[RE-wrenches] Brush sparking damaged motors running "sun-direct"

[Mick Abraham]

Hello, Motorheads~ and with a cc to SolarConverters since I'm casting in
all directions for answers.

Another "solar guy" left our town & abandoned a pond aeration system. The
owners asked me to be their "new guy", but we're struggling with motor
problems way too soon.

+++++++++++++++++++++++

Original setup: Twelve 36 cell (~60 watt) Solarex modules wired all
parallel, driving a SolarConverters PPT12/24-30 "power pump tracker". The
original array wiring work had to be redone. The S-C controller was
configured to MPPT the ~17 volt array input and the control was factory
defaulted to clamp the motor output at no more than 15vDC, as appropriate
for the original "12 volt" air motors.

The Gast DC air motors are the type that pump up the airbrakes in big
trucks. Each compressor head is direct coupled to a permanent magnet motor
with the familiar look of old school Pacific Scientific aka Honeywell
motors. The original 12v motor-compressors were well worn so we rewired the
array ~34v MPPT, jumpered the SolarConverters controller to match, and
bought the 24 volt version of the same type air motors. Another jumper in
the S-C controller sets it to clamp the output at no more than 30v DC.

Gast says no problem if their "24 volt" air motor is fed 30 volts since
they assume the truck alternator can push the voltage that high. We're
loading those motors way lower than what they're rated for since we're only
pushing air to the bottom of a nearby pond. The existing 30 amp controller
seems "super sized" compared to what the motors need and the PV array is
also way bigger than what the motors really need. My clamp meter is broke
right now--which makes me feel like an amputee--so I can't get fresh
numbers today but each motor only drew a few amps @ 30 volts when last I
checked.

Two motors are connected in parallel to the output of the single
SolarConverters motor controller so *maybe that's a key to our
problem:*the motors do not start reliably and this misbehavior is very
unpredictable. When the 24v motors were new--around 2011--these problems
did not present until after several months had passed. Tapping on a motor
shell (as one may do with a balky car starter) sometimes gets it to spin
up, so I think they're not commutating properly. We took one motor apart
and the commutator bars looked badly chewed up. The brushes looked OK where
they rub on the comm and the brushes still have some length left. Some
cleanup effort on the comm bars improved things with that motor.

++++++++++++++++++++++++++++++

With both motors wired parallel--but obviously they won't commutate in
synch with one another--I wonder if back emf from one motor (maybe when the
brush crosses the commutator gap) might be spiking the other motor with a
voltage surcharge that causes arcing at the brush/commutator interface? My
DC voltmeter shows 30 volts steady at the motor screws but it's not a fancy
meter and it may not be able to capture a higher voltage flicker.

My client and I are frustrated, hoping for a logical explanation, and also
looking for plan C to implement. The fish endure stress leading to death
whenever the aeration fails, so I hope to get past these nervous times soon.

* Could we add capacitance across each set of motor screws...or a zener
diode? If so, how would I determine values to try?

* Should we special order the SolarConverter with a flat 24 volt voltage
clamp set by the factory instead of their standard 30 volt clamp?

* Should we split things up with a dedicated controller for each motor? In
that event, must we dedicate part of the PV array to each SolarConverter or
can two controllers share the same overall solar array power supply? (All
the glass is in the same plane, and partial shade only occurs during
sunrise and sunset times.)

Thanks as always for being the "Motorheads Nonpareil".

Jolliness,

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675
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