On 5/15/15 7:38 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.
what about mechanical limit switches on the rotator motor?
Since it's a DC motor, you put a diode in series with each of the two
switches, which are then paralleled.
there are a variety of clever schemes for allowing "a bit more than 360
degrees" rotation, mostly relying on a sort of freely rotating collar
around the shaft with a tab. The rotating shaft has a tab, and that
pushes the tab on the collar. When rotating CW it pushes on one side of
the tab, and when rotating CCW it pushes on the other side of the tab.
so the tab width (and where you set the mechanical stop and/or limit
switch) sets the "overtravel" limit.
You can also do some sort of gear/belt drive with a reduction, and then
use conventional cam/limit switch schemes.
BTW, don't forget that MOVs wear out. You're in a lightning-ey area,
and every time that MOV clamps a transient, it dies a little: increasing
the leakage current.
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