Project
The design of our shaded pole motor starts with
two AC relay coils. For ease of construction, the
relay coils can be obtained from RadioShack (part
number 275-217), but other coils will work. These
relays are Omron LY-2-AC 110/120 coils rated at 1.1
VA at 110 to 120 VAC. Typically, the coils draw
about 10 mA when the contacts are closed. Some
relays operate at 2-3 VA, especially 3PDT and larger
relays that will produce larger magnetic fields for
larger gaps or that can operate a motor with greater
bearing friction. The plastic case, the hinged metal return
that supports the contacts, and the return force spring
can be discarded.
Carefully place the coil in a vise and cut the rear
of the magnet return down low with a fine-toothed
hacksaw blade, as illustrated in Figure 4. As you cut
through the steel return, it is advisable to place a 1/16”
thick plastic sheet between the steel and the coil to
prevent the hacksaw blade from breaking through and
damaging the coil. In our application, the magnetic
path is essentially through an air gap that reduces the
magnetic field and the self-inductance of the coils. This
results in larger coil current. The open circuited coil will
typically run about 25 ma and will, thus, run thermally
warmer.
The plastic terminal block has to be cut down,
similar to the metal magnet return, as illustrated in
Figure 4. The solder contact just above the relay coil
terminals can be removed by unsoldering the attached
wires and pulling the pins outward. It is convenient to cut
at this same level. Again, it is helpful to put a piece of
plastic between the coil and contact block to prevent
the saw blade from damaging the coil. Also, use extra
caution on this side of the coil because the fine coil leads
are also located here.
All the metal parts — including the aluminum sheet,
brass sheet, tubing, and rods — were obtained from a
hardware store. The magnetic return for the two coils was
fabricated from a 3” x 3” x 3/4” steel corner bracket. The
bracket was bent in the vise, as illustrated in Figure
5. The spacing is not too critical, but the poles of the
coils should be spaced on the order of 3/16”. The
coils were epoxied to the steel return using a five
minute epoxy, such as Loctite. It is always a good
practice to roughen the surfaces to be epoxied with
either a file or sandpaper to provide a rough, clean
surface for the adhesive. This completes the shaded
pole coil assembly.
The rotor for our motor is fabricated from 1/16”
thick aluminum. A compass or divider can be used
to inscribe the circle with a 2.5” diameter. A sharp
pair of tin snips can be used to cut along the mark
to produce the rotor. The main thing is not to distort
the aluminum. It must be relatively flat to run
between the gap of the pole pieces. Various size
rotors have been employed in different designs.
JANUARY 2005
Figure 4. Preparation of the AC coil.
fields by means of the shading coil. An example of this
type is illustrated in Figure 2, where a shorted coil (the
shaded pole) encircles part of the magnetic pole. Since
the current induced in the shorted turn is a function of the
rate of change of the main pole flux, it is out-of-phase and
lags the main field.
John Fleming introduced the design of the shaded
pole motor around 1890. Fleming would go on to help
Guglieimo Marconi design his equipment for the first
transatlantic wireless message in 1901 and he would
later patent the first vacuum tube (a “thermionic
valve”) in 1904. About this same time, Elihu Thomson
patented the shaded pole design in the US and, in
1892, his company merged with the Edison General
Electric Company to become the General Electric
Company.
Interestingly, the design of the shaded pole motor to
produce a rotating magnetic field is also employed in the
design of AC relays and contactors. You can identify AC
relay coils from DC designs by looking at the shaded
pole, as illustrated in Figure 3. The shaded pole design for
AC relays is used to delay one component of the magnetic
field to prevent the relay from chattering. Without the
shaded pole, an AC relay contact would chatter every
time the AC current goes through zero and the magnetic
field would be unable to hold the contacts closed against
the spring. The delayed field continues to hold the relay
contacts closed while the main field goes through zero
and visa versa.
Figure 5. A steel bracket for magnet return, pivot, and bearing.
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