[TowerTalk] Rotator Lightning Protection

[Jim Lux]

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.