A Simple Shaded Pole AC Motor
Larger diameter rotors tend to wobble more and may
brush against the pole and stop turning. Rotors as thin as
0.003” have been made and also work. Material of this
thickness can be cut with an ordinary pair of scissors.
Thinner material will not work because the resistance of
the material becomes too high and the material also
becomes mechanically unstable. A 1/8” diameter hole can
be punched in the rotor at the center of the circle. Either
use a hand punch or drill the hole by sandwiching the
aluminum between two heavy sheets of plastic to prevent
the drill from ripping through.
The central rotor bearing was made from a 1/4” long
piece of 1/8” brass rod, as seen in Figure 5. The rod was
drilled with a 0.040” diameter drill bit to a depth of 1/8”.
Most general-purpose drill bits have a 118° point. The
central bearing was placed in the central hole of the
aluminum rotor and a piece of Scotch tape was placed
on the underside to hold the bearing in position and to
seal the 0.040” hole. Epoxy was applied with a toothpick
to glue the bearing and rotor together. Since the mass of
the aluminum disc is about 1/8” below the support pivot,
it will be unconditionally stable and will not fall off
the pivot.
As an alternative, some models of this motor have
been built using the plastic top of a “three in one”
multi-purpose oil can ( 3 fl oz size) in place of the brass
bearing. The closed end of the red plastic spout was cut off
with a razor blade about 3/16” from the top and used as
the bearing. The brass bearing is, of course, more rugged
and provides a better bearing if the hole is drilled true. A
small lathe is useful here, if you have one.
The pivot point was fabricated from a 1/32” brass
rod, as shown in Figure 5. The point of the rod was
ground by turning the rod against fine emery paper. The
rod was held at about a 25° angle above the paper to
produce the point. This produces a point of about 50°,
as compared to the drill bit point of 118°, to ensure
a point of contact and lower friction. In another version
of the motor design, a steel needle was used as the
pivot point.
The completed motor can be seen in Figure 6. The
mounting base was made from a piece of 1/16” thick
brass that is 4” square. A metal base is useful to help
dissipate the additional coil heat. At the center of the
base, a 4-40 brass nut was soldered. To the center of the
nut, a 3/32” O.D. brass tube was soldered and the 1/32”
brass pivot rod was soldered into this tube. The tube
helped to stiffen the smaller pivot rod and also allowed
less heat to be applied to the pivot rod for adjustment of
the height of the pivot to locate the rotor between the
pole pieces.
Other models have been built with a wooden base that
employed the steel needle pivot mentioned earlier. In such
a design, the needle can be pushed further into the wood
to adjust the pivot height to center the rotor between the
pole pieces.
JANUARY 2005
Figure 6. The completed AC motor construction.
Once the final position has been determined, the pin
position can be secured with a drop of epoxy between the
base and pin.
After the rotor has been assembled and a pivot rod is
available, you can test the bearing and rotor run out. Softly
blow on the underside of the disc to see that it spins easily.
Also notice if the disc tends to always stop with one side
low. It is helpful to mark the rotor with a pen to provide a
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