CORONA MOTORS

A number of experiments with Poggendorff type corona motors were reported in 1921 by V. E. Johnson. One of his motors was similar to Ruhmkorff's motor. This was a horizontal mica disk, about 6 inches in diameter, supported on a vertical needle point. At the edge of the disk there were two insulated corona-producing points tangential to the disk and oriented in the same rotational direction. Another of Johnson's motors was a big machine consisting of two stationary glass disks and a three-piece glass rotor that could rotate on ball bearings between the stationary disks. The rotor was made of two large glass disks separated by a smaller disk so as to have a deep slot along its edge. Two flat sharp-point combs were inserted diametrically opposite to one another into the slot of the rotor. Johnson estimated that his motor could run at about 2000 rpm and had a power of about 90 watts.

Just as Poggendorffs motor was derived from Holtz's electrostatic machine, a series of electrostatic motors were similarly derived from Wimshurst's electrostatic machine.

In 1891 five such motors were constructed by William McVay of New York City. McVay's first motor (Fig. 33) consisted of two horizontal glass disks, about 12 inches in diameter, one stationary and the other rotating on the vertical axis just above the first. The lower disk had two quadrants of tinfoil, and the upper disk had 16 tinfoil sectors, as shown in the figure. The power (from a Wimshurst machine) was delivered to the motor by means of two insulated arms, each of which terminated in two brushes, one touching continually one of the lower quadrants, the other charging a sector on the upper disk just clear of the edge of the quadrant. Charges of the same polarity were thus deposited on the quadrant and on the sector, causing them to repel each other. An "equalizer" reduced the charge of the sectors before they passed over the further edge of an oppositely charged quadrant, thus reducing the back torque on the rotating disk. An important new feature of this motor was the simultaneous charging of the stationary quadrants and of the moving sectors, which assured a relatively strong starting torque and, together with the neutralizing system, assured a reliable unidirectional operation. McVay also constructed motors of cylindrical geometry. In this motor the quadrants were located on the inner cylinder, while the charging and neutralizing brushes were on the neck of the outside cylinder.

McVay's first motor was later modified for charging by combs, rather than by brushes. The motor then operated essentially as Poggendorff's corona motor, retaining, however, its self-starting and unidirectional qualities. Instructions for building a Poggendorff motor may be found in the May 1971 issue of Popular Science. Instructions for building a simple McVay motor may be found in the January 1914 issue of Electrical Experimenter (the author is grateful to Mr. Thorn L. Mayes for this information).

Since a corona discharge in air at atmospheric pressure requires a minimum voltage of about 3000 volts, ordinary corona motors can operate only from sources capable of supplying a voltage of this magnitude. Such voltages are, of course, easily attainable at the present time, and since the corona motors are very simple and efficient devices, they have been further developed and studied in recent years....

CAPACITOR MOTORS

In 1889, Karl Zipernowsky, a Hungarian engineer (co-inventor of practical electrical transformers), constructed a new type of electrostatic motor, which was derived from Thomson's quadrant electrometer. The rotor of this motor (Fig. 36) consisted of two pairs of aluminum sectors insulated from each other and from the rest of the apparatus. The stator consisted of four double (hollow) sectors of brass enclosing the rotor. The rotor was fitted with a commutator in four parts, by means of which the sectors of the rotor were charged oppositely to those sectors of the stator into which they were entering and identically to those sectors of the stator which they were leaving. An interesting property of this motor was that it could operate from high-voltage dc as well as from high-voltage ac sources.

Inasmuch as Zipernowsky's motor operated as a result of the electric forces exerted by one charged conducting plate upon a second charged conducting plate (which are the same forces that act upon the two plates of a capacitor) it constituted what is now called an electrostatic "capacitor motor".

Since capacitor motors do not require sparks or a corona discharge for their operation, they can operate, at least in principle, from as low a voltage as one desires to use. This is one of their important advantages and is one of the reasons that such motors have been given considerable attention in recent years. Furthermore, as already indicated, capacitor motors can operate not only from dc sources, but also from ac sources. Finally, when powered by an ac source, they can operate both as synchronous and asynchronous motors (Zipernowsky's original motor operated from ac as an asynchronous motor).

A synchronous capacitor-type electrostatic motor is merely a multi-electrode capacitor motor without a commutator, the proper charging of the rotor being accomplished by continuously supplying an ac voltage of proper frequency between the stator and the rotor. It is easy to see that if the rotor moves by one electrode in one period of the supply voltage, then the ac voltage accomplishes the same effect as that accomplished by a do voltage with a commutator. The synchronous velocity is therefore 2 &pi ƒ/N, where ƒ is the frequency of the supply voltage and N is the number of the electrodes....