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1. 11:41 AM - Re: Re: OVM14 MkIII, rev P1 (Robert L. Nuckolls, III)
2. 10:05 PM - Re: OVM14 MkIII, rev P1 (Eric Page)
Message 1
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| Subject: | Re: OVM14 MkIII, rev P1 |
>After reading the datasheet again, I can see that protecting the non-inverting
>inputs is a non-issue. The Vcc-2V figure is an operating recommendation; the
>absolute maximum is 38V. In the case of your P3 schematic, it would take 247V
>on the bus to exceed that limit on U16A!
Sure . . . in terms of avoiding catastrophic effects.
It seems more righteous to design for operational robustness.
So if the device is to avoid 'going blind' then Vcc-2 is
the target. Our voltage divider factor is 0.156 so
16/0.156 yields 51 volts. Well outside the expected stresses
described in Mil-STD-704.
>Got it. Exact delay value is unimportant, as long as it responds
>fast enough to
>protect and slow enough to avoid nuisance trips.
Yup
> > ...the weakest link in contemporary ov RELAY designs was relay CONTACTS
> > failing to break the inductively stabilized ARC.
>
>OK, here's where I have another question. My application for OV
>protection is on
>a Rotax 912iS, which has a built-in 450W 3-phase PM stator. Since I
>can't corral
>a stator by blowing a breaker, I'm adapting your design to drive relays.
YES!
>I've chosen an automotive relay with integral coil-suppression diode
>and contacts
>rated for 75VDC at 40A:
That will work. Those relays COULD be the
ALT ON/OFF control devices powered through a breaker
just like the field of a wound-field alternator.
The control switch would pull to ground. The
CBOVM module would tie to that supply breaker without
modification.
>Do you think that relay is likely to have any trouble breaking each leg of the
>Rotax stator feed? I'm guessing not since it's an AC circuit, but
>guessing has
>a poor track record...
Not at all. Remember that relay are rated
based on laboratory tests proving service
life on the order of tens of thousands of
cycles.
Okay, suppose you fly twice a day . . . how
long would you expect the relay to last?
I caution against using the normally closed
contacts of your control relays to open
2 of the three stator leads. Failure of
a relay coil offers an opportunity for a
latent failure not readily pre-flight
detectable.
Failure due to wear-out on electrical hardware
is more likely due to environmental stress with
age, brushed motors excluded!
Bob . . .
////
(o o)
===========o00o=(_)=o00o========
< Go ahead, make my day . . . >
< show me where I'm wrong. >
================================
In the interest of creative evolution
of the-best-we-know-how-to-do based
on physics and good practice.
Message 2
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| Subject: | Re: OVM14 MkIII, rev P1 |
Bob Nuckolls wrote:
> Those relays COULD be the ALT ON/OFF control devices powered through
> a breaker just like the field of a wound-field alternator. The control switch
> would pull to ground. The CBOVM module would tie to that supply breaker
> without modification.
Hmmm, yeah, that would work too... I've got a MOSFET switching the low
side of the relay coils, and an illuminated (sorry, I'm in the blinky light
camp!) reset/isolate pushbutton on the panel that will indicate a tripped
condition and provide pilot control or reset.
Since I'm not blowing a breaker, I'm just pulling the 2nd comparator's
reference input to ground to form a latch.
> Okay, suppose you fly twice a day . . . how long would you expect the relay to
last?"
Well, the relay's electrical life is 100,000 cycles at rated load. If you assume
each flight is one hour in length and there will be an OV event once every
1,000 hours (wildly pessimistic), then the relays will reach end of life in
>136k years. That should be long enough!
> I caution against using the normally closed contacts of your control relays
> to open 2 of the three stator leads. Failure of a relay coil offers an opportunity
> for a latent failure not readily pre-flight detectable.
The relays are SPST-NO type, and I was planning to use three of them in
parallel, switching all three phases of the stator's output. I won't pretend to
be knowledgeable about 3-phase AC systems; is there a better way to do this?
Assuming the regulator isn't designed to quit when one stator phase drops
offline, and it continues working from the remaining two phases, how would
one detect -- or pre-flight test for -- a single relay failure?
The only thing that comes to mind is to monitor all three phases after the
relays, and if one is missing, light a "Phase Missing" indicator. It's an
interesting problem to think about when you have no idea how much AC
voltage might be present on each phase over varying engine speed and
regulator load...
Thanks, Bob.
-Eric
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http://forums.matronics.com/viewtopic.php?p=511559#511559
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