It may be  more of a switching supply in a cheap stereo that creates noise. The switching frequency can develop harmonics in the RF range. This can overload the RF input and  be detected and amplified as background noise in the speakers. (same principle as ignition noise on the am band). Sometimes various voltages from a switiching supply are 1/2 wave rectified with barely adequate filtering. This may also generate noise on the circuits powered by these voltages.

Alternator Ripple Output

Reply #5 –
But it's not the radio loading down the battery - that would be the headlights, blower, wipers, rear defogger, fuel pump, etc. The cheap-ass radio just allows you to hear it :). Higher quality stereos have more filtering than my $75 CD/MP3/USB/XD reader, I recon :hick:

Case in point: The soon-to-be-replaced cheap stereo in my T-Bird, which has a very annoying whine that changes pitch directly with engine RPM. With the engine running at idle with no accessories on (but of course all the EFI stuff loading up the battery) the whine is steady. If I step on the brake pedal the whine gets louder but maintains the same pitch. If I turn on a signal light the whine's amplitude changes in time with the flasher, but its frequency stays the same. The frequency only changes as the engine RPM's change. Oddly, the stereo's volume control does not affect it - the whine is a steady volume unless I change the electrical load on the car. The fact that the volume does not affect it is the only thing that has thus far prevented me from booting it - I just turn it up until the music drowns out the whine :hick:

The stereo did the same thing in all three vehicles I've had it installed in (the T-Bird, the '96 Cherokee, and a '96 Town Car). One of those noise eliminators helped a bit but not much.

BTW, the Saturn dealer I worked at 13 years ago had an AVR tester that did show the ripple on a screen - it came in handy diagnosing Saturn alternators with a dead phase, which was quite common. You could hear the car coming a mile away, its alternator just howling...

...And speaking of ignition noise on the AM band: If you're ever (un)fortunate enough to find yourself in a cop car, have the cop turn on the strobe lights and switch the car's radio to the AM band. Sounds like a Star Wars space battle :D
2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

1988 5.0 Thunderbird :birdsmily: SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣

Alternator Ripple Output

Reply #6 –
Okay,
I haven't played around much with late model stereos to observe that effect. The most I've heard is the turn signals in my 8 track!

Interesting the scope would show an open winding or diode across the battery. I could see the voltage dropping to say 12.5 V, but I would have thought the battery would have smoothed things out under minimal load conditions

The noise of the strobes in the cop car is likely due to the switching of the strobes. Circuit probably uses a N channel mosfet and pulls the inductive load down with a square wave generating all sorts of electrical noise.