How alternator diodes do their thing

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From: Jack Morgan <jmorgan1023 [at] comcast.net>
Sender: <marv [at] lancaironline.net>
Subject: How alternator diodes do their thing
Date: Thu, 25 Oct 2012 09:21:41 -0400
To: <lml [at] lancaironline.net>

Hi all,


I was with Motorola during the original design cycle for the first alternator diodes for Chrysler. Shouldn't admit to being that old but I can't resist putting in two cents here. I subsequently spent more years than I will admit (with many) bringing electronics into the automobile from the first alternator through the electronics evolution resulting in more than 30 micro controllers in the basic car. It was a fun ride... obviously retired now.

Brent's description is correct... as the three windings produce their sinusoidal outputs (each 120 degrees out of phase with one another) only one winding at a time is producing the voltage required to cause conduction through it's respective diode pair. I will leave the wye versus delta differences to those who care. Brent's description is easiest but you engineering types may want to draw three sine waves on a piece of paper..... the tops of the sine waves show the ripple that is present on an open circuit alternator. The alternator is actually a current generator so there is actually negligible noise created by this voltage ripple since it is clamped by the battery and the phases smoothly take turns supplying the output current as controlled by the field current. Thus no need for a large filter capacitor. The reason that a failed diode created radio noise is that the smooth transfer of current is disrupt by the offending phase.

Paul's comments are also correct with one subtle nit. The alternator whine is due to diode switching as Paul describes. Paul is correct that the diodes have a turn on delay, however, the source of the noise is due to turn off time of the diodes (reverse recovery time for the techies). It takes time(micro seconds) for the output current in the diode to clear and the diode appears shorted during this time. The resulting reverse current spike creates the whine. The small capicator in the alternator goes a long way to filtering out this high frequency spike.

Sorry to bore most of the readers. Please send me an off list email if you would like to pick bones or want more detail.

Jack Morgan
IV P serial 603 with 45 hours.


On Oct 23, 2012, at 6:00 AM, Lancair Mailing List wrote:




From: Gary Casey <casey.gary [at] yahoo.com

>
Subject: Re: TCM 60A alternator--diodes
Date: October 22, 2012 10:13:51 AM EDT
To: lml [at] lancaironline.net





Paul,
I was trying to say what Brent said, but he said it much more elegantly.  It really all depends on the methodology used in the rating of the diodes.  Is the diode package rated at the current of the individual diodes in the package?  Or is it rated at the current produced by the alternator into which it is intended to be mounted?  And the the rating is also "derated" as a function of temperature, and at the core each diode is rated depending on the junction temperature within the diode.  What I'm saying is that it gets complicated.  I assume a front-mounted alternator runs very cool, so that's a good thing.  As I recall, most alternators put out a frequency that is 7 times their rpm (why I remember that I don't know :-) and the alternator runs a a speed that I assume is at least twice crankshaft speed.  But, the electrical noise you hear is not the ripple from the windings, but the voltage transient caused by the turn-on delay of the diodes.  Diodes don't turn on instantly, so the voltage in a winding will rise above battery voltage and then suddenly drop as the diode turns on, and that happens three times per cycle.  That will give you the frequency of the "whine" you sometimes hear in the headset, which using those numbers comes out to at least 1,680 Hz at 2400 rpm.  If there is a capacitor in the alternator, that's what it's for, not to reduce the ripple.  Look carefully at the alternator voltage (also at battery voltage) and you'll see small, very short spikes in the voltage caused by the diodes.  A "picket fence" look.
And I wasn't trying to be judgmental or anything - I was trying to point out that our conservatism in any design is a function of intended usage.  If it is only for day VFR in sparsely-used airspace the whole electrical system is a convenience and one might not be too concerned about the reliability of the alternator.  So the choices one makes are a function of intended usage.
Gary

Gary: I have to disagree and I don't think this alternator works the way you describe.  This is multi-phase high frequency alternator that has multiple poles and three windings.  The result is that all the circuits are producing AC high frequency 3-phase rectified output and the output is indeed additive from the 6 diode outputs in the sinusoidal sense. Plus, we're not running at 60Hz but a far higher frequency with six pulses per pole, multiple poles (12 or 14) and somewhere around 7000rpm at cruise.

The result in a very high effective DC output voltage with limited ripple. That's why it gets dumped right into the aircraft buss with only the small capacitor located inside the alternator  All the diodes are working all the time and they don't "take turns" in sequence.  Each diode is rated at 50A continuous and the three phases are actually additive, overlapping sine waves.   Take away a phase by removing a winding and you will generate less amperage.  The output from this configuration is about 1.73x the peak phase voltage and I extrapolate that to be directly proportional for current also.  See the diagram attached for reference. Therefore, I calculate each phase only needs 19.5A (100A over 3 phases divided by 1.71) to produce 100A output.   That 19.5A maximum per phase is far less than the 50A rating we're discussing and suggests a large margin exists. Realistically, 100A units are run somewhere less than 100%  max continuous load calculations which suggests even more margin is built in.

Having said that, the unit described is typically use for high output amperage truck alternators in the 90A range and you can do your own research to verify the applications. You might also want to check the diode sizing in the TCM 60A alternator to see how that compares with the 50s I replaced those with.

Your last comment seemed to suggest some form of risky behaviour was being contemplated by this thread. Some nonsense about night IFR or whatever.  I think it was unnecessary and a pointless addition to what was intended to be an informative discussion.

Paul
Legacy, Calgary

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