It's true a lot of little events will cause an MOV to eventually fail
shorted, except where it's on a line supplying power, a a direct hit,
or even a close strike. In the case of the line supplying power, it
depends on how that line is protected. A 50 A buss, or even 10 A buss
may clear the short in the time it takes the fuse to open. Except for a
really close strike or direct hit, signal lines such as position
indicators on rotators, the odds are far in favor of it failing
shorted. A direct hit, or the induced voltages from a close strike will
likely open the MOV and take out from a few to many associated components.
It doesn't have to be lightening. Motors, relays, arcing connections,
or anything that can cause voltage spikes may cause an MOV to fail.
It's the energy in the spike that does the damage. As the MOV ages
while in service it takes less energy to do damage to the MOV. The MOV
is really nothing more than many thousands of very tiny diodes. Look up
their construction with a search. Years ago, GE had a very nice manual
devoted to the construction and use of MOVs. It's still relevant today.
http://pml.nist.gov/spd-anthology/files/GEMOV_saga.pdf I have or had
one of those first edition manual;s listed as a collector's item. Hope
it didn't get thrown out,
73
Roger (K8RI)
On 8/27/2015 8:31 PM, john@kk9a.com wrote:
MOVs degrade with each lightning event and will eventually fail. Typically
they fail shorted. I have had many PolyPhaser rotator protectors fail or
start to leak. Often the MOV damage is visible when you open the cover. It
is certainly easy to disconnect the device to test the rotator and MOVs.
The MOVs inside the Array Solutions NEMA enclosure are easy to replace.
John KK9A
To: TowerTalk@contesting.com
Subject: [TowerTalk] Array Solutions AS 8 SP
From: Joe Feustle <jafeustle@outlook.com>
Date: Thu, 27 Aug 2015 17:00:15 -0400
Does anyone on the list have any experience with this rotor control line
device going bad? I'm working my way through a process of elimination to
find out why my TailTwister no longer indicates direction nor will turn my
beam. I've replaced the auto-start capacitor (no change) and hooked my box
up to a known, working rotor. That works just fine: releases the break and
turns the rotor. The surge protector is the next item in the line of
inquiry.
Thought I'd post this before disconnecting all the wires from the SP and
reconnecting them in their proper order of color coding.
TIA and 73
Joe Feustle, N8JF
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73
Roger (K8RI)
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