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Contactor clicks but doesn't make electrical contact

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed May 08, 2013 4:03 pm Post subject: Contactor clicks but doesn't make electrical contact

At 08:51 AM 3/25/2013, you wrote:


<mrspudandcompany(at)verizon.net>



Bob,



Re: the contactor that we were discussing a week or two ago.  I 

believe that it is the original assembly in a C177B.



I removed it yesterday and replaced with a 3 terminal, same as was 

originally installed, unit and it works perfectly.



The junk item went in the mail this AM, so you should have it in a 

couple of days.  It will be interesting to find out what you observe 

as the failure when you tear it down.



Thanks for what you do!



Roger

   Thanks for sending me the carcass. I've disassembled the

   remains. The failure mode in this case is quite clear

   and consistent with your narrative for intermittent

   functionality.  The photos at



http://tinyurl.com/cp4xkv3

http://tinyurl.com/d95qhst

http://tinyurl.com/bwqxm5v

http://tinyurl.com/bocgcaa



   . . . show effects of selective corrosion.



   The next photo http://tinyurl.com/butbm2m shows that

   all conduction surfaces are bright and clean having

   been properly torqued for gas-tightness.  External

   joints on wires brought to the contactor were

   properly installed and not contributors to this failure.



   This photo of contactor ring http://tinyurl.com/cm4ncou

   suggests that the ring was free to rotate as evidenced

   by the relative uniformity of the oxidation pattern

   in the contact area.



   The obverse side of the contactor ring http://tinyurl.com/d8pgygg

   shows no evidence of localized heating. The c-clip that retains

   the closure pressure ring shows evidence of localized corrosion.



   http://tinyurl.com/bu3paoo here we see the opening spring

   is uniformly rusted. http://tinyurl.com/d6wo24e The rust

   is finely grained and easily transfers to the touch.



   The drain hole in the contactor cap is open and clean. The inside

   surface of the cap http://tinyurl.com/c6fk5kh shows evidence

   of moisture pooling.



   The outside surface of the cap http://tinyurl.com/cn8t7rl shows

   evidence of condensation drip down the outside surface that

   concentrated as surface wetness.



   This is a good example of a 'soft' failure. The corrosion

   patterns suggest mild effects over a very long period

   of time. Concentrations and locations suggest the contactor

   had been properly installed with the drip-hole down.



   The intermittent nature of the malfunction suggests that

   rust-dust off the spring would fall onto the upper side

   of the contactor ring and provide a poor to no-conductivity

   contaminant that would prevent the ring from making good

   connection with the terminals. That when contact was good,

   any dust in the gap would fuse to become part of the

   discoloration on the contactor ring . . . which was free

   to rotate and bring a less-contaminated area into service.



   This failure was a very slow progressing event promoted

   by years of normally deposited condensate. Condensate that

   would occasionally produce low-point concentrations

   of moisture on the end of the armature, upper inside

   surface of the cap, and on the lower outside surface

   of the cap.



   Given that the failure was not gross and catastrophic,

   it appears that everything was functioning to design

   goals and that the contactor had simply reached the

   end of service life for the conditions under which

   it operated.



   Bob . . .

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Eric M. Jones

Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts

Posted: Thu May 09, 2013 5:30 am Post subject: Re: Contactor clicks but doesn't make electrical contact

I don't agree that this is just normal wear and tear. The Type-70 contactor is unsuited to use in aircraft, and the proof is in the photos. (Good pix Bob!).



I don't see any mention of coil suppressors. There should be some and they should be bidirectional zeners (or at least not p/n diodes). Potter and Brumfield, etc. have campaigned for this for years. The reason for coil suppression is that it speeds up opening and minimizes arcing and welding when opening.



Better yet, check the data sheet for these contactors and see that they have a maximum ambient of 122 degrees F., and G-sensitive, not sealed, high hold current, poor capability opening under load...then ask yourself why you are using them. They're cheap? Clique and Claque says, "It the stingy man who pays the most."



I'd go with using either NO contactors, solid state contactors, or Kilovac EV200 or Gigavac MX-11 or similar. There are lots of good products out there.



Hint: Buy EV200's where the leads are too short. It is simple to unscrew the cover and reinstall long leads.

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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net

nuckolls.bob(at)aeroelect
Guest

Posted: Thu May 09, 2013 8:13 am Post subject: Contactor clicks but doesn't make electrical contact

At 08:30 AM 5/9/2013, you wrote:

 	  Quote: 	
		  



I don't agree that this is just normal wear and tear. The Type-70 

contactor is unsuited to use in aircraft, and the proof is in the 

photos. (Good pix Bob!).


	


   But Eric, I think we were told that this was the

   factory installed contactor . . . Given the

   demonstrated longevity of this part and

   the fact that it failed gracefully suggests

   that the contactor not over stressed nor

   was it mis-applied to tasks beyond design goals

   of the manufacturer.



 	  Quote: 	
		  I don't see any mention of coil suppressors. There should be some 

and they should be bidirectional zeners (or at least not p/n 

diodes). Potter and Brumfield, etc. have campaigned for this for 

years. The reason for coil suppression is that it speeds up opening 

and minimizes arcing and welding when opening.


	


   Yes you have complained . . . and I have explained

   and conducted demonstrations on the bench to (1)

   show that there IS value for coil suppression with

   respect to life of the battery master switch and

   (2) shown that such suppression in the form of

   plain vanilla rectifiers -OR- zeners has no measurable

   influence on service life of the contactor itself.



   These contactors, applied as battery contactors

   in airplanes, DO NOT MAKE or BREAK heavy loads.

   As long as you don't drown them in water or oil,

   their service life is exemplary with respect to their

   cost.



   I have further debunked the notion that contacts

   weld "on opening". I've studied many cases of relay

   failures for sticking contacts and deduced the

   "on closure" sticking modes for relays both by

   analysis of carcasses -AND- laboratory experiment.

   Failure to close is ALWAYS a function of contact

   contamination or degradation due to mis-application.



   I've repeatedly measured the contact spreading

   velocity of relays and contactors with plain

   vanilla diode coil suppression, sophisticated

   coil suppression and NO suppression and found

   no practical differences. The various modes

   might affect contact opening DELAY but not

   the spreading velocity. Go to the bench, get

   out your 'scope and prove these assertions

   wrong.



   This contactor WAS fitted with a plain-vanilla

   rectifier as a coil spike suppressor. What evidence

   is visible in the photos that an alternative form

   of suppression would have made a difference?



 	  Quote: 	
		  Better yet, check the data sheet for these contactors and see that 

they have a maximum ambient of 122 degrees F., and G-sensitive, not 

sealed, high hold current, poor capability opening under load...then 

ask yourself why you are using them. They're cheap? Clique and 

Claque says, "It the stingy man who pays the most."


	


   What evidence do you see in the photos that suggest

   this part was thermally abused? What evidence do

   you see that suggests this contactor failed

   to function for any reason other than the

   'dusting of contact surfaces' with materials

   detrimental to good contact (I.e. spring rust)?



 	  Quote: 	
		  I'd go with using either NO contactors, solid state contactors, or 

Kilovac EV200 or Gigavac MX-11 or similar. There are lots of good 

products out there.



Hint: Buy EV200's where the leads are too short. It is simple to 

unscrew the cover and reinstall long leads.


	


   An what is the return on investment? There have

   been hundreds of thousands of airplanes fitted with

   contactors not unlike the device under discussion.



   If the airplane is fitted with an e-bus, then

   contactor failures of any nature have a back-up

   plan for comfortable termination of flight. Even

   THIS contactor did not produce an in-flight

   condition that threatened life, limb or aluminum.



   Indeed, battery contactors are a maintenance

   item not unlike tires, switches, light bulbs,

   batteries, oil, Plexiglas windows, paint and

   the occasional smoothing of nicks in the prop.

   If one were to get a prop crafted from say,

   titanium, perhaps one could dispense with

   the file.



   Yes, a $high$ contactor might prove to offer

   a longer service life but it has zero effect

   on system reliability. That vulnerability

   to g-loading is another widely circulated

   myth having no foundation in physics or practical

   application.

   Bob . . .

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