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Possibilities of High Frequency Currents


By John E. Pritchard





In all branches of human endeavor there has been that characteristic trait to attain perfection. This condition has never been reached and never will; but like a variable approaching a limit, the difference between perfection and non-perfection can be made very small.

In the early development of electrical application, it was generally conceded that direct current transmission was a marvelous advance toward the ideal method of power delivery. Its flexibility over all others was evident, at that time. This advancement opened a new field of industrial engineering. Investigators were spurred on by successes, so naturally the field became extensive. Then the disadvantages of direct current became prominent. Economy demanded that power units be centralized.

It was found that to transmit large currents long distances at low voltage, required very large conductors to reduce the heat losses, which are equal to the current squared, multiplied by the resistance. This draw-back was partially surmounted by the high voltage direct current generator; but due to the mechanical as well as current transformation difficulties, it has not been widely adopted.

LOW FREQUENCY ALTERNATING CURRENTS

Low frequency 60 cycle A. C. machinery, thru persistent research, has to-day reached a high degree of perfection. But even with our great power houses and their long transmission lines lighting great cities, propelling ponderous electric trains over high mountains and furnishing energy for every conceivable form of prime-mover in the industries, there exist many factors detrimental to perfection.

The reason for the great use of alternating currents is the ease with which they can be transformed from one voltage to another. They can be generated at a comparatively low voltage and stept up to a higher voltage. In the transmission of great power at high voltage there is required but a small current, and this, in turn, requires only conductors of small cross-section.

RESONANCE IN A. C. CIRCUIT

Transformation is accompanied with heat, magnetic leakage, hysteresis and eddy current losses. First class design has reduced these to a minimum. Also, there are disadvantages caused by inductance and capacity in every circuit. This lowers the power factor. The lower this is, the more current is required to produce power at a given voltage. This gives rise to excessive heat losses. The power factor may be made unity or nearly so, by balancing the reactive effects of inductance and capacity.

When this condition exists, resonance is said to take place in the circuit and then Ohm's law holds true, C=E/R. It is very desirous not to have this in practise, due to the great strain placed on the circuit. The voltage, as well as the current, may reach abnormal values under such conditions. In fact, the potential on a long transmission line may be 10 per cent higher at the distant end than at the generator end, due to these resonance conditions. In one case a 10,000-volt A. C. transmission line showed 11,000 volts at the distant or load end of the circuit.

This phenomenon may be illustrated by the mechanical analogy of a pendulum six feet long, on the foot of which is hung a weight of one hundred pounds. Strike the weight with a force of ten pounds.

Call this the positive impulse. When it has traveled the full length of its arc, give it another ten pound blow. This is the negative impulse. Now the pendulum will oscillate freely. The time for a single vibration will always be equal, regardless of the length of its arc; therefore the impulses will occur at equal intervals of time and will be analogous to an alternating current delivered to a resonant circuit. The amplitude of the swing will get greater and greater until finally the energy represented in the moving weight is many times larger than the individual impulses. These are called free vibrations. But if the forces are not applied at the instant of the weight's return to center, part of the energy will be lost in heat. These are forced vibrations.

Alternating currents, in present day practice, applied to induction motors and transformers, are impeded by resistance, inductance, and capacity; giving rise to these forced vibrations. This condition causes voltage drop, low power factor, hysteresis or iron losses, and eddy current losses. A synchronous motor on the line with an overexcited field, furnishes an equivalent of capacity and tends to neutralize inductance and thereby makes the vibrations free. A condenser would do the same, but it would be too large to be practical.

HIGH FREQUENCY ALTERNATING CURRENTS

High frequency alternating currents and machinery for producing them are comparatively new. This field of application is not extensive. There has been considerable development on the large scale in wireless telegraphy and telephony. But here the power has not, up to the present time, exceeded a few hundred kilowatts, and it could hardly be called power transmission, for only a very small portion of the original energy eventually reaches its destination in any case.

Very few investigators have given any thought to the utilization of these currents, for great power distribution. Very new and strange phenomena are apparent when these currents are working. For instance, currents can be induced efficiently in circuits without iron as a magnetic medicine. Hysteresis losses are practically eliminated. Without iron in a transformer, it would be lighter for a given power than the present 60 cycle ones. It would be more compact, thereby reducing magnetic leakage. There is no saturation point to consider because of an air core. The reactive effect of a few turns of wire is very great; but a small condenser will cause resonance. Therefore the power factor can be easily made unity.

Among the few original engineers in this field of high frequency A. C. power transmission is Prof. Dr. E. F. Northrup, who was assisted by the writer at the Palmer Physical Laboratory, Princeton University, in extensive research along these lines. With these currents, it was possible to get results never before obtained by any other investigator.

Advantages of High Frequency Transformers With an air core transformer, having a resistance load, more efficient results were noticed than could be procured with transformers having iron for a core. The losses were mostly due to the resistance of the conductors; but they were very small, as the circuits were of large cross-section.

The hysteresis was negligible, due to the absence of iron. Magnetic leakage was reduced to a minimum because the coupling between primary and secondary was close. The capacity was 10 kilowatts, with three turns primary, insulated for 7,200 volts, and one turn secondary, reducing to 2,400 volts. The circuit was resonant; therefore the power factor was unity, or numerically 1.

It remained for Prof. E. F. Northrup to critically study these currents at low voltage. He found they could be confined in a mass of metal, thereby producing a terrific heat in a very simple and efficient manner.

The present day development in wireless has been to raise the voltage and frequency, because under these conditions maximum radiation of energy is brought about.

After several years of careful experimentation, a high frequency, electric induction furnace was produced that was applicable for the production of heat in either a conductor or non-conductor. Its extreme simplicity, owing to the absence of iron and a few turns in the inductor coil, leads one to believe that along these lines lay perfection in electrical mechanics.

At present there is in actual operation under industrial conditions a 20 K.W. furnace of this type. It is used for studying glass at high temperature in vacuum or under pressure.

A few years ago applications of this sort were purely imaginary. There is now under test a 50 K.W. unit. Most all tests were carried on with the damped oscillations of condenser discharges up to 50 K.W., but the undamped oscillations of the high frequently alternator will be used for greater capacity.

With the spark method for producing these currents, a two phase field was used, no doubt for the first time. Altho nothing other than a half revolution of a circular conductor on an axis in this field was noticed, the writer is firmly convinced that, with proper apparatus and research, an induction motor could be made utilizing these currents. A motor of this sort would be WITHOUT IRON as a magnetic medium and therefore extremely light. Proper design would undoubtedly make it efficient.

High frequency lighting has already given promise to be eventually the means whereby the much heralded "cold light" may be produced.

The fundamental principles involved in producing these currents has been firmly establisht in "wireless" and with these methods, slightly changed, it is possible to investigate a brand new field of "endless wonders."

In the last decade there has been marvelous advances in electricity; but these have only paved the way for far greater things.



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