Having worked with literally thousands of MOV,s from little ones up to
50,000 Joul ratings, They will typically fail shorted, BUT I have seen
many fail open. It depends on the power contained in the "final spike"
that causes them to fail. From power line spikes and nearby lightening
strikes the ODDS ARE it/they will fail shorted. If enough power/energy
is contained in that final spike at failure time, it becomes likely they
will fail open and that anything on that line will fail as well.
Based on experience, you can expect the MOV to fail shorted and it will
PROBABLY protect any electronic device connected to it. That being the
case, the prop pitch motor will PROBABLY need some sort of "fail safe"
to prevent over rotation.
That's a lot of probables, but it's unfortunately a fact of life,
leaving you playing the odds which favor the MOV failing shorted, but
with no guarantee it will do so. If the MOV works as we all hope, then
you will need some sort of "fail safe"
Since the big ground system was installed my tower has been hit at least
7 times that can be verified with nothing disconnected and NO MOVs in
the rotator lines. No damage was seen.
I can not say the same for my computers and network. A nearby strike
did nothing to either station, but it sure cost me a lot in network and
computer damage. The CAT-5 lines go through protection at the UPS for
each computer. The connectors were melted and the wire contacts welded
together at one UPS. The router, 8 port switch and most NICs were shot!
So, lightening and MOVs are a crap shoot. They will protect you from
the smaller stuff, but cause other problems. They are unlikely to
protect you from the larger stuff.
In the end, I'd use them, plan on the MOVs failed as a short, and hope
for the best There is no guarantee, but you have tilted the odds in
your favor..
73
Roger (K8RI)
.
On 5/15/2015 10:38 AM, Paul Christensen wrote:
N4CC and I have finally completed our remote Internet station in
Hilliard, FL. Probably half our time spent before and during
construction was studying and implementing lightning abatement.
Ground rings were created around each tower, and the communications
shelter. All grounds, including the adjacent electrical service
ground are brought together at an external ground bus (EGB). In
total, 55+ ground rods are used with four rods each 24 ft. Lines are
bonded at the top and bottom of the towers. We do not disconnect
anything. Everything stays up and running 24/7 regardless of the WX.
Poyphaser rotator MOV protection devices are installed at the base of
each tower. At the EGB, we're using an Array Solutions model. In
looking at all the failure modes, I missed one: If the MOV on the
rotator return lead was to short to ground, it creates a disastrous
situation where the prop pitch will turn freely beyond the electrical
stop point. The MOSFET is controlled by a PWM circuit and the
duty-cycle is what varies motor speed. A shorted MOV will cause the
lines to break apart up the tower at the coax loop. The prop pitch
would stop at nothing and keep turning. Do the MOVs in these units
generally fail open or closed?
We're using a pair of M2 PCX2800 controllers. To help protect the
MOSFET device from a similar "short-to-ground failure" during a
lightning event, I designed a circuit that engages a vacuum relay such
that the MOSFET is only exposed to the outside world during
rotation. It's not a 100% guarantee against MOSFET failure, but
should help to mitigate damage. This is an add-on that's similar to
what K7NV did with the Green Heron prop pitch controller model.
Here's my thought for a potential fix: While another vacuum relay
could be added on the +48V supply side to the prop pitches, I could
isolate circuit ground from chassis ground on the secondary side of
the controller's power transformer. The primary would still be safety
protected and meet UL. Isolating circuit ground from chassis ground
would inhibit rotator turning in the event of a MOV failure on the
return line.
Anyone been through this? I welcome comments on any better ways to
manage this.
Paul, W9AC
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73
Roger (K8RI)
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