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DrElch
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Joined: October 22, 2007
Posts: 82
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 Posted: Thu Oct 08, 2015 12:54 am Post subject: Alternating Current in system while alternator is charging |
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Well, how does the old saying go? If you can't fix it with a hammer, you've got an electrical problem. So here it is...
I've got a type IV engine 2.0 with the 55A alternator.
A while ago my generator lamp started flickering and turned solid a few miles later.
The alternator output current was way below 13v, so I suspected the brushes.
I replaced them, but figured that the used brushes were nearly the same length as the new ones. and didn't look too bad.
However the ceramic brushholder was broken and I fixed it.
While I was there, I checked the whole alternator. I checked all diodes: All worked as expected. Stator and rotator also had no short circuits or interruptions. Everything just seemed fine.
So I put that thing back in and this time my current was somewhat better. 13.5V charging and 13V with headlights, wiper, emergency lights on. Not perfect, but enough for the gen lamp to shut up.
However I got curios and asked myself why things are still not perfect.
So I checked the B+ cable for alternating current. My multimeter read 30V alternating current, which is super strange. Where the hell does that one come frome?
If the diodes where broken i'd say ok, but they are fine, all 9 of them.
Could this be a regulator issue?
Any other cases where AC might leak into the system?
Thanks,
Michael |
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Wildthings
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Joined: March 13, 2005
Posts: 50352
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| Posted: Thu Oct 08, 2015 2:06 am Post subject: |
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| You meter probably always reads that there is alternating current when there is actually just DC current. Also since the DC current is not super steady then the variation in the DC current will read as AC. I would not have thought that it would read as high as 30V though. |
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DrElch
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Joined: October 22, 2007
Posts: 82
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| Posted: Thu Oct 08, 2015 4:04 am Post subject: |
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So looking for AC with a multimeter is stupid and leads to mistakes?
Is there a way I could find this out though?
I have talked to a alternator expert and he told me to check for AC without being any more specific.
he said that 0.5V AC should be the threshold for a healthy system.
But we can agree that 13.5V is too low when the alternator is running or is this...well still ok? |
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morymob
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Joined: November 09, 2007
Posts: 4683
Location: east-tn
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| Posted: Thu Oct 08, 2015 4:19 am Post subject: |
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| Should be 14.2-14.4 to fully chg batt, your alt regulator probably going bad or some windings in field leaking current &about to short. If u trust your local flaps with his 'magic'load tester, it's free it will tell if 4 sure alt is not up to par. Is battry getting old, may not be able to get to a norm full chg, as usual no warranty offered at this time. |
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sjbartnik
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Joined: September 01, 2011
Posts: 5998
Location: Brooklyn
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| Posted: Thu Oct 08, 2015 6:55 am Post subject: Re: Alternating Current in system while alternator is chargi |
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| DrElch wrote: |
| The alternator output current was way below 13v, so I suspected the brushes. |
That's voltage, not current.
| Quote: |
| Stator and rotator |
rotor
| Quote: |
So I checked the B+ cable for alternating current. My multimeter read 30V alternating current, which is super strange. Where the hell does that one come frome?
If the diodes where broken i'd say ok, but they are fine, all 9 of them.
Could this be a regulator issue?
Any other cases where AC might leak into the system? |
AC is not getting into the system. You are setting your meter to read AC in a DC system so of course you are getting unreliable info from your meter because it's expecting to see AC and you are feeding it DC.
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sjbartnik
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Joined: September 01, 2011
Posts: 5998
Location: Brooklyn
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| Posted: Thu Oct 08, 2015 6:56 am Post subject: |
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| DrElch wrote: |
| So looking for AC with a multimeter is stupid and leads to mistakes? |
In a DC system, yes.
| Quote: |
Is there a way I could find this out though?
I have talked to a alternator expert and he told me to check for AC without being any more specific. |
If you're going to check your alternator's AC output you have to do it before the rectifier/regulator.
| Quote: |
But we can agree that 13.5V is too low when the alternator is running or is this...well still ok? |
It's pretty close to spec, I think 13.8v is what you're ideally looking for.
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Wildthings
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Joined: March 13, 2005
Posts: 50352
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| Posted: Thu Oct 08, 2015 8:29 am Post subject: |
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| DrElch wrote: |
So looking for AC with a multimeter is stupid and leads to mistakes?
Is there a way I could find this out though?
I have talked to a alternator expert and he told me to check for AC without being any more specific.
he said that 0.5V AC should be the threshold for a healthy system.
But we can agree that 13.5V is too low when the alternator is running or is this...well still ok? |
If you were reading 14 VDC with a 0.5 VAC fluctuation I would say things were fine. I don't know your meter and thus have not idea how it might actually read in this situation, some digital meter just can not read a non steady voltage well. To know what was really happening you would have to hook your alternator up to an oscilloscope.
Take your alternator in to a Mom and Pops automotive electrical shop and let them figure out what is going on. |
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kreemoweet
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Joined: March 13, 2008
Posts: 3899
Location: Seattle, WA
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| Posted: Thu Oct 08, 2015 9:38 am Post subject: |
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| DrElch wrote: |
| So looking for AC with a multimeter is stupid and leads to mistakes? |
Not at all. Every alternator has a little bit of fluctuation in the DC output. This is widely referred to as "AC" ripple, but that is really an abuse
of terminology. Most (but not all) multimeters are able to measure that fluctuation, at least approximately. http://www.youtube.com/watch?v=Nhc5VqlUrY0
However, your result of 30V is quite outlandish. I suspect you're misreading the MM. A normal reading would be less than 100 millivolts.
I'm also wondering just how you determined the diodes were all OK. Checking them properly involves unsoldering their connections to isolate each one for testing.
You can't tell much of anything about them just by looking.
There's nothing abnormal about an alt. output of 13-13.5 Volts. My VW specs say 12.5 - 14.5 V @3000 rpm and 35 Amps load.
Here are some other specs I have for the 55A alternator:
Stator winding resistance: 0.13 - 0.14 ohms
Rotor winding resistance: 4 - 4.4 ohms
slip rings minimum diameter: 31.5 mm
wear limit of brushes: 14 mm
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DrElch
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Joined: October 22, 2007
Posts: 82
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| Posted: Thu Oct 08, 2015 10:06 am Post subject: |
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| Quote: |
| That's voltage, not current. |
Sorry, my mistake, I blame it on being German 
| Quote: |
| I'm also wondering just how you determined the diodes were all OK. |
My multimeter has a diode testing mode that can tell you which direction current is allowed to flow. I did it following the VW handbook regarding alternator testing.
My multimeter has a mode to measure AC, but maybe it doesn't work for AC ripple situations? I will double check anyway.
The more important takeaway seems to be that 13.5 Volts is within the tolerance. I will recharge my battery over night and give it another chance. I'll report afterwards |
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mikedjames
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Joined: July 02, 2012
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Location: Hamble, Hampshire, UK
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| Posted: Thu Oct 08, 2015 10:26 am Post subject: |
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When cheaper (sub $100's) meters with AC ranges are used on DC they will read crazy values because they assume they are measuring rectified half cycles of sinewaves.
What you can try is to connect the +probe in series with a condensor , and then it will read the alternating part of the voltage (AC ripple) - the condensor blocks the DC.
It still will not be accurate but if it reads several volts then there is a problem.
In general if the alternator is connected by solid wiring to the battery then having a high AC voltage would indicate a bad battery or bad grounding.
I have a Fluke scopemeter 95 and this indicates DC voltage and displays a little graph of the voltage at the same time so you can see the AC ripple - I think there are now some much cheaper products around (it was $250 on eBay at 20 years old), things like the Tenma UNI-T 81B are more like $150.
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Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
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telford dorr
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Joined: March 11, 2009
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Location: San Diego (Encinitas)
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| Posted: Fri Oct 09, 2015 12:30 pm Post subject: |
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Agreed.
As a sanity check, connect your meter, set to the AC mode, to the battery with the engine NOT running. As there is no AC ripple now, the meter should read zero. If it doesn't, then your meter can't handle a mixed AD/DC voltage, and you need a series capacitor to block the DC voltage (say, 22 uf / 25 vdc should do it. Mind the capacitor polarity marks...)
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'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
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DrElch
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Joined: October 22, 2007
Posts: 82
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| Posted: Fri Oct 23, 2015 4:54 am Post subject: |
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Current status:
I recharged the battery and put the alternator back in.
Right now I've got between 13.5 and 13.9 volts with the engine on.
Generator Light is virtually off. So things look good for now... |
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Vince Waldon
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Joined: November 07, 2010
Posts: 451
Location: Edmonton, Alberta
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| Posted: Fri Oct 23, 2015 10:10 am Post subject: |
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| kreemoweet wrote: |
| DrElch wrote: |
| So looking for AC with a multimeter is stupid and leads to mistakes? |
Not at all. Every alternator has a little bit of fluctuation in the DC output. This is widely referred to as "AC" ripple, but that is really an abuse
of terminology. \ |
Getting into the weeds a bit, but we also need to remember that it's called an alternator because it generates alternating current internally... which is then converted into DC using the internal diode bridge.
A leaky or shorted diode (or two) and a sinusoidal waveform will be superimposed on the DC output... which some multi-meters will probably detect. Certainly your ears will detect it if you've ever had an old car with aging alternator diodes and poor noise suppression on the AM radio. 
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Tcash
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Location: San Jose, California, USA
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| Posted: Fri Oct 23, 2015 3:22 pm Post subject: |
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Had to look that one up.
Sinusoid
The sine wave or sinusoid is a mathematical curve that describes a smooth repetitive oscillation.
Certainly your ears will detect it if you've ever had an old car with aging alternator diodes and poor noise suppression on the AM radio.
Ah that brings back memories. Gives me an idea for a homemade diode tester.
GD
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Wildthings
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Posts: 50352
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| Posted: Fri Oct 23, 2015 5:55 pm Post subject: |
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The bottom pattern is what the three phase rectified voltage put out by an alternator is going to look like. As you can see there is still a small AC component.
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Vince Waldon
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| Posted: Sat Oct 24, 2015 8:22 am Post subject: |
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Great graphs!!
Even more in the weeds... somewhere I've a schematic for a simple circuit that detects the overlaid AC component... for use with plug-into-the-lighter-socket accessories that you want to have power on/off automatically with the engine running.
Sorry OP, will pull up from this diversion.
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kreemoweet
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| Posted: Sat Oct 24, 2015 9:43 am Post subject: |
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Terrible graphs! I see no "rectification" going on there. And WTF is that black line doing up at square-root-of-3 times V(peak)? Makes no sense at all!
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Tcash
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| Posted: Sat Oct 24, 2015 9:46 am Post subject: |
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Ran across this.
Tcash |
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Wildthings
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Joined: March 13, 2005
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| Posted: Sat Oct 24, 2015 10:44 am Post subject: |
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| kreemoweet wrote: |
| Terrible graphs! I see no "rectification" going on there. And WTF is that black line doing up at square-root-of-3 times V(peak)? Makes no sense at all! |
The black line is the rectified voltage and the [(square root of 3) x V] is its value assuming "V" is the maximum AC voltage relative to ground. |
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telford dorr
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| Posted: Sat Oct 24, 2015 12:48 pm Post subject: |
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More exactly, square root of 3 x Vpeak is the expected voltage where Vpeak is the voltage of one alternator phase relative to the centerpoint of the "Y" connected alternator windings. Not important for normal repair work, as we generally don't care about the internal alternator working, and the actual output voltage is overwhelmed by the alternator winding inductance anyhow. The only point of interest is these factors are what allow us to connect the alternator output directly to a battery without blowing up either the battery or the alternator...
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'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST) |
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