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

[Dick Green WC1M]

My MD-750 is 350 feet from the shack, behind a stand of trees. I installed three conduit runs: one for RF coax/hardline, one for control cables and one for 240VAC (#10 conductors.) I put a subpanel at the tower to split the 240VAC into two 120VAC circuits for the tower and an outdoor AC receptacle. At the time, I didn't realize that the UST control box runs off 120VAC. In addition to the voltage drop due to the long run, putting an AC cable through a conduit with low voltage cables is a code violation. There wasn't enough room to run a thick control cable through the 1" AC conduit, so I built a small 12VDC relay box to control the AC relays at the tower. That box lives in the relay cabinet at the tower, and I have a simple switch box in the shack that's powered by 12VDC. The control cable runs to the tower through the low-voltage control cable conduit. This has worked great for 22 years.

As for raising/lowering the tower remotely, early on I had a near-disaster when the coax snagged on the motor housing. That problem was solved by reconfiguring the cable run, adding another cable standoff, and putting a shield around the bottom of the tower. All this is well-documented in a TowerTalk post I did back then, probably in 1997 or 1998. I haven't had any issues with remotely operating the tower in 22 years, other than having to be careful about not letting the cable get stuck under ice and snow in the winter. 

73, Dick WC1M

-----Original Message-----
From: john at kk9a.com <john at kk9a.com> 
Sent: Wednesday, January 2, 2019 10:51 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] US Tower Up/Down Raising Fixture (MD-750) Motor Switch

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