If the ball bearings in the rotator and "thrust" bearing are operating
as intended then an oil film is insulating the races from each other and
from the balls. Every rotator I've disassembled has no electrical path
around the bearings.
I've watched the arcing across a motor ball bearing oil film on a scope,
and seen it in disk drive read signals. If a polymer "thrust" bearing
is used on a tower then even higher voltage isolation is present at that
point. The charge comes from air friction against the antenna or disk.
The oil film breakdown is on the order of 10's of volts. VFD powered
motors are known to have potentially serious bearing problems from the
high frequencies the VFD's put into the windings, coupled to the shaft
and the potential is created to the frame across the bearing.
http://est-aegis.com/TechPaper.pdf
The consequence of not grounding around the rotator bearing is pitting
of the balls and races by the arcs and also RF noise generated near the
antenna. Also, having a heavy gauge wire from mast to tower top is
likely to be helpful in a strike. The tower probably also acts somewhat
as a Faraday cage to shunt currents to the outside of the structure and
away from the rotator.
MOVs provide overvoltage protection but have a downside as they degrade
with every snubbing shunt and eventually fail. On an AC power strip, a
failed to short MOV burned up a nearby ham's shack here when the plastic
power strip case ignited. If on antenna control lines, sufficient
degradation can be experienced over time to prevent normal operation.
As one antenna manufacturer discovered before they switched to TVS
diodes. Of course any transient suppression device can be vaporized
with sufficient power surges, but some pretty hefty capacities are
available.
http://www.littelfuse.com/products/tvs-diodes.aspx
Grant KZ1W
On 12/24/2016 16:18 PM, john@kk9a.com wrote:
Unless you enjoy repairing rotators, I would. I have either MOVs or GDTs on
all of my motors and indicating sensors. I have not made any additional
effort to ground the mast, it seems like the bearing and rotator housing
would both already have that covered.
John KK9A
To: towertalk@contesting.com
Subject: [TowerTalk] Lightning Protection for Orion Rotor
From: William Saulnier <caseydogmusic@gmail.com>
Date: Fri, 23 Dec 2016 11:16:44 -0600
At the risk of starting yet another discussion of lightning protection in
general I do have the following specific question.
In addition to an overall sound design for grounding, etc. is there a
significant advantage to having rotor control line surge suppressors at the
top of the tower? I have read that its a good idea to ground the mast to
the tower to bypass the rotor and I was already planning to have
suppressors on the control lines at the bottom of the tower and at the
entry to the radio room. The intent would be to have better protection for
the rotor itself especially its apparently sensitive reed switch.
Thanks.
Bill N5VR
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