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MOV on switch contacts?

 
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gregmchugh



Joined: 03 Apr 2012
Posts: 42

PostPosted: Fri Jun 29, 2012 9:39 am    Post subject: MOV on switch contacts? Reply with quote

Bob,

I notice on some of the documents from AeroElectric there is a
Metal Oxide Varistor across the switch contacts on some of the
examples of switches controlling battery contractors but they
are not shown on other examples. I was wondering when the addition
of a MOV needed / recommended, where are they placed
when used, and what specific part is recommended? I don't
recall seeing them used in any other of the examples but I
have seen the use of diodes on the battery contactors to
suppress transients. Are the MOV's and diodes alternate
options for transient suppression?

I picked up a couple of the Tyco Kilovac EV200
contactors on eBay for $60 each that I am planning to use in
my system and my understanding is that they do not need
any external protection for transients. I am planning on a
standard Odyssey main battery and an aux battery (probably
a lightweight Lithium Iron battery from Shorai designed for
engine cranking). This is a motor glider application so the
engine will be off for long periods of time. I will have the
main battery online powering the system with the aux
battery offline during gliding in reserve in case the main
battery is allowed to drain too far or if there is a main
battery problem. Backup power for engine restart seems
like a prudent approach even though one should always
be planning for the possibility that the engine cannot
be restarted in a motor glider with a safe alternate landing
location available at all times.

Greg McHugh


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nuckolls.bob(at)aeroelect
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PostPosted: Fri Jun 29, 2012 10:54 am    Post subject: MOV on switch contacts? Reply with quote

At 12:39 PM 6/29/2012, you wrote:


Bob,

I notice on some of the documents from AeroElectric there is a
Metal Oxide Varistor across the switch contacts on some of the
examples of switches controlling battery contractors but they
are not shown on other examples. I was wondering when the addition
of a MOV needed / recommended, where are they placed
when used, and what specific part is recommended? I don't
recall seeing them used in any other of the examples but I
have seen the use of diodes on the battery contactors to
suppress transients. Are the MOV's and diodes alternate
options for transient suppression?

Those were on the original Z-figures. In fact, I went
to some pains to acquire MOV transient suppressors that
were rated to conduct at low voltages characteristic
of DC systems in vehicles.

The rationale for using them was based on the fact
that they are bi-directional, no polarity. The builder
couldn't put them in wrong. But they were more expensive
than a diode, much harder to find in low voltage ratings
and I eventually changed my mind about suggesting them.

I picked up a couple of the Tyco Kilovac EV200
contactors on eBay for $60 each that I am planning to use in
my system and my understanding is that they do not need
any external protection for transients. I am planning on a
standard Odyssey main battery and an aux battery (probably
a lightweight Lithium Iron battery from Shorai designed for
engine cranking). This is a motor glider application so the
engine will be off for long periods of time. I will have the
main battery online powering the system with the aux
battery offline during gliding in reserve in case the main
battery is allowed to drain too far or if there is a main
battery problem. Backup power for engine restart seems
like a prudent approach even though one should always
be planning for the possibility that the engine cannot
be restarted in a motor glider with a safe alternate landing
location available at all times.

Okay. Are you aware of the discussion we had a few months
ago about the LiFe battery fire at Cessna . . . and the
fact that Cessna withdrew their highly vaunted LiFe
battery from the market?

I'm not suggesting that you should not be doing what
you're planning . . . just making sure you've got
all the data.
Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Fri Jun 29, 2012 11:03 am    Post subject: MOV on switch contacts? Reply with quote

P.S.
I picked up a couple of the Tyco Kilovac EV200
contactors on eBay for $60 each that I am planning to use in
my system and my understanding is that they do not need
any external protection for transients.

Not a big deal but be aware that the EV200 has
been noted to be a low frequency noise generator
in the 200 Hz range. When in the duty-cycle limited
hold mode, it's axiomatic that artifacts of that
switching frequency will show up on wires that
come out of the product.

I've had a couple builders not that they could
hear this noise on some intercom installations.
I don't think it was loud enough to be heard
in flight but it was strong enough to be noteworthy
during ground tests. I'd be interested in your
observations and experience . . .

Bob . . .


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gregmchugh



Joined: 03 Apr 2012
Posts: 42

PostPosted: Fri Jun 29, 2012 3:07 pm    Post subject: Re: MOV on switch contacts? Reply with quote

Bob,

Thanks for the info. I recall a story of a Cessna fire related to ground
power and a Lithium Ion battery and then an FAA AD for these batteries,
but I was not aware of a fire involving a Lithium Iron Phosphate battery.
Was this also Cessna approved battery?

Greg McHugh


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PostPosted: Fri Jun 29, 2012 4:58 pm    Post subject: MOV on switch contacts? Reply with quote

At 06:07 PM 6/29/2012, you wrote:
Quote:


Bob,

Thanks for the info. I recall a story of a Cessna fire related to ground
power and a Lithium Ion battery and then an FAA AD for these batteries,
but I was not aware of a fire involving a Lithium Iron Phosphate battery.
Was this also Cessna approved battery?

Yes. It almost got 'covered' with a mandatory
service bulletin but it seems a small number of
the airplanes fielded with LiFe batteries were
out of the country. Hence the A/D to make sure
they were taken care of. I've not heard whether
the LiFe effort will be resumed.
Bob . . .


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