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PM capacitor inrush

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jan_de_jong(at)casema.nl
Guest

Posted: Fri Feb 25, 2011 6:48 am Post subject: PM capacitor inrush

Every time we close the master switch of a PM alternator system we 

connect the battery right across the large capacitor, charging it at a 

current far exceeding its ripple current rating. If in the architecture 

the switching relay is not selected for carrying starter current the 

current will exceed its rating too.



The term "right across" is ofcourse inaccurate - the capacitor gets 

charged in a series RLC connection.

In trying to get an idea of current versus time I found the following 

calculator sites.

For the inductance of lengths of wire:

http://www.consultrsr.com/resources/eis/induct5.htm

For the transient response of a series RLC system:

http://sim.okawa-denshi.jp/en/RLCbpkeisan.htm

See "low pass filter tool".



For each of R and C I used 2 reasonable values: "best" and "marginal".

For R: 0.015 Ohm and 0.060 Ohm

For C: 0.050 F and 0.015 F



For L I used 3 values to cover: battery next to capacitor 0.0000005 H 

(18" roundtrip), battery behind seats 0.0000050 H (10' roundtrip), 

battery in the tail 0.0000150 H (30' roundtrip).



The transient response graphs show voltage across the capacitor as a 

function of time after 1 volt is suddenly applied at time 0.

To get current from that you have to make an estimate of rate-of-change 

from the graph: I = C dV/dt - volts per second times capacitance in 

farads equals momentary current in amps. Our step size is 12V instead of 

1V ofcourse.



For the 12 R, C, L combinations I estimated below the following items:

- duration of the steepest 2/3 of the ramp (in seconds)

- average current during the steepest 2/3 of the ramp (in A)

- overshoot (in V)



ohms, farads, henrys: seconds, amps, volts

 

0.015, 0.050, 0.0000005: 0.00080, 500, 0

0.015, 0.050, 0.0000050: 0.00100, 400, 0.4

0.015, 0.050, 0.0000150: 0.00180, 280, 2.6

0.015, 0.015, 0.0000005: 0.00025, 480, 0

0.015, 0.015, 0.0000050: 0.00060, 250, 2.9

0.015, 0.015, 0.0000150: 0.00080, 225, 5.5

0.050, 0.050, 0.0000005: 0.00260, 155, 0

0.050, 0.050, 0.0000050: 0.00270, 150, 0

0.050, 0.050, 0.0000150: 0.00300, 135, 0

0.050, 0.015, 0.0000005: 0.00080, 150, 0

0.050, 0.015, 0.0000050: 0.00080, 150, 0

0.050, 0.015, 0.0000150: 0.00090, 133, 0.2



Ranges

Time: 0.25 to 3 ms

Current: 133 to 500 A

Overshoot: none to 5.5 V



Just another data point...



Cheers,

Jan de Jong

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Feb 25, 2011 11:36 am Post subject: PM capacitor inrush

At 09:39 AM 2/25/2011, you wrote:

 	  Quote: 	
		  



Every time we close the master switch of a PM alternator system we 

connect the battery right across the large capacitor, charging it at 

a current far exceeding its ripple current rating.


	


    Correct . . . but ripple current is not a transient

    event. When used as a smoothing element in the

    design of a DC power system, ripple current squared

    times ESR yields a value for internal energy dissipation that

    warms up the capacitor. It's this warming effect that

    sets operating limits for ripple current.



    The warming effect of a micro to millisecond transient,

    while  many times the ripple current rating of the device,

    is of not significance in terms of abuse to the

    capacitor.



 	  Quote: 	
		    If in the architecture the switching relay is not selected for 

 carrying starter current the current will exceed its rating too.


	


    Unlike the selection of fan motor sizes, fuse sizing,

    battery sizing, hydraulic pump sizing, etc. etc.

    the performance numbers for switches are not the

    convenient ideas we would like them to be . . .



http://www.aeroelectric.com/articles/Switch_Ratings.pdf



http://www.aeroelectric.com/Reference_Docs/Switches/CH_Switch_Training_Manual.pdf



     Then there are switch failures that masquerade

     as a ratings issue . . . but in fact are

     based on other simple-ideas . . .



http://www.aeroelectric.com/articles/Anatomy_of_a_Switch_Failure/Anatomy_of_a_Switch_Failure.html



     The documents cited above are a minuscule fraction

     of the published works exploring simple-ideas

     that drive switch and rely contact performance. At

     the same time, we find   See page 11-16 of . . .



http://www.aeroelectric.com/Reference_Docs/FAA/AC43.13-1B_Ch11_Electrical.pdf



     where the authors cite nameplate and spec sheet

     ratings values as the holy grail of guidance

     for switch replacement. If only those who offer

     advice based on experience were writing the rules.



     Then you have some guy come along with installations

     like this . . .



http://aeroelectric.com/Pictures/Switches/miniswitches.jpg



     Here we see landing and taxi lights controlled

     with miniature toggle switches that are in no

     way 'rated' to the task. The owner/pilot reports

     these switches have been performing to design goals

     for years of service in his airplane.



     The numbers that are almost never cited in support

     of published switch ratings are the "operating cycles

     at tested load". Spec sheets never speak to "de-rating"

     a switch or relay based on severity of neglect for

     failure to observe the ratings. Question: Suppose you

     KNEW that a 5x abuse of some 'rating' reduced the

     projected service life of a device from 50,000 cycles

     down to 1,000 cycles. How would that influence your

     decision to use the device in your airplane? Assume

     further that you've exercised due diligence in the

     design and operation of your electrical system. I.e.

     failure tolerant in every regard. Now, all failure

     and wear-out issues are reduced to cost-of-ownership

     and maintenance issues . . . and are NOT the seeds

     from which dark-n-stormy night stories are grown.

 	  Quote: 	
		  The term "right across" is ofcourse inaccurate - the capacitor gets 

charged in a series RLC connection.

In trying to get an idea of current versus time I found the 

following calculator sites.

For the inductance of lengths of wire:


	


     You are to be commended for considering the question

     and going into the level of investigation. Indeed, there

     are observable effects for the LRC qualities of

     power system configuration . . . but in this case,

     they are not that significant with respect to longevity

     of switches and relays.



     The reactive qualities of system components are

     responsible for the 'real' spikes in a power

     distribution system. As I explained in the chapter

     on noise, true 'spikes' are indeed over (or under)

     voltage events. They are VERY short in duration

     measuring perhaps tens of microseconds. They are

     exceedingly easy to accommodate in the qualification

     of 99.9% of all electronics designs . . . but on

     occasion, will rise up to become a nuisance.



     EVERY nuisance tripping event I've had occasion to

     deal with for OV protection system had roots in

     these kinds of events. Not dangerous to electronics . . .

     no energy involved. But they were responsible for

     nuisance trips.



     Having thus armed yourself with some notion of scale

     (energy, amplitude, duration, shape) of the transient

     events, you're ready to go back and look at their

     significance with respect to switches, OV relays,

     annunicator panels, etc.



     I can assure you that gremlins of this pedigree

     are of no practical significance when it comes to

     relay or switch life.



     An interesting side note speaks to the original

     premise of this topic. Positioning the PM alternator

     control relay to the AC output of the alternator

     moves it from a DC, large transient-current

     world to an AC, current-limited domain where

     relays and switches thrive.

   Bob . . .

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jan_de_jong(at)casema.nl
Guest

Posted: Sat Feb 26, 2011 1:57 pm Post subject: PM capacitor inrush

I was not too worried about the once per flight high current event, but 

having derived some numbers I thought I 'd share them anyway (I found 

the 45% overshoot from a 15 mOhm 30' wire to 15 mF interesting - glad 

I'm not building a 747).

But thank you for your comments!

Jan de Jong

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Feb 26, 2011 6:01 pm Post subject: PM capacitor inrush

At 04:54 PM 2/26/2011, you wrote:

 	  Quote: 	
		  



I was not too worried about the once per flight high current event, 

but having derived some numbers I thought I 'd share them anyway (I 

found the 45% overshoot from a 15 mOhm 30' wire to 15 mF interesting 

- glad I'm not building a 747).

But thank you for your comments!


	


    To be sure, the severity of the 'problem' increases

    with physical size of the system (long runs of wire)

    and complexity (lots of electrical wigglers).

    I think one of my Florida seminar attendees was

    a space shuttle techi . . . he commented that it

    was the electrically noisiest vehicle he'd ever

    worked on.



    But there are ways to cope. For example, the signals

    from GPS satellite signals are so weak at the earth's

    surface, they cannot be distinguished from atmospheric

    noise were it not for a predictable pattern in their

    emissions. When you know precisely what you're looking

    for, the extraneous stuff can be filtered/ignored as

    long as it's not overwhelmingly strong.



    Knowing what's on the power bus is the first step toward

    being sure that you can artfully reduce its effects

    to insignificance.



    Bob . . .



 	  Quote: 	
		  Jan de Jong



-----

No virus found in this message.

Checked by AVG - www.avg.com


	




   Bob . . .

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