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Spark suppression diode on starter MOTOR

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tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Sat Apr 02, 2022 5:06 am Post subject: Spark suppression diode on starter MOTOR

It's common (or maybe not so common) knowledge that spark suppression diodes are a good idea on relays and contactor coils.  But I've found precious little discussion about putting one on the starter MOTOR (B lead).  The motor windings have the same issue of collapsing magnetic fields.  Is the battery supposed to absorb that energy?  This is more of an academic question than a practical one, since airplanes aren't falling out of the sky without them.



Anyone?

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Sat Apr 02, 2022 9:48 am Post subject: Re: Spark suppression diode on starter MOTOR

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

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

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

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

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Apr 03, 2022 9:55 am Post subject: Spark suppression diode on starter MOTOR

At 08:06 AM 4/2/2022, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "tomcostanza" <Tom.Costanza(at)gmail.com>

 	


 P.S. Heres an article I posted about 22 years

 ago on the topic of spike suppression . . .



  https://tinyurl.com/y79ht8zm



 



 

 

   Bob . . .



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

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

   out of that stuff?"

tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Mon Apr 04, 2022 3:44 am Post subject: Re: Spark suppression diode on starter MOTOR

Thanks for that Bob.  I read the article in its entirety.  But I didn't see where it answered my question.  To be clear...I understand why we put diodes across the coil of a contactor or relay.  My question is, why don't we put diodes across the starter MOTOR?  That has collapsing magnetic fields also doesn't it?  



My conjecture tells me that:

The starter contactor has already disconnected the starter motor from everything.  So no harm done if there is a flyback current.  But I'm not an engineer, and would very much appreciate your educated explanation.



I did see one or two articles for Ford Mustang owners to add a diode here, but that's all.  And it didn't explain why; just do it.



Thanks again Bob for all you have and continue to contribute to our knowledge of all things electronic.



Stay healthy everyone.

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

Ceengland

Joined: 11 Oct 2020
Posts: 391
Location: MS

Posted: Mon Apr 04, 2022 5:52 am Post subject: Spark suppression diode on starter MOTOR

On Mon, Apr 4, 2022 at 8:20 AM tomcostanza <Tom.Costanza(at)gmail.com (Tom.Costanza(at)gmail.com)> wrote:



 	  Quote: 	
		  --> AeroElectric-List message posted by: "tomcostanza" <Tom.Costanza(at)gmail.com (Tom.Costanza(at)gmail.com)>

 

 Thanks for that Bob.  I read the article in its entirety.  But I didn't see where it answered my question.  To be clear...I understand why we put diodes across the coil of a contactor or relay.  My question is, why don't we put diodes across the starter MOTOR?  That has collapsing magnetic fields also doesn't it?  

 

 My conjecture tells me that:

 The starter contactor has already disconnected the starter motor from everything.  So no harm done if there is a flyback current.  But I'm not an engineer, and would very much appreciate your educated explanation.

 

 I did see one or two articles for Ford Mustang owners to add a diode here, but that's all.  And it didn't explain why; just do it.

 

 Thanks again Bob for all you have and continue to contribute to our knowledge of all things electronic.

 

 Stay healthy everyone.

 

 --------

 Clear Skies,

 Tom Costanza

 -- in year 17 of a 3 year project

 

 	
Hoping Bob won't mind if I take a swing at rearranging his answer...

Perhaps it's your starting point that's the problem. It isn't 'everything' that a flyback diode protects; it's the controlling switch's contacts that need protection from the arc that forms as they open.The suppression diode on the coil of the contactor protects the (relatively) weak pair of contacts in the switch that controls it, from the relay coil flyback current. With the starter (obviously a much larger, higher energy device than the relay coil), engineers designed its 'switch' (the starter relay) with *two* pairs of contacts, and a much stronger spring to open them than is seen in other relays. The combination of rapid opening due to the strong spring, and the division of load and arcing between the two pairs of contacts, and the overall extremely robust nature of the contacts, means that a flyback diode isn't needed to protect the relay contacts.  

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

_________________
Charlie

tomcostanza

Joined: 19 Oct 2008
Posts: 49

Posted: Mon Apr 04, 2022 6:50 am Post subject: Re: Spark suppression diode on starter MOTOR

Thanks Charlie.  So if I understand you correctly, there still is a flyback current from the motor, it just isn't an issue because the contacts on the starter contactor are more robust than a normal relay or battery contactor.  Did I paraphrase your answer correctly?

_________________
Clear Skies,
Tom Costanza
-- in year 17 of a 3 year project

Ceengland

Joined: 11 Oct 2020
Posts: 391
Location: MS

Posted: Mon Apr 04, 2022 7:24 am Post subject: Spark suppression diode on starter MOTOR

On 4/4/2022 9:50 AM, tomcostanza wrote:

 	  Quote: 	
		   



 Thanks Charlie.  So if I understand you correctly, there still is a flyback current from the motor, it just isn't an issue because the contacts on the starter contactor are more robust than a normal relay or battery contactor.  Did I paraphrase your answer correctly?



 --------

 Clear Skies,

 Tom Costanza

 -- in year 17 of a 3 year project

Yes, that's a pretty good paraphrase of my paraphrase (which I *hope* is 
	


a good paraphrase) of Bob's original explanation.







Charlie



-- 

This email has been checked for viruses by Avast antivirus software.

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_________________
Charlie

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Mon Apr 04, 2022 8:51 am Post subject: Re: Spark suppression diode on starter MOTOR

If it is desired to put a diode in parallel with the starter motor, the easiest way is to connect the banded

end to the downstream fat terminal of the starter contactor and the other end to ground.

 It is really hard on a start contactor to chatter while attempting to crank the engine with a weak battery.

_________________
Joe Gores

Ceengland

Joined: 11 Oct 2020
Posts: 391
Location: MS

Posted: Mon Apr 04, 2022 12:35 pm Post subject: Spark suppression diode on starter MOTOR

On 4/4/2022 2:31 PM, Robert L.       Nuckolls, III wrote:

     

      	  Quote: 	
		                

         

                    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 . . .

            	
     EEYouch!

     At those prices, a switch would be cheaper. (Unless it was a     'certified' switch...)

     	          		Virus-free. www.avast.com 		 	 [url=#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2] [/url]

_________________
Charlie

bob.verwey(at)gmail.com
Guest

Posted: Tue Apr 05, 2022 12:12 am Post subject: Spark suppression diode on starter MOTOR

..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: 	
		     	  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?"

 

  	


-- 

Best Regards,Bob Verwey



082 331 2727

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