[TowerTalk] US Tower Up/Down Raising Fixture (MD-750) Motor Switch

[john at kk9a.com]

A long long time ago I had a US Towers 89' crankup. I had purchased a new
home and moved the tower from my former residence. I installed the tower
350 feet from my home and could not get the motor to run. Obviously I
should have ran larger conductors. So I rewired the motor for 240 volts
and it worked perfectly this way for several years until I replaced it
with a guyed tower. I am not aware of a starting winding or anything else
on the tower requiring 120 volts.

John KK9A


Richard (Rick) Karlquist wrote:


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