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paulm(at)olypen.com
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 Posted: Tue Aug 22, 2006 8:42 pm Post subject: MOV's/ opening arc |
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I disagree that opening time is not significant as related to an aircraft
application.
I have done dozens of repeatable tests where the contact opening rate was
the subject and have demonstrated the duration of the contact opening arc is
extended and the arc intensity is much greater, specifically if you are
looking at a big Contactor opening an inductor or generator like an
alternator. This when the common diode is used across the coil.
The issue of "opening time" is very important as the contact suppressor
Transorb must absorb the energy from the alternator load dump over 50 ms vs.
10 ms (typical) and this means the transorb must be 5X bigger in heat
rating. (The need for a contact suppressor is below a ways)
The specific test set up is the contactor has either the diode or the
bi-directional transorb across the coil and there is a uni-directional
transorb clamping the alternator load dump on the alternator side of the
contacts. The alternator is either properly running or has been failed to
produce full output with no voltage control.
Perhaps not clearly stated above, but the testing was done several years ago
to simulate internal regulator alternators where the "B" lead was opened to
protect the system (Or the pilot simply opened the "B" lead on a healthy
alternator for what ever reason including the false triggering of a OV crow
bar circuit. From the point of the opening of the relay coil to the end of
the time the relay contacts are fully open the Transorb clipping the OV was
tasked with clamping the voltage.
The point is that with out the transorb to clamp the alternator output, the
opening arc voltage jumped to over 100V and the open contacts continued to
arc after they were fully open (this happens regardless of any OV crowbar
present as the crow bar is on the other side of the open contacts). The
Transorb on the alternator side clips the arc voltage during the opening
process and only after the contacts were fully open and unable to restart an
arc did the transorb fail short and then open. Thus the time from relay coil
power off to final contact open determines the power rating of the transorb
and using a diode across the coil added 40+ ms to the total time which is
typically 5 times longer that the transorb must keep the alternator voltage
in line if the transorb was across the coil. Further the duration and
intensity of the arc was increased significantly. If the transorb was not
rated high enough it failed during the process and the arc continued after
the contacts were fully open. For example 50 amps for 50 ms is a lot of
power for the common transorb and they fail short and then quickly fail
open.
Its well known that relays that are required to break DC voltages above the
40-50V range must use a arc quencher circuit or they usually will simply
never stop arcing. Thus something is required to keep the arc voltage under
say 35V until the contacts are well open where the contact gap is big enough
to prevent a restart of the arc.
My conclusion to the above tests is opening a failed internally regulated
alternator where the failure was unregulated high voltage output is not as
simple as some would believe and the "B" lead contactor (if not special
design) has the potential to continue to arc after any type of OV protection
has opened the contactor.
Paul
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