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Re: [TowerTalk] U.S. Tower MDP-750 motor failure and related prolems

To: Victor Morris <>
Subject: Re: [TowerTalk] U.S. Tower MDP-750 motor failure and related prolems
From: "Richard (Rick) Karlquist" <>
Date: Sun, 19 Dec 2010 20:36:55 -0800
List-post: <">>

On 12/19/2010 7:31 PM, Victor Morris wrote:
> Aloha everybody,
> I'm new to this group and I'm looking for some help. Earlier this year I 
> purchased a TMM-433 HD tower from U.S. Tower. I chose the option with an MDP 
> 750 motor, inside control box, and outdoor relay box.
> Almost from the beginning I've had trouble. It started with what appeared to 
> be an intermittent problem with a stuck outdoor relay. Sometimes the tower 
> would not raise when e I turned on the power. However if I went outside and 
> tapped the box, the tower went up fine. I thought this was just a nuisance, 
> but I may have been wrong.
> About 3 weeks ago a circuit breaker tripped one time when I tried to lower 
> the tower. I reset the breaker and the tower lowered properly.

> Vic, AH6WX

The biggest user caused problem with these things is insufficient 
wiring.  If the wiring is marginal, the breaker may or may not
trip depending on temperature and other conditions.  The motor
draws 20A @ 120V.  That is a 6 ohm load.  The round trip resistance
of wiring back to your *service entrance* should be only 5% to at
most 10% of this.  For 10%, this works out to 200 ft of 12 AWG wire.
And that's marginal.  I use doubled up 10 AWG wiring for mine, and
it just barely starts on hot days.  I believe it is well known
that insufficient voltage on motors is bad for them.  Of course
leaving the power applied to a motor with a dead starting cap
can't be good either.  Starting caps go out all the time.  I
used to have a pool pump motor that went through caps at the rate
of a new one every year.

I have traced out the circuit for my MDPL1000 and can send it to
you if it is helpful.  You will probably have to use it as a guide
and trace out your own version.  There are two relays in cascade.
The timed ones seem to be the most troublesome.  I had to replace
both of mine.  However, I cannot think of any way the relays could
damage the motor.

I always stand outside by my tower and watch it go up and down.
I just don't trust these things.

Rick N6RK

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