Nice tutorial Matt. I have archived this posting in my tower folder.
I always wondered why UST fooled with running the motor on 120VAC,
and thought about rewiring it for 240VAC. I had no idea about
the start winding, etc and letting the smoke out of the motor :-).
An alternative to rewiring the motor is to use a 240V to 120V
transformer located at the tower. This can be constructed as
an autotransformer using a 10A 120V to 120V isolation transformer
wired with the two windings in series. That doubles its power
rating while giving up isolation; but you don't need isolation.
Transformers can sometimes be obtained for a song at flea markets,
surplus stores etc. The price tends to be inversely related to
the size=weight! Bring a dolly to cart the thing home. I have
several 5kVA transformers that they threw out at work. Doing this
saves 75% of the cost of the copper wiring.
One additional comment: using a motor switch overrides the delay
relays. This leaves it up to the operator to let the motor stop
turning completely before reversing the direction. Otherwise it
will run in the wrong direction. But on my tower, the delay relays
are ineffective when going down, now that I put "improved" oil in
the gear box and the motor takes up to a minute to coast to a
stop on a really hot day. Another reason I like to be at
the tower and baby sit things rather than have remote control.
73
Rick N6RK
On 1/2/2019 12:29 AM, Matt wrote:
4 switch conductors are required for running a reversing induction motor at
120VAC. There are drum type switches available as well as motor rated
3-position 3PDT toggle switches (9 terminals), or you can combine a
2-position DPDT toggle switch for reversing and a 2 position (I like
momentary) DPST toggle switch for engagement, all motor rated of course.
Drum switches are more reliable than toggle switches. With toggle switches
I like to use a local disconnect upstream in case the contacts arc and
stick. Regarding conductor length, there are standard wire gauge charts
available from a number of online sources based on motor HP. Be sure to
use the charts based on voltage drop because of distance.... Also be sure
to leave the local thermal overload in series with the line in at the motor
to protect your motor.
You can also wire most fractional motors for 240 VAC operation to reduce
current load, conductor size, and easier starting, but it typically requires
one additional switch wire to derive 120VAC from the center tap of the
series run winding to use for the start winding (do not use 240 VAC on the
start winding unless you want smoke...). You can alternatively derive 120
VAC from the source by using the available neutral and it only requires 4
switch conductors to the motor - however it's generally a good idea to be
sure you have your overload protection arranged to disengage both lines
simultaneously if you do this. Switch requirements are similar but wiring
is different.
Email me if you need a wiring diagram.
You may also want to consider using 24VAC coil contactors. I have built
some tower motor control boxes using Packard brand contactors and
transformers which are very economical and work fine. You can do some
pretty neat stuff with these using limit switches and ladder logic wiring -
or just to avoid having to run large line conductors back to a remote
switch.
Hope this info helps & good luck on your project.
73
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Patrick Greenlee
Sent: Tuesday, January 01, 2019 8:40 PM
To: Richard (Rick) Karlquist <richard@karlquist.com>;
towertalk@contesting.com
Subject: Re: [TowerTalk] US Tower Up/Down Raising Fixture (MD-750) Motor
Switch
I should have not replied in haste to something of which I was not
properly informed. Fast on the trigger but slow on the draw. That's
no toes Pete.
If the length of the run and required wire gauge were within reasonable
limits, especially monetary limits and there were no strong contrary
electronic negatives I would certainly consider going without a relay.
I have been accused of being a belt and suspenders kind of guy.
Patrick
On 1/1/2019 5:28 PM, Richard (Rick) Karlquist wrote:
As built, the switch on the control box energizes the coil of a relay
that carries the motor current. That is the relay that can stick.
Are you saying you are going to run the motor current (not just the
relay coil current) through your remote switch and bypass the built in
relay?
Not following you...
73
Rick N6RK
On 1/1/2019 1:21 PM, Patrick Greenlee wrote:
I did not propose a relay. I was thinking of up-sizing the wire
sufficiently to obviate a relay. I concur with your low opinion of
the A/C contactors. I have replaced some of those too. Brute force
switching and HD wire run. Reliable and not prone to failure.
Patrick NJ5G
On 1/1/2019 1:03 PM, Richard (Rick) Karlquist wrote:
On 1/1/2019 10:45 AM, Patrick Greenlee wrote:
As you mention, upsize the wire and operate in the dry. This
assumes that you aren't at the ragged edge of too much wire loss
now. Probably not but you could grab a meter and make sure.
Patrick NJ5G
The trouble with what you describe is that if the relay sticks on,
you are too far away from the tower to kill the power and stop the
motor. The type of relay used is the same as is used in air
conditioners, and AC relays are famous for sticking on. On my
HDX-5106, the relay has stuck on twice.
73
Rick N6RK
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