John,
Thanks for the added info. Last time I looked, GH didn't publish the full
schematic set for the K7NV version of the rotator. But, I suspect it's
doing the same thing: using a DPDT relay to isolate both the DC supply line
and the DC return to to protect the power MOSFET and guard against the
effect of a shorted MOV. However, I feel much more comfortable using
high-quality vacuum relays, rather than frame relays for this purpose. IF
GH is using a frame relay, it may be more susceptible to contact flashover,
thus undermining the relay's purpose.
In looking at high-quality DPDT vacuum relays suitable for the job, the
choice is limited and very expensive -- typically USD $500 ea. In this
case, adding the second SPST vacuum relay can be accomplished at a
significantly lower cost. Due to the large sixe of the DPDT vacuum relays,
using a pair of the SPST types allows me to sneak-in the second relay where
there's no room for the larger type.
Paul, W9AC
-----Original Message-----
From: john@kk9a.com
Sent: Saturday, May 16, 2015 3:01 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Rotator Lightning Protection
Physically unplugging things on a remote station is not a practical
solution.
I believe that MOVs typically fail shorted and actually they start to leak
voltage after some use. This can affect the ramping on some controllers. I
would either use a Green Heron controller or duplicate his isolation relay
circuit to minimize the chance of free spinning.
John KK9A
To: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Subject: Re: [TowerTalk] Rotator Lightning Protection
From: Bry Carling <bcarling@cfl.rr.com>
Date: Sat, 16 May 2015 01:59:14 -0400
My solution is to pull all AC plugs from the wall and disconnect all
antennas
whenever a storm is coming. That pretty much takes care of most problems.
My ground system is going to be improved from the current setup but I'm not
going to the kinds of extremes that some people have, and assuming that will
be
a perfect system of protection.
Best regards - Brian Carling
AF4K Crystals Co.
117 Sterling Pine St.
Sanford, FL 32773
Tel: +USA 321-262-5471
.
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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