Jim & Gerald,
Tnx for the info. Will need to look at limit switches for the future.
Paul, W9AC
-----Original Message-----
From: Jim Lux
Sent: Friday, May 15, 2015 12:01 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Rotator Lightning Protection
On 5/15/15 8:15 AM, Paul Christensen wrote:
Jim,
"what about mechanical limit switches on the rotator motor?"
A possibility but that's probably best done when these units are back on
the ground for service. That shouldn’t be for quite a long time.
"Since it's a DC motor, you put a diode in series with each of the two
switches, which are then paralleled."
Maybe I'm just being dense this morning, but I don’t see how a
series/parallel diode combination helps in this case. I just drew-out
the electrical diagram but don't see how the diode would limit current
during an MOV fault while still allowing rotator current during a normal
turning operation.
I was assuming (incorrectly, as it happens) that your motor was
basically a DC permanent magnet motor with two leads, and you switch
polarity to switch direction.
Let's say that lead 1 is positive for cw rotation, and lead 2 is
positive for ccw rotation.
The limit switch that goes open to stop CW rotation has a diode in
series with it for that direction of current. The limit switch that goes
open to stop CCW rotation has a diode that is opposite.
The two "switch+diode" assemblies are in parallel, and that is in series
with the motor.
That way, if you hit the limit in one direction, you can apply voltage
of the opposite polarity and move off the limit switch.
(Man, this is hard to describe in words and trivial in a diagram)
A bit more info: The prop pitch uses two windings: one for CW and the
other for CCW. A common lead connects to the ends of each and
ultimately, this common lead routes to the controller's MOSFET.
Oh, that makes it easy.. limit switch for that direction in series with
each winding. No diodes, etc.
The M2
controller uses a P&B frame relay that directs +48V to either the CW or
CCW winding, depending on the called direction. The relay's NC contacts
are on one or the other windings. I forgot which. But the MOV failure
would result in current in this NC winding since its connected to +48V
via the NC relay contact.
Paul, W9AC
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