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Spark suppression diode on starter MOTOR

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Apr 03, 2022 9:53 am Post subject: Spark suppression diode on starter MOTOR

At 12:48 PM 4/2/2022, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>



 Batteries don't absorb the energy from inductive voltage spikes. 	


   Correct. A battery's ability to mitigate

   hazardous stimuli in the electrical system

   is limited to standing off a runaway power

   generation system for the very short period

   of time necessary for the o.v. protection

   system to operate (hundreds of milliseconds).



   This is a HIGH energy event (sometimes exceeding

   10,000 watt-seconds).



   "Spikes" are short duration (less than 10mS)

   excursions generated by release of reactively

   stored energy (usually inductive, sometimes

   capacitive) and is a very low energy event

   measured in millijoules (1/1000th of a watt-

   seconds).  These events are never a source

   of catastrophic system damage. They will

   sometimes manifest in transient mis-behavior

   of a system that has NOT been qualified to

   live in the legacy, aircraft electrical

   environment.

   

  	  Quote: 	
		  The controlling switch contacts do.  As the switch contacts open,

 an arc jumps across the opening contacts.  The energy is dissipated as heat. 	


   Some years back, we had some deep-dives into

   physics and terminology of electrical transients

   here on the List. I produced a number of bench-top

   demonstrations illustrating the sources, effects and

   mitigation of low energy transients (true spikes).

   The attached plot was taken from one of those

   experiments.



   An S701 battery contactor coil collapse

   spike was plotted while being controlled by

   a toggle switch and NO coil suppression. Note

   that the maximum excursion was on the order of

   250 volts impresses across the spreading

   contacts gap in the switch. An arc formed

   between those contacts which persisted for

   about 1.4 mS at which point the arc broke

   and remaining energy took the transient to

   its maximum excursion.



   Energy dissipated in this even was on the order

   of tens of millijoules. No single event

   being hazardous to the switch . . . but hundreds

   of events would eventually erode the switch's

   contacts.



   I couldn't quickly find examples of how much energy

   from such events propagates out onto the system during

   a de-energized non-suppressed coil event . . . but

   I have measured it. It's TINY . . . 99.9% of the

   energy is spent on the controlling device unless

   managed with appropriate suppression. Even if the

   battery WERE a valuable member of the spike suppression

   community, there wouldn't be anything for it to do!



   Two other images illustrate a study I did

   at Beech on another contact failure investigation.

   You can see the difference in burning and metal

   transfer of controlling contacts with no

   transient suppression on the loading device.



  	  Quote: 	
		  Starter motors have counter EMF that might limit inductive voltage spikes. 	


   True . . . but they are still quite stressful on

   their controlling device. This is why starter contactors

   are specially crafted for DOUBLE break contacts

   to increase contact spreading velocity for early

   break of the inevitable albeit CEMF mitigated

   arc.



   Perhaps one MIGHT improve service life of a starter contactor

   with some purposeful suppression of the arc but

   the contactor designers have pretty well slain

   that dragon. 



 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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Un-Suppressed_Battery_Contactoir_Coil_Spike.jpg

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100k_Cycle_Contact_NO_suppression.jpg

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Apr 04, 2022 11:45 am Post subject: Spark suppression diode on starter MOTOR

 	  Quote: 	
		  

 Viewed another way, if we used a robust enough switch to control the relay,

 with really fast acting contacts, we might be able to eliminate the

 flyback diode from the relay coil. But the diode is much cheaper/simpler

 than a switch that could handle the arc issue without damage.



 Charlie 	


   Dead on. The only thing I would expand upon

   is the term 'damage'. 'Wear' is the operative

   term. I recall a demonstration I did many moons

   ago where I switched a 1.5 volt flashlight cell

   across a 1 ohms resistor.



   Approx 1+ amps of current flow. I did this with

   a sub-miniature, 'micro' switch with the side

   sanded off so that one might observe the inner

   workings.



   While watching the contacts under a microscope

   in a dark room, I could see a tiny arc between

   the spreading contacts. The color of the arc

   was blue . . . HOT. This suggests that for

   even the smallest energy levels, the potential

   for erosion of metal off the contact surfaces

   is not zero; hence wear is not zero either.



   Engineering applications data for all switches

   will include predicted SERVICE LIFE values

   for switches under various operating conditions.

   See:



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



   Diodes didn't even start to show up in TC aircraft

   until the late 60's.  I was at Cessna when we

   started fabricating diode assemblies to facilitate

   installation in various places about the systems

   including contactor coils.



   I remember Gordon Wood (PhD in physics)

   bought an HP peak-reading voltmeter and discovered

   that some contactor coil spikes exceeded 400-500

   volts! The fact that such a potential existed

   was contrary to our pledge of compliance with

   Mil-STD-704 . . . so began a program to mitigate

   our 'transgression'. It had nothing to do with

   life of the battery contactor switch . . . 

   that astounding spike that just had to be dealt with.



   Don't recall any discussions about ENERGY nor

   how it might have propagated into the system . . .

   but we sure stomped out those spikes. Here's

   a constellation of Cessna diode assemblies

   dating from the 1960's to present day.



  http://aeroelectric.com/Pictures/Cessna_Diode_Assy/



   Are we not grateful NOT to own a TC aircraft?



   Competent design is based on properties of materials,

   management of energy and refinement of process.

   But as we may observe in the photos cited above,

   'refinement' hasn't really moved in the

   right direction.



    



 

 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Apr 05, 2022 7:26 am Post subject: Spark suppression diode on starter MOTOR

At 03:11 AM 4/5/2022, you wrote:

  	  Quote: 	
		  ..and one should consider duty cycle?



 On Mon, 4 Apr 2022 at 23:29, Robert L. Nuckolls, III < nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

  	  Quote: 	
		   

  Viewed another way, if we used a robust enough switch to control the relay,

  with really fast acting contacts, we might be able to eliminate the

  flyback diode from the relay coil. But the diode is much cheaper/simpler

  than a switch that could handle the arc issue without damage.



  Charlie 	
 	


   Absolutely. Service life on switches (and relays) is

   predicated on the type of load being managed by the

   device. Those loads are divided into resistive (heaters),

   inductive (coils and un-terminated transformers),

   and lamp (high inrush like incandescent lamps, motors,

   capacitive). There can be a huge variance of predicted

   service life on the same switch over that range of

   loads.



   But note: Service lives are generally given in thousands

   or tens of thousands of operations. After all, who would

   even consider a switch rated for only 500 operations?

   Service life numbers are critical to revenue generation

   and risks for loss-of-vehicle (and the folks inside).

   Hence, folks who design and maintain air transport

   or military aircraft tend to migrate to the higher

   numbers. It's not only a matter of safety but cost

   of operation. The boss is really unhappy when a $million$

   machine is down for lost functionality in a $10 switch.



   We who fly around in machines with lower criticality

   can reduce hazard with failure mode effects analysis

   while mitigating expense with owner built and maintained

   aircraft.



   Back to that duty-cycle thingy. Over the lifetime

   of the airplane, how many times will the most exercised

   switch be operated? Hmmm . . . battery master? Landing

   gear? Trim switch? Last stat I read (many moons ago)

   was that the typical light aircraft is flown 50 hrs

   a year. How many flight cycles is that? What's

   the probability of using up all nine-lives of the

   most severely stressed switch in the airplane?



   Those are all questions for which we can apply no

   specific numbers . . . our designs and utilization

   goals are anything but 'average'. But with attention

   to simple details and diligent preventative

   maintenance, risk for an unhappy day in the cockpit

   due to electrical issues is exceeding small compared

   to fuel exhaustion, weather, poor mission

   planning, etc.



   Bottom line is that pleasures to be secured

   with the operation of your airplane has much

   more to do with being a pilot than with worrying

   about diodes on the relays. 



 

 

   Bob . . .



   Un impeachable logic: George Carlin asked, "If black boxes

   survive crashes, why don't they make the whole airplane

   out of that stuff?"

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