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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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 Posted: Tue Nov 13, 2007 5:47 pm Post subject: Master Relay Switching High Current |
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I now have a hundred hours on my Lancair Legacy wired using Z13-8. I'm happy that I've only had a few electrical issues, none of which were significant. But I'm wondering about my master relay, a continuous duty one from B&C.
My airplane uses a hydraulic pump to raise and lower the landing gear. The pump is powered by an electric motor protected by a 40 amp ANL fuse. I haven't measured it, but factory specs indicate that current peaks at close to 100 amps just before the pump shuts off as it produces peak pressure.
The hydraulic pump automatically turns on and off to maintain set pressure whenever the master switch is on. When the airplane is parked overnight, hydraulic pressure bleeds down enough that the automatic pressure switch closes. Of course, since the master switch is turned off, the pump doesn't turn on.
But when I turn on the airplane's master switch, the hydraulic pump immediately runs for a split second to restore hydraulic pressure. I think that the master relay is therefore switching approximately 100 amps. My understanding is that a continuous duty relay, like the master relay, can carry large current loads once it is closed, but it isn't good at switching large loads.
Should I expect premature failure of my master relay? If so, I'll just carry a spare. They're reasonably cheap and light weight.
Thanks,
Dennis Johnson
Lancair Legacy #257
[quote][b]
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mprather(at)spro.net
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| Posted: Tue Nov 13, 2007 6:23 pm Post subject: Master Relay Switching High Current |
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There may be some concern with making the connection (turning on the
system including the pump), but I'd guess the real limitation is on
breaking the connection. It wouldn't surprise me if your master relay
lives a normal lifespan under this kind of use. I'll be interested in
what the experts come up with.. 
Regards,
Matt-
| Quote: | I now have a hundred hours on my Lancair Legacy wired using Z13-8. I'm
happy that I've only had a few electrical issues, none of which were
significant. But I'm wondering about my master relay, a continuous duty
one from B&C.
My airplane uses a hydraulic pump to raise and lower the landing gear.
The pump is powered by an electric motor protected by a 40 amp ANL fuse.
I haven't measured it, but factory specs indicate that current peaks at
close to 100 amps just before the pump shuts off as it produces peak
pressure.
The hydraulic pump automatically turns on and off to maintain set pressure
whenever the master switch is on. When the airplane is parked overnight,
hydraulic pressure bleeds down enough that the automatic pressure switch
closes. Of course, since the master switch is turned off, the pump
doesn't turn on.
But when I turn on the airplane's master switch, the hydraulic pump
immediately runs for a split second to restore hydraulic pressure. I
think that the master relay is therefore switching approximately 100 amps.
My understanding is that a continuous duty relay, like the master relay,
can carry large current loads once it is closed, but it isn't good at
switching large loads.
Should I expect premature failure of my master relay? If so, I'll just
carry a spare. They're reasonably cheap and light weight.
Thanks,
Dennis Johnson
Lancair Legacy #257
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nuckolls.bob(at)cox.net
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| Posted: Wed Nov 14, 2007 6:59 am Post subject: Master Relay Switching High Current |
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The data sheet for the Stancore-WhiteRogers contactors
can be viewed at:
http://www.aeroelectric.com/Mfgr_Data/Contactors/Stancor-WhiteRogers/Stancor_70-Series.pdf
The p/n of interest to us is the 70-902 which is
said to have a service rating of 80A Continuous and
150A inrush. Further down in the data sheet, we're told
that the manufacturer signs up to 100,000 cycles
of performance at rated load.
You have correctly observed that the added load
of a hydraulic pump motor becoming active at the same
time the battery contactor is closing (contacts
bouncing) is more stressful than if the contactor
is allowed to close and become stable before the
added stress of starting a motor is added
to the mix.
However, the stresses are not extra-ordinary with
respect to the device's ratings. Having said that,
it's a given that your contactor isn't going to last
100,000 cycles in your airplane . . . with or without
added motor startup loads. Laboratory confirmation of
service life was accomplished at 4 cycles per minute
so the whole 100,000 cycle test took 25,000 minutes or
about 17 days in the relatively pristine environment
of the test lab sans humidity and temperature
cycles applied over years of service.
In your airplane, the effects of TIME will prove more
severe than effects of utilization and it would not
be surprising for you to experience a contactor failure
on a par with experience in the field with the same
contactor on other aircraft. Replacement every 5-10
years is average. I had two Cessnas in my rental fleet
need new contactors in the 6-months I owned the airport
and I have no idea how long these were in service but
it's a certainty that neither device had more than
a few thousand cycles on them.
Suffice it to say at about $18 each ( http://alliedelec.com
and http://newark.com ) the 70 series devices are a good value
on spam-cans and even better value on OBAM aircraft
architectured for failure-tolerance.
It's a good that you're perceptions of "added
stress" are correct and that you've raised the question.
The answer is, "I see no return on investment of the $time$
needed to mitigate these stresses." Yes, in a fleet of
100 aircraft wired and operated just like yours, you
MIGHT see a observable decrease in service life in battery
contactors for ships that DO get the battery-on motor
inrush load as opposed to those that DON'T, But in the
grand grand scheme of things, the differences will be
insignificant.
At 07:22 PM 11/13/2007 -0700, you wrote:
| Quote: |
There may be some concern with making the connection (turning on the
system including the pump), but I'd guess the real limitation is on
breaking the connection. It wouldn't surprise me if your master relay
lives a normal lifespan under this kind of use. I'll be interested in
what the experts come up with..
|
YES! A study of relay and contactor switching physics
readily demonstrates that for most applications, contact
opening is the more stressful activity. But a feature
of switch physics oft overlooked is contact BOUNCE that
occurs every time the switch closes. See:
http://www.aeroelectric.com/Pictures/Curves/switch_transition_CK7201.jpg
This is the transition/conduction trace for a miniature
toggle switch like:
http://www.aeroelectric.com/Pictures/Switches/miniswitches.jpg
http://www.aeroelectric.com/Pictures/Switches/CK7000.jpg
Note that total transition time from first motion
of closed contact until stable condition against
the opposite contact is small . . . about 1.4 milliseconds.
Note further that the contact closes, opens and re-closes
about 8 times in the exemplar trace. This is TYPICAL of
virtually all mechanical switches, relays and contactors.
This BOUNCE means that the contact OPENS under inrush loads
multiple times for each action that goes toward moving the
switch to a new, CLOSED position. I have numerous failure
studies on aircraft where especially unique conditions
NOT specified in manufacturer's data sheets precipitated
tense failures for crew on relays that had VERY FEW total
operating cycles on them. The stressor exhibited only during
contact "closure" when in fact, the multiple micro-events
during closure bounce provided part of the recipe for
sticking the relays (and a runaway trim actuator).
I mention this only to point out that while manufacturer's
data is useful and quite probably truthful, it cannot
and does not cover all the simple-ideas that feed
recipes for success . . . or failure. Too many of our
contemporaries are content to rely on the snapshot of
knowledge offered by the data sheets but find they
lack understanding when things are not behaving as
the data sheets suggest they should.
Bob . . .
| Quote: | Regards,
Matt-
> I now have a hundred hours on my Lancair Legacy wired using Z13-8. I'm
> happy that I've only had a few electrical issues, none of which were
> significant. But I'm wondering about my master relay, a continuous duty
> one from B&C.
>
> My airplane uses a hydraulic pump to raise and lower the landing gear.
> The pump is powered by an electric motor protected by a 40 amp ANL fuse.
> I haven't measured it, but factory specs indicate that current peaks at
> close to 100 amps just before the pump shuts off as it produces peak
> pressure.
>
> The hydraulic pump automatically turns on and off to maintain set pressure
> whenever the master switch is on. When the airplane is parked overnight,
> hydraulic pressure bleeds down enough that the automatic pressure switch
> closes. Of course, since the master switch is turned off, the pump
> doesn't turn on.
>
> But when I turn on the airplane's master switch, the hydraulic pump
> immediately runs for a split second to restore hydraulic pressure. I
> think that the master relay is therefore switching approximately 100 amps.
> My understanding is that a continuous duty relay, like the master relay,
> can carry large current loads once it is closed, but it isn't good at
> switching large loads.
>
> Should I expect premature failure of my master relay? If so, I'll just
> carry a spare. They're reasonably cheap and light weight.
>
> Thanks,
> Dennis Johnson
> Lancair Legacy #257
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Dennis Johnson
Joined: 10 Jan 2006
Posts: 89
Location: N. Calif.
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| Posted: Thu Nov 15, 2007 2:46 pm Post subject: Master Relay Switching High Current |
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Hi Bob,
Thanks for answering my question about the effect of the hydraulic pump motor on the master relay. Your answer is also consistent with the fact that I haven't heard of any problems in the field, but I have the new model hydraulic pump that has a significantly higher current draw than most of the existing ones. I won't worry about it.
Thanks,
Dennis Johnson
Do not archive
[quote][b]
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