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OV Protection for Three-Phase PM Alternator

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nuckolls.bob(at)aeroelect
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Posted: Mon Jan 25, 2010 9:37 pm Post subject: OV Protection for Three-Phase PM Alternator

At 07:56 PM 1/25/2010, you wrote:




I've been considering how to use a three-phase permanent-magnet 

alternator instead of the single-phase units shown in the Z-figures 

and discussed in the AeroElectric book.  I'd like to put overvoltage 

protection upstream of the regulator-rectifier but the three-phase 

output is delivered to it via three feed-wires instead of the two 

from a single-phase PM stator.  It seems that OV disconnect relays in 

any two feed-lines would interrupt current from all three stator-coil 

sets; the two AEC 9024-20 modules operating the relays would probably 

be triggered at slightly different voltages due to component 

tolerances but an OV event in the narrow window between those two 

voltages seems unlikely.  I have three questions:



     (1)  Have I overlooked something that makes the basic concept unworkable?



   Don't want 2 ov sensors for reasons cited . . .



     (2)  What would be the consequences of an OV event that happened 

to open one relay but not the other?



    Not easily predicted . . .



     (3)  Is it possible to control two relays with one AEC 9024-20?



    Yes, or get a two-pole relay. But two relays is workable too and

    probably less expensive.



If necessary I could go with a single OV disconnect relay downstream 

of the regulator but that would leave that not-cheap 

regulator-rectifier vulnerable.  Twinned OV protection is cheaper 

than one fried regulator...



    Agreed.

Rick Nordgarden

Council Bluffs IA





Read this topic online here:



http://forums.matronics.com/viewtopic.php?p 83444#283444





Checked by AVG - www.avg.com

01/25/10 19:36:00

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jan 26, 2010 6:57 am Post subject: OV Protection for Three-Phase PM Alternator

At 11:30 PM 1/25/2010, you wrote:


<etienne.phillips(at)gmail.com>



Hi Rick



Putting an OV module upstream of the regulator when using a permanent

magnet alternator is very likely to trip incessantly, as the regulator

keeps the voltage correct by dissipating the extra energy as heat,

unlike a field-coil alternator that actively modifies the voltage

generated by the alternator.



     Etienne's assertions are based on the earliest marketplace

     examples of rectifier regulator for PM alternators. The PM

     alternator goes wAAAyyyy back in the history of electrical

     systems on small engines where folks wanted to add things

     light lighting and self starting to scooters, bikes,

     and lawn care equipment.



     Indeed, these alternators were seldom rated for more than

     5-10 amps at max RPM (in a 6v system). The simplest means for

     controlling their widely variable output voltage was to simply throw

     a shunt load on the device to prevent excess energy from

     making it out onto the system.



     Of course, this means that the rectifier/regulator had to

     be capable of turning 100% of the alternator's output

     into pure heat under all conditions. So the heat dissipating

     abilities of the R/R had to accommodate 50-100 watts of

     output.



     As the systems needing simple, low cost power matured the

     the alternators were up-sized to accommodate design goals.

     One can buy 3-phase, 30A PM machines today . . . but the

     regulators are most certainly not capable of sinking all

     that power. The reason is that SHUNT regulation has been

     pretty much abandoned for SERIES regulation wherein

     excess energy is kept off the system not by sinking it

     to ground but by disconnecting the energy source from the

     rest of the system.



     This means that alternators are now taxed only as

     the system demands can use the energy . . . before, the

     alternator always ran flat-out and was necessarily

     hot all the time. Exemplar schematics for modern rectifier/

     regulators can be viewed here . . .



http://aeroelectric.com/Pictures/Schematics/3-Phase_PM_Rectifier_Regulator.gif



http://aeroelectric.com/Pictures/Schematics/PM_Regulator/Kubota_Schematic.jpg



For that reason, I wouldn't think that it's possible to have an OV

event, other than with a regulator failure, and the only way to catch

that would be to stick the OV relay and module downstream of the

regulator, after the rectification has happened, and when you're

dealing with DC.



     The original idea for adding OV protection to the SD-8

     DID put the disconnect relay downstream of the R/R. But

     this didn't really comport with design goals for all

     other OV protection systems . . . shut off the flow of

     energy at the source. This is what prompted moving

     the relay from the downstream side of R/R to the upstream

     side in the AC wiring.



Now on a different note, wouldn't it be a good idea to stick the OV

relay upstream of the smoothing capacitor? I've seen one of those

70,000uF caps explode due to too much juice, and it's quite

impressive! My thinking is that if your regulator goes off the hook

and supplies the cap with 40V (or whatever the permanent magnet

alternator can generate), and that thing explodes in the cockpit, the

pilot is going to need a new set of underwear...



     You need to abuse these guys pretty badly to achieve

     such spectacular failures. Certainly a runaway 30A

     alternator would be classified as pretty abusive.

     Hence, putting the OV disconnect in the upstream

     AC wiring affords the capacitor the same degree of

     OV protection as the rest of the electro-whizzies

     on the airplane.

     Bob . . .

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grnord

Joined: 23 Jan 2010
Posts: 5
Location: Council Bluffs IA

Posted: Tue Jan 26, 2010 8:46 am Post subject: Re: OV Protection for Three-Phase PM Alternator

Bob and Etienne, thanks for your replies.  You've shed some light on a controversy that erupted on the CorvairCraft list (on mylist.net) early this month: somebody started bad-mouthing PM alternators and as evidence he cited several instances of blown smoothing capacitors on Rotax installations.  The thread got bogged down in ad hominem attacks and counter-attacks and eventually was dropped without resolving the question of why the capacitors had blown, but it appears in light of what you two have pointed out that those setups must have had the capacitors upstream of their OV protection -- or no OVP at all...



...Which leaves me puzzled with respect to the Z-figures' treatment of smoothing capacitors with PM alternators: only Z-9 and Z-16 show them downstream of the OVP; Z-10/8, -13/8, -17, -20 and -21 all have the capacitor upstream of the OVP.  The  guy stirring things up on CorvairCraft confirmed that failures of these caps are indeed spectacular.  How likely to hear such a bang are users of Z-10/8 et al?



Am I right in concluding that my best bet for minimizing the probability of an unhappy ending with a three-phase PM alternator is to put a single disconnect relay in the DC line downstream of the R/R but upstream of the smoothing capacitor, leaving the R/R to fend for itself?

Rick Nordgarden

Council Bluffs IA

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jan 26, 2010 9:12 am Post subject: OV Protection for Three-Phase PM Alternator

At 10:46 AM 1/26/2010, you wrote:

 	  Quote: 	
		  



Bob and Etienne, thanks for your replies.  You've shed some light on 

a controversy that erupted on the CorvairCraft list (on mylist.net) 

early this month: somebody started bad-mouthing PM alternators and 

as evidence he cited several instances of blown smoothing capacitors 

on Rotax installations.  The thread got bogged down in ad hominem 

attacks and counter-attacks and eventually was dropped without 

resolving the question of why the capacitors had blown, but it 

appears in light of what you two have pointed out that those setups 

must have had the capacitors upstream of their OV protection -- or 

no OVP at all...


	


   Hard to tell . . . mostly because they were using

   hardware that has not been as well evaluated for

   behavior as the SD-8 and regulators supplied by

   B&C.



 	  Quote: 	
		  Am I right in concluding that my best bet for minimizing the 

probability of an unhappy ending with a three-phase PM alternator is 

to put a single disconnect relay in the DC line downstream of the 

R/R but upstream of the smoothing capacitor, leaving the R/R to fend 

for itself?


	


   No . . . best-we-know-how-to-do is disconnect the

   offending energy flow AT ITS SOURCE . . . break the

   AC leads. The Z-figures have not been all updated

   to this idea . . . but unlike the larger, 3-phase machines,

   and the 18A system on Rotax,  the SD-8 on a vacuum pump

   pad doesn't represent much of a hazard to the smoothing

   capacitor. Alternators that run much faster (higher

   open circuit voltage) are another matter entirely.



   By the way, you can leave this capacitor off if you

   wish . . . it's value is problematic. See:



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



   Note that some of the plots are at 50 mv/division vertical

   scale, others are at 200 mv/division.

   Bob . . .

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etienne.phillips(at)gmail
Guest

Posted: Tue Jan 26, 2010 1:26 pm Post subject: OV Protection for Three-Phase PM Alternator

On 26 Jan 2010, at 7:10 PM, Robert L. Nuckolls, III wrote:

 	  Quote: 	
		  

   . . . best-we-know-how-to-do is disconnect the

  offending energy flow AT ITS SOURCE . . . break the

  AC leads.


	


Hi Rick and Bob



I realize now that I didn't get the picture in my mind across very  

clearly at all! Agreed that the actual break in the circuit should be  

as far upstream as possible, but the sensing should happen downstream  

of the regulator, as you have explained. Apologies for confusing the  

matter...



As for the cap disintegrating, my fears are put at ease if the relay  

disconnects the source upstream of the regulator.



Thanks

Etienne

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