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SD-8 PM Alternator

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mcculleyja(at)starpower.n
Guest

Posted: Thu Apr 13, 2006 6:58 pm Post subject: SD-8 PM Alternator

Bob N,et al,



There has been periodic discussion on this list in the past about the

SD-8(& maybe other permanent magnet alternators)not being capable of

coming on line if the battery or other power source to the buss fails or

is not otherwise available to activate the SD-8.



I have experimented with a simple idea that solves this situation and

want to share the info to solicit comments on the suitability or any

downside features anyone may offer.



The circuit consists of tapping into the SD-8 regulator output and 

feeding that through a diode to the positive side of an electrolytic 

capacitor, grounding the negative lead of the capacitor and connecting a 

spring loaded,normally-open push-button switch across the diode.



After engine start-up and during the subsequent pre-flight system check 

of the SD-8, it charges the capacitor and the diode prevents loss of 

this charge thereafter even if the SD-8 is not left switched on. If 

total loss of electrical power to the buss occurs during flight, the 

SD-8 will come on line by momentarily activating the push button switch 

while also closing the SD-8 switch to the buss.



Through both ground and flight testing, I found no situation where

it failed to work. An unexpected finding is that the SD-8 will activate

with as low as 1.0 volt charge on the capacitor! Another bonus is that

the capacitor leakage rate is so low that from an initial charge of only

12.5 volts, it requires 2 hours short of 3 full days to leak down to 

10.5 volts! (average of 0.0286 volts per hour). Of course, this rate is 

progressively less as the charge voltage drops further, so if someone 

left the Master on and killed the battery and then wanted to prop the 

engine many days later(more than 2 weeks!)the SD-8 could then be brought 

on line!! BTW,I'm using a 56kmf,16V(20Vsurge)capacitor from Digi-Key, 

P/N P6878-ND, at less than $8. Less capacity should also work 

adequately,but the already small size, weight and cost doesn't 

contribute enough to cause concern.



What puzzles me is how the SD-8 and its regulator can respond to a

voltage as low as 1.0. Can someone explain this?



Jim McCulley

Tailwind

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nuckollsr(at)cox.net
Guest

Posted: Tue Apr 18, 2006 5:49 am Post subject: SD-8 PM Alternator

At 10:50 PM 4/13/2006 -0400, you wrote:



 	  Quote: 	
		  

<mcculleyja(at)starpower.net>



Bob N,et al,



There has been periodic discussion on this list in the past about the

SD-8(& maybe other permanent magnet alternators)not being capable of

coming on line if the battery or other power source to the buss fails or

is not otherwise available to activate the SD-8.


	


    <snip>



   You've done a nice piece of work here sir and I want you to

   know that my silence thus far is for want of all the materials

   I'm looking for (and time) to give you the reply it deserves.

   I'm running one foot in the bucket and the other in a fire

   right now . . . but I did find the schematic for a PM alternator

   that I'd misplaced. More on this soon . . .



   Bob . . .

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nuckollsr(at)cox.net
Guest

Posted: Sat Apr 22, 2006 4:56 am Post subject: SD-8 PM Alternator

At 10:50 PM 4/13/2006 -0400, you wrote:



 	  Quote: 	
		  

<mcculleyja(at)starpower.net>



Bob N,et al,



There has been periodic discussion on this list in the past about the

SD-8(& maybe other permanent magnet alternators)not being capable of

coming on line if the battery or other power source to the buss fails or

is not otherwise available to activate the SD-8.



I have experimented with a simple idea that solves this situation and

want to share the info to solicit comments on the suitability or any

downside features anyone may offer.



The circuit consists of tapping into the SD-8 regulator output and

feeding that through a diode to the positive side of an electrolytic

capacitor, grounding the negative lead of the capacitor and connecting a

spring loaded,normally-open push-button switch across the diode.



After engine start-up and during the subsequent pre-flight system check

of the SD-8, it charges the capacitor and the diode prevents loss of

this charge thereafter even if the SD-8 is not left switched on. If

total loss of electrical power to the buss occurs during flight, the

SD-8 will come on line by momentarily activating the push button switch

while also closing the SD-8 switch to the buss.



Through both ground and flight testing, I found no situation where

it failed to work. An unexpected finding is that the SD-8 will activate

with as low as 1.0 volt charge on the capacitor! Another bonus is that

the capacitor leakage rate is so low that from an initial charge of only

12.5 volts, it requires 2 hours short of 3 full days to leak down to

10.5 volts! (average of 0.0286 volts per hour). Of course, this rate is

progressively less as the charge voltage drops further, so if someone

left the Master on and killed the battery and then wanted to prop the

engine many days later(more than 2 weeks!)the SD-8 could then be brought

on line!! BTW,I'm using a 56kmf,16V(20Vsurge)capacitor from Digi-Key,

P/N P6878-ND, at less than $8. Less capacity should also work

adequately,but the already small size, weight and cost doesn't

contribute enough to cause concern.



What puzzles me is how the SD-8 and its regulator can respond to a

voltage as low as 1.0. Can someone explain this?


	


    First, let me commend you for the time and effort to perceive

    some design goals, craft and experiment and publish the results.

    Your efforts and questions have prompted me to consider the

    self-excitation deficiencies in SD-8 alternator regulators.



    Someone sent me a schematic of a Kubota regulator some years

    ago. This regulator includes a warning light function and I

    believe is typical of most PM regulators including the Ducati

    regulators supplied with Rotax engines. I've published the

    schematic at:



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



    If one deletes the features associated with the warning circuit, you get

    this:



http://www.aeroelectric.com/Pictures/Schematics/PM_Regulator/Kubota_v-reg.jpg



    If you focus just on hte warning circuit, here's the details.



http://www.aeroelectric.com/Pictures/Schematics/PM_Regulator/Kubota_Alt-Warn.jpg



    You'll note that the warning circuit turns out the light if

    the alternator is generating voltage and is not disconnected

    from the regulator. This is typical of MOST built-in 'idiot'

    lights on alternator regulator systems (and generators) since

    day-one. These lights are not definitive annunciations of alternator

    performance.  There are failure modes that do not light the

    light. Hence my long standing recommendation for ACTIVE NOTIFICATION

    OF LOW VOLTAGE independent of the alternator's built in features

    as being the definitive annunciation for loss-of-alternator.



    Your experiments have shown that the regulator will come alive

    with a very low voltage available at the 'b-lead'. A study

    of the schematic for the Kubota regulator leads one to

    conclude that once you have just enough voltage to forward

    bias some junctions (The b-e junction of Q3 through R3

    and D6) there is a potential for triggering the SCRs and

    having the system wake up. I suspect the B&C regulator

    is similar.



    The system you've crafted has demonstrated some utility

    for overcoming the SD-8 regulator's inability to self-excite.

    After some thought on the design goal, I thought of another

    approach to suggest. Since you're set up for and

    have the mind-set to conduct the necessary experiments,

    please consider . . .



http://www.aeroelectric.com/Pictures/Schematics/PM_Regulator/Self_Excitation_Experiment.jpg



    What we'd like to do is not let the b-lead voltage out

    of the regulator be 100% dependent upon triggered SCRs in

    the regulator. Suppose you added a couple of diodes

    from the alternator winding along with a resistor to

    ground to build a very inefficient keep-alive rectifier.

    The two diodes parallel the SCRs but with a conduction

    impedance too high for the system to deliver significant

    power. We then need some small, fixed 'load' across the

    output filter capacitor to prevent the unloaded alternator

    voltage (as high as 40v) from charging the capacitor

    to a level dangerous to the cap and system equipment

    in case the alternator control relay is closed while the

    capacitor is charged up.



    I've shown a pair of 1K resistors. Use mechanically

    hefty resistors . . . the 2W isn't need for electrical

    reasons, just mechanical. Adjust the size of the series

    resistor at the diodes to achieve a couple of volts or

    more (but not greater than 14) across the capacitor

    at max engine rpm with the alternator system OFF.

    I suspect this resistor might be as high as 3 to 10K

    and still make the system sleep with one eye open.



    The pilot-operated push-button is not an unreasonable

    solution too. At Cessna many moons ago, we crafted

    two systems for the same purpose. One used an array

    of d-cells, a diode and a push button to provide

    the pilot with a means of kick-starting a stalled

    alternator that did not have access to the ship's

    battery. On the military versions of the 337, we

    had tach generators (3-phase PM devices) that were

    fitted with rectifiers and push-buttons to do the

    same thing and eliminate the need for batteries.



    Here, we have an opportunity to go a step further

    and modify the regulator's performance such that

    it never quite goes to sleep and eliminate the

    need for pilot intervention to kick-start the

    system.  If you can assist in conduction of this

    experiment, we can prove/disprove my hypothesis

    and perhaps generate an article that will help

    others slay this dragon too.



    Bob . . .

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