14-10-2012, 01:45 PM
If the contacts are switching an inductive load they'll be arcing and a capacitor or something across them may well help.
- Joe
- Joe
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Contactor Issues
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14-10-2012, 04:10 PM
The general-purpose airframe-mounted relays with a 28v DC coil, used on many modern aircraft for all sorts of switching applications, normally as standard have a diode built-in across the coil to protect whatever switch contacts are operating the relay coil. The relay coil is then polarity-sensitive - it must be connected so the diode is reverse-biassed when the coil is energised. When the controlling switch opens the diode becomes forward-biassed and dissipates the energy stored in the relay coil.
It's probably worth following this practice, add a diode across the contactor coil rather than the switch contacts - and make sure its the right way round! The back-emf can be a surprisingly high voltage, so don't skimp on the diode ratings. Andy
14-10-2012, 04:52 PM
Photo 1 shows load being switched, photo two shows contacts taken from burnt out teddington, photo three shows a teddington unit minus capillary.
14-10-2012, 06:14 PM
I have to admit that I've never worked with this level of voltage and current. However, I have had some experience with smaller relays in various applications.
Several theories spring to mind, most of which are mentioned above:- 1. Something is missing. 2. Something is faulty. 3. They apparently have limited life but that's because they don't operate very often in normal service. 4. They are designed and rated like that because they HAVE to work when required, and operational life is immaterial. This is interesting - http://relays.te.com/appnotes/app_pdfs/13c3236.pdf Alan P.S. None of the Markup Icons at the top of the Post box - Bold, Underline etc - appear to work, at least not for me 
14-10-2012, 06:31 PM
Hmm, [undefined=undefined][undefined=undefined]interesting[/undefined][/undefined].
That's Bold and Underlined. What am I missing? Alan
14-10-2012, 06:37 PM
I think option 4 is most likely, bust there is a second teddy that takes all power away in the event the first teddy fails or the cooling is not enough, belt and braces comes to mind.
14-10-2012, 06:38 PM
I'll have to e-mail you as if I type the instructions here they'll activate rather than show.
The easy way is highlight what you want to change and then click the appropriate icon. - Joe
14-10-2012, 06:44 PM
Alan.
Interesting read, some arcing *can* be benificial and the section on nickle silver contacts does reflect the pitting seen on the contacts. I'm on the lines of thinking that supressing the arc but not fully extinguishing it would be a better option in order to keep the contacts clean other than relying on the self wiping nature of the switch. |
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