Nuts & Volts - January 2005

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.