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Syncing Motors (was Re: new coil specs)
Hi Stan, Grayson,
Comments interspersed.
> Original Poster: Grayson B Dietrich <electrofire-at-juno-dot-com>
> Since in the conversion, a good deal of that hp will be lost.
This is more or less really due to the fact that normal
induction motors rely on the "slip" difference between
mains frequency and rpms. A normal, unmodified
induction motor possess a great amount of torque,
due to this slip phenomenon (even at start up). The
greater the difference between rotational speed and
mains frequency, the greater the resulting torque is
(all else being equal). This is why induction motors
are esp. good in applications, which require a high
amount of torque during start-up. As such a motor
doesnīt posses any sort of poles, it canīt sync into
an exact position, which makes it undesirable for
"pacemaker" (fragile) xformers (like NSTs). Although
a VERY lightly loaded (unmodified) induction motor
can indeed actually sync into the mains frequency (but
it will NEVER sync into the same position each time)
> And the modification:
> After removing the rotor, there will be a fairly large, regular
> cylndrical section that is normally in close proximity to the coils
> withing the motor case. This is what needs to have four "flats"
>machined into it. I used a mill to cut them, but a lot of people
>The material is mostly aluminum, so it will be easy to machine.
>four flats, evenly spaced around the curved surface of the cylinder,
>should be cut about 1/16" to 1/8" in depth. This means that four
>lengthwise flat sections will be filed into the rotor, with 1/16" to
>1/8" of material removed. light colored bands of metal will be
>seen at an angle after the top payer of the rotor is filed off. Try
>not to cut completely through any of them.
Whoah! Stop. This may be true for your motor, but cannot be
universally true. The number of flats depend on the motor
speed. For a 3450 (modified: 3600) rpm motor you only need
two flats. The depth of the flats is more or less given by design.
You will need to measure the exact width of the dead poles
(center to center).These are the poles through which ONLY the
start windings or NO windings at all run.
Using a mill (for best balancing) you want to mill 4 flats at 90°
(or 2 at 180° for 3600 rpm motors) that are exactly as wide as
these dead poles are. Since you are milling a flat of given width
on a round surface, the depth of the cut is automatically pre-
defined. More material is removed from the center of the flat
and less from the left/right ends of flat. What you are essentially
doing is creating "physical" poles, which will sync to the magnetic
field that the mains create in the run windings.
There will no longer be any slip between mains frequency and
rotational speed, which is why these motors donīt have a great
amount of torque, once they achieve their "new" top speed.
Coiler greets from Germany,
Reinhard