International projectionist (Jan-Dec 1957)

efficiencies of 80-85%. Motor-generators, however, require the use of a ballast resistor for each arc lamp. Use of ballast rheostats cuts the efficiency of the motor-generator down to 60-70%. The Ballast Rheostat An arc-lamp ballast rheostat closely resembles an ordinary electric heater. In fact, the rheostat generates heat by impeding the flow of current. The heat represents wasted power; but in this case the waste is absolutely necessary. The voltage characteristics of multiple-arc generators are such that some means of limiting the amount of current (amperes) is mandatory. The carbon arc offers less resistance to the flow of current as its temperature rises; and if there were no ballast resistance in the circuit to keep the resistance of the arc, itself, in check, more and more current would flow until the generator became virtually short-circuited. An overloaded generator demagnetizes itself and stops generating current. Too great a ballast resistance results in an unnecessarily great waste of power. Insufficient ballast results in an erratic unstable arc and a flickering light on the screen. Arc circuits supplied with DC by rectifiers need no ballast resistance. The inductive reactance of the AC transformer functions as a ballast which, happily, consumes no power and produces no heat. So while rectifiers and generator sets have about the same intrinsic efficiency, the use of generators requires power-wasting ballast rheostats not necessary with rectifiers. Electrical efficiency, however, is not the only factor to be considered when choosing an AC-to-DC current-converting device for powering projection arcs. Stability and reliability are just as important. The loss of power in the ballast rheostat is smaller than the losses occurrring in those stack rectifiers which have deteriorated through age, and is admittedly a small price to pay for satisfactory performance. Popularity of Rectifiers Rectifiers are popular because of their low initial cost and silent operation. Moreover, selenium rectifiers are now made with sufficient power-handling capacity to operate even the most powerful arcs. But as every projectionist learns by experience, the power requirements of high-intensity arcs are very exacting. The carbon arc is a device that tolerates no irregularities in the current supplied to it. By rectifying the alternating current furnished by an inefficient power company a rectifier passes on to the arc the ups and downs of the supplied AC and, in the case of polyphase current, fails to compensate for phase imbalance. The rectification may be 3-phase, 6-phase, or even 12-phase. but the ripple in the DC output is nearly always a 60-cycle ripple. A 60-cycle ripple in the current fed to the arcs interacts with the 48-cycle projector-shutter frequency to produce a 12-cycle "beat frequency" clearly visible as flicker when the ripple factor exceds 3 or 4% of the total output voltage (Fig. 3). Since the ripple originating in the rectifier itself may amount to as much as 5%, and because phase imbalance of the AC input may be as great as 10%, flicker of the projected pictures when rectifiers are used is often extremely annoying. Tungar-tube and selenium-stack rectifiers are favored at the present time, the latter for arc currents exceeding 60 or 70 amperes. The advantage of J\ /\ . /\ /\ /\ /\ r / 60-cycle current Single-phase rectified current (DC) without 60-cycle ripple 3-phase rectified current without 60-cycle ripple 1 I ffff I 'Mill Hffli IIM|H UM i 3-phase rectified current WITH 60-cycle ripple FIG. 3. The production of 12-cycle flicker by 60-cycle ripple in rectifier output current. the selenium type is its long life (estimated to be more than 50,000 hours) and consequent elimination of the need for tube replacements. Selenium rectifiers also have a lower inherent ripple factor. Motor-generator sets, in spite of their lower power-conversion efficiency when used with the necessary ballast resistors, have a long life with little or no deterioration of output. They require only infrequent repairs (replacement or brushes, smoothing of the commutator in a lathe, etc.) and are tolerant of abuse and neglect. The initial cost of a good motor-generator set is nevertheless higher than that of a rectifier, and all except the models having sealed bearings require lubrication about once a year in the case of those having grease-lubricated ball or roller bearings, once a month if oil-lubricated sleeve bearings are used. The outstanding advantages of motor-generator sets consist of their ruggedness and unsurpassed ability to deliver a ripple-free direct current of constant voltage at all loads. Even large mains-voltage fluctuations and phase disparities do not affect the DC output when the generator drive motor is of the 3-phase induction type. Such fluctuations, if very large, temporarily reduce the power-converting efficiency of the set (by heating the drive motor) , but do not disturb the constancy of the generated DC. Other Links in Chain Regardless of whether generators or rectifiers are used, there are other links in the power-transmission chain which, if weak, will adversely affect the quality of the screen illumination. If a motor-generator set is used, the condition of the field rheostats (usually in the control cabinet) should be checked. While these rheostats and their connecting wires carry only a weak current, it should not be assumed that they are unimportant. If the generator shunt-field circuit is broken, generation of current will stop. The conducting wires which carry the DC from the generator or rectifier output terminals to the arcs are important from the standpoint of safety as well as of electrical efficiency. If these wires (two leading to each lamp) are too small in size, they will offer sufficient resistance to the current to produce heat as well as a loss of power. The fire hazard is a very important (Continued on page 26) 10 INTERNATIONAL PROJECTIONIST • JANUARY 1957