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PM ALTERNATOR SPIKE PROTECTION (NOT)

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nuckollsr(at)cox.net
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Posted: Mon Apr 23, 2007 8:26 am Post subject: PM ALTERNATOR SPIKE PROTECTION (NOT)

At 10:50 AM 4/23/2007 +1000, you wrote:



 	  Quote: 	
		  

<peterjfharris(at)bigpond.com>



Ken thanks for the lead to Bob's advice on spike protection. Re your

comments about the use of a capacitor is this connected across the two AC

output leads from the stator?

I have been told to just fit a diode from the regulator output to B+ for

spike protection but this would not enable isolation of battery from

regulator?

Thanks

Peter


	


   The capacitor is not for "spike protection". The single phase

   PM alternators are exceedingly noisy without at least a battery

   on line and the noise can be further reduced by addition of

   a capacitor. Further, if you'd like for your SD-8 or similar

   alternator to come up self-excited and run well without a battery,

   you'd be well advised to install at least the 22,000 uF capacitor

   and the resistors described in Figure Z-25 Revision L on page

   27 of . . .



http://www.aeroelectric.com/articles/Rev11/AppZ_R11J.pdf



   It would be pretty cool if we could eliminate the word

   "spike" from the vernacular of speaking of airplane

   electrical systems. The word has morphed into a dozen

   different meanings on the part of some speakers . . .

   and most are regarded as evil. The word is then picked

   up by the neophyte student of electrical systems and

   whacked about like a shuttlecock in a badminton game

   with each speaker believing that he's speaking with some

   level of understanding and each speaker having a different

   vision of what's being discussed. Sounds like Congress!



   The ONLY instance where our electrical systems are

   likely to experience a true "spike" (see definition

   in chapter on over voltage) is during the opening

   of coil on a relay or contactor. The spike CAN be

   exceedingly spectacular. See:



http://www.aeroelectric.com/Pictures/Curves/Cole-Hersee_without_Diode.gif



   but (1) even if ignored, the ONLY potential victim

   of such spikes is the SWITCH THAT CONTROLS the relay

   or contactor. The spike DOES NOT propagate out onto

   the system. Even if it did, it's of such low energy

   content that a .10 uF capacitor across the bus or the

   +14v input to an appliance would wash it completely. See:



http://www.aeroelectric.com/Pictures/Curves/CH_Bus_Noise_w_0p1_Cap.gif



   (2) by adding the diode or other spike suppression

   process, we can give the controlling switch some

   relief that results in a longer service life for

   the switch. See:



http://www.aeroelectric.com/Pictures/Curves/CH_w_1n5400.gif

http://www.aeroelectric.com/Pictures/Curves/CH_w_2x18v_Transorbs.gif

http://www.aeroelectric.com/Pictures/Curves/CH_w_120_Ohm.gif

   ALL OTHER FORMS of bus voltage perturbation come

   from instabilities of one kind or another in the

   alternator system and are not properly called

   "spikes." Regulator stability problems manifest

   themselves as small "surges" or "dynamic instabilities

   excited by transient loads". These are low energy,

   non threatening things as well. The guy we need to

   take note of is the "over-voltage" condition promulgated

   by the uncontrolled, full-bore output of the engine

   driven power source that pushes bus voltage toward

   the moon until it's brought under control by some

   means crafted for that purpose.



   Know that the capacitor in a PM alternator system

   is just there for smoothing the considerable ripple

   voltage that exists on a full wave, single phase rectified

   power output typical of the SD-8. I did some comparative

   noise studies on an SD-8 some years ago and compiled this data:



http://www.aeroelectric.com/Pictures/Curves/SD-8_Noise_Data.pdf



   Note that with a 10KuF cap (of unknown quality)

   the noise under most conditions was under 700 mv pk-pk

   except when the battery was disconnected on page

   5. Further tests with a brand new 47KuF capacitor

   brought the noise levels down nicely even with a

   battery off line as shown on page 7.



   Yeah, I know those traces look kinda "spikey" but

   let's not speak of them in that matter. The voltages

   plotted are the normal and expected performance

   characteristics of the system under study and none

   of the traces should cause an sense of evil or

   foreboding that seems to bubble up every time folks

   start worrying about "spikes".



   A few months ago we studied and solved the problem

   of getting the SD-8 to come up self-excited. This

   is where the resistors and rectifier were added to

   Z-25.  At some later time and under a different

   discussion topic, we considered moving the control

   relay to the AC side of the PM alternator's regulator

   as shown on the current revision of Z-16. However,

   this is a case of not having your cake and eating

   it too. Moving the control relay to the AC side of

   the regulator kills our ability to come alive self

   excited.



   We'll have to throw the pot back on the stove to

   see if we can achieve both qualities from a single

   architecture.



   Bob . . .

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Peter H

Joined: 20 Mar 2007
Posts: 197

Posted: Mon Apr 23, 2007 3:13 pm Post subject: PM ALTERNATOR SPIKE PROTECTION (NOT)

Bob I got a spike which destroyed my electronic ignition module when my

alternator CB popped. I was running a half baked circuit with no proper

protection, hence my journey into your excellent and informative Appendix Z.

I have ordered the OVM and now digging deeper.

Can you recommend a regulator rectifier for the Jabiru, one that can be

started like Z25 without a battery?

Thanks

Peter H 



--

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jmfpublic(at)comcast.net
Guest

Posted: Tue Apr 24, 2007 12:59 pm Post subject: PM ALTERNATOR SPIKE PROTECTION (NOT)

Bob,

  

 Regarding the PM alternators, you wrote:

  

 A few months ago we studied and solved the problem

   of  getting the SD-8 to come up self-excited. This

   is where the  resistors and rectifier were added to

   Z-25.  At some later  time and under a different

   discussion topic, we considered  moving the control

   relay to the AC side of the PM alternator's  regulator

   as shown on the current revision of Z-16.  However,

   this is a case of not having your cake and  eating

   it too. Moving the control relay to the AC side  of

   the regulator kills our ability to come alive  self

   excited.

  

    We'll have to throw the pot back on the stove  to

   see if we can achieve both qualities from a  single

   architecture.

  

 Here is a possible entry for the pot.  Design so that the relay and  the S1 controlling switch get power either from the battery or the same bridge  circuit of Z25.  A diode would be placed in series from the main battery  bus to the fuselink, and the output from the diode bridge would also be fed  by a series diode to the upstream (bus) side of the circuit breaker.   Diodes would need to be in the 100 to 200 volts range, but that is not  difficult.  If smaller breakers than 5 amps are available that would be  better, too.

  

 A more elegant solution would use a thyristor triggered by a pulse from the  overvoltage module, but I haven't tried to draw that one.  This is looking  like a job for a small printed circuit board again.

  

 Jim Foerster,  J400 slowly wiring

    [quote][b]

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