[GreenKeys] Fwd: Re: ASR LMU12 Motor

[Steve Garrison]

The short was against the fan.  The cable from the LESU to the printer was
resting against the fan and I didn’t notice it.  That’s what I guess I get
for moving things around so much and not being extra cautious.

 

The fan rubbed its way through the sheathing on the cable bundle and then
through the insulation on at least one wire.  I haven’t even gotten to the
cable bundle repair yet.  The motor frame case was grounded as I found that
it wasn’t just a day or two ago and put the ground strap back under the
screw head where it belonged.

 

The damaged bearings sounds to me to be the most likely “funny sound” I’m
now hearing.

 

Thank you for the insight Bruce, I’ll admit I never would have thought of
that problem.  And thanks to the others that have responded as well.  As all
of you have commented, it is the line frequency that sets the speed of a
synchronous motor.  Why that fact didn’t pop to the front of my mind I’ll
never know, but as they say you learn something new every day!

 

Steve G

 

From: greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net>
On Behalf Of Bruce Gentry via GreenKeys
Sent: Sunday, January 3, 2021 4:28 PM
To: greenkeys at mailman.qth.net
Subject: [GreenKeys] Fwd: Re: ASR LMU12 Motor

 

I fear the short damaged bearings in the motor or shafting. The motor mount
is resiliant rubber, and if the motor frame was not grounded, the current
may have passed through the bearings. This arcs and etches the bearing balls
and races. If the motor frame was grounded, did the wire touch the motor
shaft, fan, or anything metal connected to it?.� Any current passing
through� any bearings in the machine can do the damage. The synchronous
motor in a 28 is 3600 RPM, that is the fastest a synchronous motor can go on
60 cycles. No sort of short or other damage or change can increase the speed
beyond that.� If� you were dealing with� a goverened motor, that is a
totally different matter.

����� Bruce Gentry, KA2IVY

 

On 1/3/21 14:38, Steve Garrison wrote:

OK, folks what can go wrong with a synchronous motor that can make it seem
to be turning at a much higher RPM than it should be?

�

I had a mishap that shorted a wire against the motor that caused a spark, a
pop, and the fuse in the LESU blowing.� After removing the short,
replacing the fuse, then powering back up, the machine was running but
�sounded funny!�� After removing the typing unit and powering backup I
still had the 
�funny� sound, just not quite as loud.� Without the typing unit it was
just the motor and the reperf churning.� The TD drive shafts had been
removed earlier.

�

My feeling is the �funny� noise is the mechanisms running at a higher
RPM than designed.

�

I still need to repair the shorted wire, but it isn�t part of the motor
circuit anyway.

�

Steve G./N4TTY

�






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