Low Cost Apparatus Competition

Apparatus Title: An AC, Shaded Pole, Induction Motor for the Overhead projector or table top display.

Abstract: Early AC induction motors used a two-phase "rotating pole" mechanism to produce rotation. Single phase, self-starting motors make use of a variety of techniques to produce a torque to begin rotation. In the "shaded pole" type of motor, off-set, closed coils, called shaded poles produce a slight torque to begin the rotation. This demonstration apparatus uses the stator and windings of a 2 pole motor and a hand made brass cage to act as the rotor. The apparatus demonstrates several aspects of induction, Lenz's Law and phase relationships in "rotating pole" type motors.

Equipment required to construct apparatus:

• AC motor stator with windings (used or scrap, under $15),
• miscellaneous sections of brass or copper rod and telescoping tubing (under $ 10),
• miscellaneous scraps of Plexiglas,
• nuts and bolts and solder. See description of construction details

Sketch of the apparatus: Find figures for this talk and captions at;

http://www.geocities.com/b5lazer/Physics/ .

A complete description of the apparatus is desirable: See construction details in the following article.

An AC, Shaded Pole, Induction Motor for the Overhead Projector or Table Top Display

Introduction

With the serendipitous discovery of electromagnetism by Hans Christian Oersted in 1820, began a revolution to invent electrical rotation devices. Just one year later Michael Faraday had invented the first primitive electromagnetic motor.[1] Faraday's subsequent invention of the DC generator set off a flurry of experimentation worldwide. The history of the development of electric power and electric motors is a sinuous trail with hundreds of players on two continents racing to generate and use electric power. Many researchers, entrepreneurs and tycoon barons worked independently and sometimes-in concert to make the electric to make electric power and the electric motor a tool of society.

In the 1840's several independent workers developed DC motors. It was the dream of Nikola Tesla to create brush-less dynamos and motors using Alternation Current that served as the historic focus of a turning point in history.2 After demonstrating a polyphase AC motor in 1884 Tesla was granted a broad patents for the use of "rotating magnetic fields" in electrical devices in 1887.

Tesla sold his patents to George Westinghouse in 1888 thus beginning the "War of the Currents" pitting Thomas Edison's DC power systems with Westinghouse backed AC power generation and distribution.

The single-phase induction motor was an important goal of the Westinghouse group because Tesla's original [2] phase designs were not compatible with existing dynamo equipment. Although Tesla had developed the "split phase" motor years earlier, he worked with the Westinghouse group as a consultant through 1889 to help refine the design which was already covered by his earlier patents.1 Following this time the trend was toward building bigger and more powerful 3 phase motors for industrial use. Development of smaller motors including the "Squirrel-cage" continued for decades.[1]

Fractional horsepower motors were needed in household applications. Small versions of bigger motors were used but by the 1940's simple motors such as the shaded pole design were produced in mass quantities for use in house hold appliances such as washing machines, ventilators and record players.[3]

Operation of Shaded Pole Motors.

The operation of all single-phase AC motors depends on a primary winding and an auxiliary or secondary winding to provide a "starting torque". The auxiliary windings are located 90 electrical degrees out of phase with the primary winding.[4] The resulting shift in phase provides the torque needed to make the motor self-starting.[5] (See Figure 2).

In a shaded pole motor the auxiliary winding is replaced by one or two closed turns of heavy copper wire embedded in one side of each stator pole.[4] (See figure 3). That part of the stator that is not encircled by the copper turns is the main stator pole and that part that is covered by the copper turns is termed the "shaded pole" The operation of the shaded -pole motor is described in "Electric Motor repair".[4]

"On starting, a current is induced into the shaded poles from the main poles. The shading coils establish a magnetic field that is out of phase with that established by the main fields, and a shifting field is produced sufficient to give the desired starting torque. When the motor reaches speed the effect of the shading coils is negligible."[4]

Using current probes and an oscilloscope the changing currents in the main pole and shaded pole illustrates the phase relation of the currents producing the "rotating" magnetic field. (See figure 4).

Building the Apparatus

The first prototype of my apparatus used the stator of a 4 pole motor along with a hand-made cage made of copper. I used a hand made wooden gig to hold the 8 rotor arms for soldering. It operates very smoothly and can be seen as (See figure 13). The main drawback was that the shaded poles are completely covered by the windings and can be viewed only if the stator is held at an angle.

In later versions I decided to use a two-pole stator because the shaded poles are clearly visible. The stator I chose for my competition machine came from a toy rock tumbler purchased for $1 at a garage sale.

In choosing a stator the thickness of the laminated portion gives a rough indication of the power of the motor. The general rule is the thicker the stator the greater the power. I could compare stators by measuring the no load resistance of the primary coil. I found that stator coils of 5 or 6 ohms were too powerful and tended to heat up very quickly. Stator coils with resistance between 30 and 50 ohms were too weak to produce good results. The competition stator coil has a resistance of about 13 ohms. (See figure 5).

The second step is building the cage itself. I chose a ribbed design that would illustrate the closed loops required for induced currents in the rotor. (See figure 6)

The choice of materials is the first step. Having run out of copper stock I chose solid brass rod of diameter 3/16's inch for the ribs. Since eight ribs have to be soldered to the hubs I calculated that the hub should be in the range of 1/4 inch in diameter. I made the hubs using telescoping brass tubing. After choosing the widest diameter tubing needed, I soldered smaller tubing into a mass, leaving the smallest tube at the center open to accept the needle (pivot)of the jewel vee bearing. I then placed a section of the tubing in a drill press and cut it off using a jewelers saw. (A little help from the machine shop would help this process.) I calculated the length of each rib segment taking into account the radius of the hub, the desired height of the cage, the radius of the hole in the stator and some clearance for rotation. The competition apparatus has rib segments that are about 11/4 inches in length. (See figure 7.)

In order to insure uniform length I drilled a hole in a milled block of aluminum 11/4 inches thick. The hole must penetrate the block for removal of the cut segments. I placed the aluminum block in a vise with a backing block to hold the brass stock. I cut each segment with a jewelers saw and filed each end of the segment flat. (See figure 8).

To bend each segment into a "C" shape I drilled a hole in an aluminum block to the depth of the desired rib radius. After inserting the segment into the hole I used a small hammer to bend the segments at the desired radius. The ribs must have right angles and be flat. (See figure 9).

To assemble the cage I had the school's machine shop make an aluminum gig with a center hole of diameter 1/4 inch and 8 radiating grooves to hold the bent rotor segments. Working from the top, I supported the upper hub with a cut segment of ceramic tubing extracted from a crucible holder donated by the chemistry department. After placing the ribs in the grooves braced against the lower hub I placed the upper hub on the ceramic segment and used fine steel wire to pull the ribs against the upper hub. After careful application of flux paste I used a pencil torch to solder upper hub to the ribs. (See figure 10). I then turned the cage over and repeated the soldering process. I used short segments of straight pins through the hubs and used epoxy to glue them in place to provide pivots on each end of the cage.

The next step was to mount the stator on a sheet of Plexiglas used as the base. I used the existing holes of the stator as drill guides to drill holes in the Plexiglas. I used nylon spacers to provide an air space between the motor and the base. To locate the position of the jeweled bearing I wrapped several lengths of paper around the cage until it fitsnugly in the hole in the stator. The pin pivot marked the drill point for the bearing. The bearing, taken from the movement of an old analog voltmeter was centered and mounted on a short nylon spacer. Epoxy was used to glue it in place. (See figure 11).

Long bolts through the stator support a strip of Plexiglas holding the upper bearing. The bearing height is adjusted by raising or lowering the nuts on the support bolts. (See figure 12).

To finish the project I wired in a momentary contact switch on to a separate shelf of Plexiglas mounted on nylon spacers. The machine can be used as a tabletop demonstration or on the overhead projector for larger audiences.

Note: stock metal, tubing and jeweled bearings are available through Small Parts Inc. 13980 N.W. 58 Court P.O. Box 4650, Miami Lakes, Fl 33014-0650 or call 1-800-2204242 for immediate service or a catalog.

Bibliography

1. Author Unknown, ITQ Historical Archive, http://pixii.com/apparatus.htm

2. Cheney and Uth. TESLA, Master of Lightning. New York: Barnes & Nobel Books, 1999.

3. Author Unknown, A Short History of Electric Machines, http://historia.et.tudelft.nl/pub/art/machines.php3

4. Rosenberg and Hand. Electric Motor Repair, Third Edition. New York: Holt, Rinehart and Winston, 1987.

5. McGraw-Hill encyclopedia of science & technology. 8th ed., "Induction Motors.

Other Useful References

Electricity: http://www.ioa.com/~micron/002-Electricity.html

Carlson Nikola Tesla: http://www.eh.net/Conference/carson.html