International projectionist (July-Dec 1934)

PROJECTION RECTIFIERS: SINGLE AND POLYPHASE TYPES IT WAS stated in this publication (Nov. issue) 1 that single-phase rectifiers are unsatisfactory for use with the new lamps burning Suprex carbons. This statement is correct; but some readers may have glossed over its limitation and concluded that this type rectifier is unsuited to any projection purpose. This is not so. Single-phase rectifiers of proper design are satisfactory for low-intensity projection work; but they are wholly unsatisfactory for either high-intensity or Suprex arc operation. Why.'' Because it is the combination of shutter action, the single-phase rectified current, and the intensity of the projected light that creates the unsatisfactory condition. In support of the foregoing the following facts are presented: With A. C. arcs using 60-cycle current, the front carbon is the only one in focus with the mirror and is alternately positive and negative 60 times a second. When the carbon is positive, light is projected to the screen; but when the carbon is negative, very little light is projected. It follows, therefore, that the screen is alternately bright and dim 60 times per second. With the shutter idle, the flicker will not be apparent because of the inability of the eye to follow such rapid changes in light intensity, the result of the phenomenon known as '"persistence of vision." Now let us consider the character of the light projected by an arc supplied with D.C., in which case the carbon in focus will be positive constantly and the projected light will be of constant intensity. Such a light, however, will appear 1 ''The Suprex Carbon Arc," J. J. Finn-, page 7. body else and then await the passage of eight or ten months before a decision is forthcoming. Usually, too, the decision would be a perfect example of straddling and pussyfooting. Numerous individual cases could be cited in support of the foregoing estimate of the NRA. but there remains to be said only that the NRA is, as the distinguished General Johnson said, dead as a dodo. All that need be done is to write its obituary notice. J. K. Elderkin FOREST MANUFACTURING CORPORATION no different to the eye than that projected from 60-cycle A. C. arc — without the shutter running. We can now consider the effect of shutter action. Having two blades and two openings, the shutter, running at normal speed of 24 revolutions per second, will cut off the light on the screen twice with each revolution. Therefore, the frequency of the projected light from the D. C. arc, due to shutter action, will be 2 x 24, or 48 cycles, per second, which, if the light be of high intensity, can be detected by the human eye. This effect may be termed inherent flicker, since it is a characteristic of the projector and bears no relation to the nature of current supplying the arc. The degree of visibility of this flicker depends upon only two things: the intensity and the color of the projected light. Effect of Shutter Action It has been noted that light projected by a 60-cycle A. C. arc has no visible flicker, but that light projected at a frequency of 48 cycles does present a visible flicker provided the intensity of the light be high enough. Now, it is obvious that the lower the frequency below 48 cycles the more noticeable will be the flicker. In other words, while projected light interrupted 60 times per second appears to be uninterrupted, and light interrupted 48 times per second is passable, if we interrupt the light at a rate considerably lower than 48 times per second, the result will be highly objectionable flicker. The logical question now presents itself: If we project a light that is interrupted 60 times per second, and then cut into this light with a shutter that in itself interrupts 48 times per second this already interrupted light, what is the result? The result is stroboscopic and the difference between the two frequencies is manifested by a very objectionable flicker. The only cure for this situation would be to either speed up the shutter to 60 interruptions per second (not practical), or to supply the arc with current having a frequency of 48-96 or 144 cycles. Or, possibly, have a difference between shutter frequency and supply current of at least 48 to 50 cycles (also not practical). The type of shutter used has no bearing on the results obtained. This writer has used all types of shutters — wide blades, narrow blades, uneven blades, "flicker blades", synchronous and nonsynchronous, and combinations of all, but with no other results than those enumerated herein. If it wTere not for the shutter, screen results would be the same irrespective of power supply, but unfortunately we must use a shutter. It was stated previously that singlephase rectifiers are acceptable for lowintensity projection but not for any other type of projection. Why? If we supply an arc with current rectified from singlephase A. C, the carbon in focus is always positive and therefore projects a light of constant intensity. However, this rectified current has a ripple corresponding to the A. C. from which it is derived. The magnitude of this ripple depends upon the design of the rectifier, some of which may have more ripple than others. If this be so, then, why is a single-phase rectifier considered as being satisfactory for any type of projection arc? The answer is: because the higher the amperage required from a single-phase rectifier, the more difficult becomes the problem of filtering or removing the ripple. If we had to deal only with current of 8 to 10 amperes at 10 to 12 volts, we could build a filter having sufficiently large chokes and condensers to practically eliminate the ripple. It is an entirely different matter, however, to filter 25 to 60 amperes at from 35 to 60 volts. This writer, with extensive experience and a wide knowledge of available filters and condensers, knows of no filter that would effectively remove the ripple from, say, a 25-ampere, 50-volt rectifier. A good single-phase rectifier, then, is one in which the ripple is reduced to the lowest possible limit, the accomplishment of which requires the use in the rectifier of sufficient inductance. The latter may be done wholly within the transformer by proper design, or it may be in the form of a choke or chokes placed in the rectifier circuit either ahead of the rectifying element or in the output circuit. What is the effect of this ripple upon the projected light? Since the carbon in focus is always positive, there is al [17]