Hi John & Rick,
Thanks to you both for making much useful information available over the
years - which I have used on many occasions.
I used the term fractional loosely - slang for small single phase cap start
induction motors.
Standard fractional (slang) motor wiring is:
P1 - thermal overload line side
P2 - thermal overload load side
T1 - 120V run winding #1 top side (often internally tied to P2 without an
external lead)
T2 - 120V run winding #1 bottom side
T3 - 120V run winding #2 top side
T4 - 120V run winding #2 bottom side
T5 - 120V start winding top side
T6 - start capacitor (tied internally to bottom side of start winding)
T7 - start capacitor
T8 - tied internally to T7 (this is effectively the bottom side of the start
winding in series with the start capacitor)
To run the motor at 120V we need both run windings operating in parallel
along with the start winding:
Connect 120V line to P1
Tie P2 +T1 (if equipped) + T3 + T5 together
Connect Neutral to T2 + T4 + T8
Connect capacitor to T6 & T7
Ground the frame
To reverse directions swap T5 & T8 (T8 to P2, T5 to neutral)
To run the motor at 240V you need the run windings in series & start winding
in parallel to one of the run winding (#2 in this case):
Connect 240V phase A to P1
Tie P2 to T1 (if equipped)
Tie T2 + T3 + T5 together (this is the center tap = ~120V when running)
Connect 240V phase B to T4 + T8
Connect capacitor to T6 & T7
Ground the frame
To reverse directions swap T5 & T8 (T8 to T2/T3, T5 to Phase B)
There is a mirror image alternative 240V wiring which places the start
winding in parallel to run winding #1 in lieu of run winding #2:
Connect 240V phase A to P1
Tie P2 to T1 (if equipped) + T5
Tie T2 + T3 + T8 together (this is the center tap = ~120V when running)
Connect 240V phase B to T4
Connect capacitor to T6 & T7
Ground the frame
To reverse directions swap T5 & T8 (T8 to P2, T5 to T2/T3)
Best 73 to both of you.
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
john@kk9a.com
Sent: Wednesday, January 02, 2019 9:51 AM
To: towertalk@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
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