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Contactor Failure

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speedy11(at)aol.com
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Posted: Tue Dec 22, 2009 6:57 pm Post subject: Contactor Failure

Bob,

 Sorry to be so long in responding.

 Very interesting analysis.

 I will follow your advice.

 Thanks,

 Stan Sutterfield

  

  	  Quote: 	
		  At 08:46    AM 12/2/2009, you wrote:

The continuous duty contactor I have installed is    temporary.  I will 

replace it with an intermittent duty one.



I    installed the Skytec starter as it arrived from the manufacturer - 

that    is, with the jumper from the contactor's control line to the 

main terminal    installed.  I left the jumper installed because Skytec 

recommend it -    see    

http://www.skytecair.com/images/Certified%20LS%20Wiring_1100.jpg.  My    

error, of course, was that I left out what they call the master    solenoid.



   I'm just guessing . . . but this configuration    was probably

   crafted to MINIMIZE changes to an existing system    in a TC

   aircraft. Adding the jumper to the starter contactor    (a

   technique adopted by B&C many years earlier) offers    three

   advantages. (1) you get a drop-in replacement of a    previously

   installed starter and (2) buffers existing ship's    wiring from

   experiencing the extra-ordinary inrush offered by    the two-stage

   solenoids as described in my article and (3) the    PM motor inrush

   current doesn't hit the system until BOTH the    standard starter

   and built-in contactors close. Since the    standard contactor

   closes first, there's a potential for    improving life of this

   contactor IF it's contacts are closed    and stable when the

   second set of contacts on the starter get    closed a few milliseconds

   later. The DOWNSIDE is the potential    for suffering delayed

   pinion dis-engagement.

This    is Van's recommended wiring    

http://www.skytecair.com/images/Van's%20Starter%20Wiring%20Lg.jpg, 

but    Skytec recommends against wiring this way.



   This is pretty    slick. The diagram suggests using the auxiliary

   "I" (ignition    boost) terminal available on most starter

   contactors to close    the starter-mounted contactor  . . . again

   with the idea    of shifting motor inrush currents to the

   starter's internal    contactor. It has the advantage of breaking

   the internal    contactor's coil current when the external starter

   contactor    opens.  This accomplishes the same goals as Z-22

   but with    an external STARTER CONTACTOR as opposed to an external

   BUFFER    RELAY.

Bob, are you implying that I should have removed the jumper    wire from 

the starter?



   Not implying, recommending . .    . based on what we learned

   about (1) delayed disengagement and    (2) higher stresses

   on starter control contactor with    substitution of PM

   motor technology. If you already have an    external contactor

   installed (a la Van's suggestion) then    making a simple change

   to remove the jumper and wire the    internal contactor control

   terminal to the external contactor    "I" terminal would be

   a good move.



   Van's    drawing or Z-22 is the elegant solution for new design

   in an    OBAM aircraft. Skytec's drawing is the less than elegant

      solution for minimizing changes to a TC aircraft when    replacing

   original starter called out on the ship's    certification documents.

   The way you had it wired ORIGINALLY    wasn't all that hard on

   the external contactor . . . assuming    that it's contacts were

   closed and stable before the internal    contactor gets closed.

   I've not had an opportunity to measure    it but I suspect this

   is the case. THEREFORE, I'm comfortable    with suggesting that

   your original sticking failure was    probably an isolated case

   and not indicative of a system    design error. ADDING the battery

   master contactor in series    with cranking currents is the

   firewall against a future repeat    causing you to burn a battery

   or starter.



      Adding the wire to conform to Van's suggested configuration

      would be a good move to take care of the potential for delayed

      disengagement.



        Bob . .  . 	


 

  

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