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questions about "filter" capacitors

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hdwysong(at)gmail.com
Guest

Posted: Fri Jun 06, 2008 1:39 am Post subject: questions about "filter" capacitors

Hello folks,



I'm wondering what drives the sizing/requirements for the caps hanging

off of our alternator feeds.  I am comparing a few different

architectures and would like to understand the subtleties associated

with each.



The first architecture is a Rotax 914 w/ standard 14V PM generator.

Suggestions from other Rotax 914'ers led me to conservatively derate

my 14V budget to ~12A to keep the Rotax (Ducati) regulator/rectifier

healthy.  Per Rotax recommendations (and supported by the Z drawings),

I have a 25V 22,000uFcap installed.  This satisfies the "rule of

thumb" from the list archives of "1,000 uF per 1A" for PM alternators,

and plots of the 14V bus voltage during engine runs indicate that all

is well with this cap installed.  The downsides are that the cap is

huge (physical size of a mini Coke can) and I have blindly applied a

rule of thumb to size the cap.  It works... but I would like to

understand why.  Why 1,000 uF instead of 100 uF or 5,000 uF?



The second architecture is my stock C150M with a 60A Ford alternator.

After noting the size of the cap installed on the Rotax I began

wondering how I could've overlooked such a monster on my 150?  Turns

out I didn't.  The Cessna schematics/SB's identified a couple of noise

filters (square box on the firewall, small can with a pigtail on the

alternator) as well as a small axial capacitor behind the instrument

panel.  Do these numerous iterative "filters" on the 150 provide the

cumulative equivalent of a single "1,000 uF per 1A" capacitor on my

OBAM bird?



Is the "1,000 uF per 1A" rule of thumb for the PM generators even

applicable to the 150?  Am I missing a corollary that defines an upper

limit where adding more Farads offers little/no return on investment?

If I had a 100A alternator would I really NEED to shop for a 100,000

uF cap or would a smaller cap suffice?



Thanks for your time/assistance/patience.  I look forward to the

education and clearing the haze.



D

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rampil

Joined: 04 May 2007
Posts: 870

Posted: Fri Jun 06, 2008 7:22 am Post subject: Re: questions about

Look at it this way:



Forget what you hear here in terms of rules of thumb.



22 mFd  is what the factory decrees, so that is what a responsible

builder should used unless there is strong evidence to the contrary.



The value of the cap is determined by estimating the resistance of the

driven system and then the desired tau (time constant) which will

give the desired degree of smoothing to the DC output.  If you don't put

a cap in, your systems will get rectified AC as the supply waveform,

not the desired DC.  (Quite possibly all modern avionics, lights, and LEDS

would run quite happily on rectified AC, maybe not electronic ignition

modules - caveat lector!!)



There is no electrical max limit on the cap value, the limit is of size 

and weight

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_________________
Ira N224XS

hdwysong

Joined: 06 Jun 2008
Posts: 21

Posted: Fri Jun 06, 2008 12:29 pm Post subject: Re: questions about

 	  rampil wrote: 	
		  22 mFd  is what the factory decrees, so that is what a responsible builder should used unless there is strong evidence to the contrary. 	




Understood.  The 22000 uFd cap is installed and will certainly remain on the 914 per Rotax schematics.  Rather than wanting to fix what isn't broken I'm simply trying to understand where these "magic" numbers come from.



 	  rampil wrote: 	
		  The value of the cap is determined by estimating the resistance of the driven system and then the desired tau (time constant) which will give the desired degree of smoothing to the DC output. If you don't put a cap in, your systems will get rectified AC as the supply waveform, not the desired DC. 	




Makes sense, Ira.  Thanks.  Somewhere there exists an "allowable ripple" spec and, given the expected/typical loads on a system, the cap is sized accordingly to satisfy the spec.



Would it be practical for us to estimate the system resistance alongside the power budget to derive an educated guess for how big the B-lead cap should be?  Seems like a science project with little return on investment (time/effort), but perhaps it will provide a reasonable ballpark #?



Alternatively, is there any harm in simply hooking things up WITHOUT a B-lead cap and measuring how "noisy" the bus is before I stick a filter cap in the system?  This is more of a build-test-fix approach but at least I would have data to substantiate the size (and weight and cost) of the cap chosen.



D

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nuckolls.bob(at)cox.net
Guest

Posted: Sat Jun 07, 2008 6:14 am Post subject: questions about "filter" capacitors

At 01:29 PM 6/6/2008 -0700, you wrote:

 	  Quote: 	

rampil wrote:

 > 22 mFd  is what the factory decrees, so that is what a responsible 

 builder should used unless there is strong evidence to the contrary.

Understood.  The 22000 uFd cap is installed and will certainly remain on 

the 914 per Rotax schematics.  Rather than wanting to fix what isn't 

broken I'm simply trying to understand where these "magic" numbers come from.

   In this particular case, I'll bet that somebody when to

   the parts store, bought something that "looks like it will

   work" and installed it. When nobody complained about noise

   after that, what ever value was in place (22KuF) was called

   "golden". It may have been that the 1KuF/Amp rule-of-thumb

   was applied. We'll never know.

 	  Quote: 	
		  rampil wrote:

 > The value of the cap is determined by estimating the resistance of the 

 driven system and then the desired tau (time constant) which will give 

 the desired degree of smoothing to the DC output. If you don't put a cap 

 in, your systems will get rectified AC as the supply waveform, not the 

 desired DC.

Makes sense, Ira.  Thanks.  Somewhere there exists an "allowable ripple" 

spec and, given the expected/typical loads on a system, the cap is sized 

accordingly to satisfy the spec.

   If we were talking about General Motors or Honda, yeah,

   there might be a house spec for maximum allowable noise

   to meet corporate design goals. There might even be an

   IEEE or SAE spec but there's no practical way of knowing if

   Rotax has a spec or consulted anyone else's documents.

   Rotax's regulators are supplied by Ducati and since these

   folks are in the vehicular electrics business, their

   recommendations may well have been the outcome of well

   considered conditions. But Rotax probably wouldn't be

   aware of it.

   In the US aerospace industry, there are generally accepted

   recommendations embodied in RTCA DO-160 and Mil-Std-704.

   Go to my website and do a site search on "DO-160" and

   then another search on "mid-std-704". These two searches

   will produce a number of hits on what these documents

   are and how they help us.

 	  Quote: 	
		  Would it be practical for us to estimate the system resistance alongside 

the power budget to derive an educated guess for how big the B-lead cap 

should be?  Seems like a science project with little return on investment 

(time/effort), but perhaps it will provide a reasonable ballpark #?

   It's a bit beyond 'estimating' . . . the range of

   bus impedances can be large depending on size and

   condition of battery, numbers of accessories and any

   associated internal capacitors, etc.

 	  Quote: 	
		  Alternatively, is there any harm in simply hooking things up WITHOUT a 

B-lead cap and measuring how "noisy" the bus is before I stick a filter 

cap in the system?  This is more of a build-test-fix approach but at least 

I would have data to substantiate the size (and weight and cost) of the 

cap chosen.

   ABSOLUTELY! But this has already been done. See:

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

   Here I loaded the SD-8 to 10.5 amps and applied

   the 1KuF/A rule-of-thumb. Bus noise from the SD-8 was

   quite nominal in comparison with the 1.5 v pk-pk

   noise that we're told is acceptable under Mil-Std-704

   design goals.

   But this was one measurement on equipment that I

   had access to at the time. The repeatable

   experiment is what validates all recipes for success

   and any data you can collect and add to our collective

   knowledge would no only be appropriate but most

   welcome. I've not had an opportunity to 'scope

   a Rotax 912 installation.

   Bob . . .

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