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=?iso-8859-1?Q?Finally_got_around_to_sync=B4ing_my_async_?= motor
Hi All,
I just wanted to give you an update on my RSG. I finally found
the time to start construction on it (which is ONE of the reasons
why I am "on and off" the list at the moment). Anyway......
This post is a little lengthy, but I thought I would share it with you,
because someone asked how to "sync" async motors and
sync RSGs have been the content of many recent posts. For those
of you with little spare time, go ahead and hit the "kill" button now.
While scrounging around at the local junkyard I found a 1/4 hp
1450 rpm (50Hz) motor that seemed perfect for a SRSG, so I
lugged it home (with lots of other electronic/pneumatic goodies).
Just as I had expected, the thing didnīt start when plugged in.
Oh well. I had "sniffed" it, before I took it home and I couldnīt
smell any burnt windings. There isnīt much more that can
break on an induction motor. So I pulled the covers and had
a look inside. One of the wires going to the run winding had
torn inside the insulation (which made it a little harder to find).
So I "quick repaired" it and gave it another try. A slight hum,
but no turning of the armature......... (:o((. Apart it went again.
This motor doesnīt use a start cap, but rather the old style
centrifugal switch. After looking at the contacts, it was clear
why it didnīt start up: They where junk. So, I made a set of
new start-up contacts and reassembled the whole thing
again. This time it started up immediately. Current was
around 1.43A (up and running). Simply wonderfull.
So, now I took it apart (again :o)) ) and proceeded to have
a real good look at the motor. It has four coils (meaning 4
90° flats are necessary). The overall condition was dirty and
used, but okay. After tearing it apart completely, I cleaned
all parts in solvent, had a look at the bearings and all parts.
Everything was okay. I cleaned up the end caps and housing
with a file and sandpaper (I hate AL castings and the ever-
present casting flash). The connections to the windings and the
start-up switch were made with new wire. I used heat shrink
to cover the new (uninsulated) connections. After masking the
windings and bearings, the motor got a fresh coat of black
and red paint. Then I proceeded to measure the width of the
dead poles. Total armature run-out was only 8/100mm, which
I deemed okay. Endplay was 0.45mm, which was a little too
large for my taste. That is why I machined a new spacing
washer, which reduced the endplay to 0.12mm. I didnīt want
to go for zero play as the parts can expand, when the motor
gets warm, which then would bind. After machining the 4
flats to measured specs and exactly 90° apart, I polished the
shaft at both ends with 2000 grit sandpaper to remove minor
blemishes. Next on the list was a thorough cleaning of the
rotor. Then I reassembled the motor.
First run was successful. The motor "kicked in" much harder
than before the modifications. From the new "sound", I was
pretty sure it was synchronizing to the mains, but I needed to
know for sure, so I chalked a cross on the rear of the shaft and
used a handheld FL-light to have a look. The FL light acts as a
sort of cheap stroboscobe (50Hz "flicker"). The chalk marks produce
a refractionating pattern (sorry I donīt know the technical name for
them). On a normal induction motor these patterns "float" slowly
around (opposite to the rotational direction). The donīt "fix" (sync)
in one spot. On my modified motor, the patterns lock into place (the
same place every time the power is turned on) very neatly. I turned
down the variac to about 110V. You could hear the motor trying to
hold itīs sync at this voltage. Every time it fell out of sync, you
could feel (and see via the FL light) the motor "kicking" back into
sync. At full voltage, I had a really hard time slowing down the
motor. As soon as I released the shaft, the motor kicked back into
sync position. Next was a speed measurement. As expected with
a 4 "pole" rotor, it now ran at 1500 rpm (opposed to the 1450 rpm
measured earlier) at 50Hz. The last measurement was a current
measurement. The motor now pulls 1.72A (instead of 1.43A). I
think the current draw is still very reasonable. Every time I slowed
down the rotor (using a rag to prevent skin burns) far enough to
get the motor out of sync, the current jumped to about 3.2A, until
I released the rotor and it synchronized to the mains again (and
the current dropped back to the 1.72A). Absolutely neat!!
I think the mods were a complete sucess and even more, the
motor now looks better-than-new. The vibrations did not increase
at all, so I guess my milling accuracy was "right on".
Having heard from a few coilers (Gary Lau being one of them),
that induction motors, modified this way, might not always start
up in every armature position, I turned the rotor in 10°
incrediments and applied power. It started up every time.
Gary:
Are you using a capacitor start motor or a centrifugal
switch start-up unit? Maybe cap start motors donīt
apply enough juice to the starting windings on modified motors
(this is the only difference I can think of) with the original cap.
So, now "all" I have left to do is build my SRS gap. Updates will
follow as construction continues........
Coiler greets from germany,
Reinhard