I recently had an interesting issue with my HDX-589MDPL tower. I am posting my
experience, hoping it saves someone from a potentially catastrophic event.
We had a rather intense storm rip through the Philadelphia suburbs last week,
so on the side of caution, I made the wise decision to run outside and lower my
tower down somewhat before the storm approached my area to the east. Good
decision - we had numerous trees down and a few broken power poles,
resulting in a 30 hour power outage.......... But the tower and antennas made
it through unscathed.
However, as I was retracting the tower, once it reached my desired level, I
went to place the UP/OFF/DOWN toggle switch in the OFF position. However, the
tower kept moving in the downward direction. In a panic, I flipped the switch
up, then back down to the OFF position, which made no change. Fortunately
the GFI outlet was within reach, so I simply tripped the GFI test button to
stop the tower. Upon resetting the GFI outlet, even with the switch in the OFF
position, I heard a loud buzz emanating from the tower motor. Not good.
However, if moved the switch to the UP position, the tower would move up,
but going back to OFF, the tower would still continue moving up. I had to pull
the power cord or trip the GFI outlet to stop the tower.
Fortunately, I had the tower down to a safe level. The storm was fast
approaching, so I called it a day and figured I would investigate further the
next day...........
In the meantime, I placed a call to US Tower. One of their electricians kindly
returned my call, explained the design of the control box, and emailed me a
block diagram.
The next day, once power was restored, I did some troubleshooting. While the
tower was moving in the downward direction, the lower limit switch was tested,
and the tower was able to be stopped by manually engaging the lower limit
switch. However, when performing the same test in the upward direction, the
upper limit switch had no effect, and the tower continued to move upward.
Fortunately I performed these tests with the tower somewhere around halfway up,
so it was not much of a concern at the time. However, had I decided to fully
extend the tower, the results could have been disastrous!
Next, I decided to pull the cover off the control box on the side of the tower.
The motor and limit switches are controlled via the toggle switch, which is a
3PDT center off switch made by Carling Technologies. Two of the poles are used
to reverse the phasing of the motor direction control leads, and the third pole
is used to supply the main neutral feed to the motor. The main hot lead is hard
wired to the motor at all times. The center of this third pole feeds the motor,
the up side connects to the return from the upper limit switch, and the down
side of the switch connects to the return from the lower limit switch. Both
limit switches are hard wire fed from the AC supply neutral. The limit switches
each provide their own neutral path back to the toggle switch. The toggle
switch simply selects which limit switch is in effect at the time. The upper
limit switch is in effect when the switch is in the up position, and the lower
limit switch is in effect when the switch is in the down position. When the
tower is NOT at either limit, both limit switches are conducting a neutral
return back to the toggle switch.
Now here is where things get bad real quick - in my case, the common terminal
and the lower terminal of the third pole of my switch failed in a SHORTED
condition! Basically, the motor was connected to neutral at all times via the
lower limit switch. Thus the reason the tower would move in the proper
direction once the switch was thrown up or down, but would not stop once turned
off. Also, if left in the OFF position, and power was applied to the AC cord,
the motor would buzz due to lack of direction control lead supply, while main
hot and neutral were now immediately present at the time of plug-in.
Fortunately, I was moving in a downward direction when the switch decided to
fail shorted in the DOWN direction. Had I fully lowered the tower, the lower
limit switch would have still taken effect and broken the neutral path.
However, had I decided to raise the tower with this switch failure condition,
the upper limit switch would have been of no use, as the lower limit switch was
still in the circuit, supplying the neutral return at the same time as the
upper limit switch! I surely would be telling a different story at this
point!
I tested further and made sure the limit switches themselves were ok, and that
the motor was still functioning properly. I then began the search for a
replacement toggle switch.
The next day I ran over to my nearest Grainger Supply warehouse, where I was
able to pick up an exact replacement switch.
For future reference, the switch is a CARLING TECHNOLOGIES HM254-73 3PDT
ON-OFF-ON SWITCH. It is also available from Mouser for a few bucks less
before shipping, but I wanted it right away.
I installed the switch in short order. UST did a nice job of using spade lugs
on all of the wires, and the switch uses brass screw terminals. $25 and a few
hours later I had the tower tested and back to normal operation.
Looking over the wiring diagram for the control box after the fact, I am
questioning why UST did not just simply wire the two limit switches in series,
then just use the UP/DOWN toggle switch to break the connection between the
series-wired limit switches and the motor. This way, both limit switches are
always inline when the circuit is completed, and in the event of a switch
failure such as I just experienced, both limit switches would still function at
all times, regardless of motor switch direction, thus avoiding a potentially
catastrophic event. If the toggle switch were to fail shorted, at least either
limit switch could break the neutral return path regardless of the switch
direction.
Summary: Do yourself a favor and regularly test the toggle and limit switches
as part of your routine maintenance inspections.........................
Eric
K2CB
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