Matronics Email Lists :: View topic - OV Protection for Three-Phase PM Alternator

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

OV Protection for Three-Phase PM Alternator

Matronics Email Lists Forum Index -> AeroElectric-List

View previous topic :: View next topic

Author Message

grnord

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

Posted: Mon Jan 25, 2010 5:56 pm Post subject: OV Protection for Three-Phase PM Alternator

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?



    (2)  What would be the consequences of an OV event that happened to open one relay but not the other?



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



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...

Rick Nordgarden

Council Bluffs IA

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

etienne.phillips(at)gmail
Guest

Posted: Mon Jan 25, 2010 9:33 pm Post subject: OV Protection for Three-Phase PM Alternator

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.



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.



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...



Thoughts?



Thanks

Etienne



On 26 Jan 2010, at 3:56 AM, grnord wrote:



 	  Quote: 	
		   

 >



 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?



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

 to open one relay but not the other?



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



 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...

 Rick Nordgarden

 Council Bluffs IA





 Read this topic online here:



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

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Jan 26, 2010 12:08 pm Post subject: OV Protection for Three-Phase PM Alternator

At 11:42 AM 1/26/2010, you wrote:




My recollection is that these Z diagrams have gone through changes over

time as design goals have evolved.  I think I remember seeing some of the

PM diagrams without capacitors even.  Some systems aren't very sensitive

to alternator noise..  Components never installed never fail.



    I don't recall that. Until I did the noise studies we just

    "assumed" that the single phase rectified output from the

    SD-8 was going to be exemplar ordinarily noisy and adding

    a capacitor was a good thing to do. Rotax and others had

    been doing it for some years prior.



    The surprise was that the SD-8 without a capacitor generated

    no more noise than other devices qualified under mil-std-704

    design goals that allow up to 1.5 v pk-pk conducted noise

    in certain frequency ranges. See figure 15 in



http://www.aeroelectric.com/Reference_Docs/Mil-Specs/Mil-Std-704_excerpts.pdf

I'd guess that the vast majority of PM alternators run for the life of the

airplane so long as the components are held to reasonable temperature and

the wiring is done correctly.  Similarly, the output voltage of the

unregulated PM alternator isn't _that_ high - <100V.  You can pick a cap

that can stand the full output and not worry about it failing should the

regulator pack it in.  I see caps rated to (and above) 100V on the digikey

site for reasonable prices.



    True. However the majority of catastrophic failures I've

    put my hands on were the result of hooking the capacitor

    up backwards. And the SD-8 turning at 4,000 rpm on a vacuum

    pump pad isn't a snarling monster either. Getting OV

    protection sensed downstream of the capacitor and cutting

    power upstream of the R/R seems to be the elegant solution.



    Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

Display posts from previous:

	Matronics Email Lists Forum Index -> AeroElectric-List	All times are GMT - 8 Hours
Page 1 of 1

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum