Motion Picture News (Sep-Oct 1923)

1240 Motion Picture News National Anti-Misframe League Forum implified Electricity for Projectionists Motor G*»npratnrs Lesson VI — Part XII mm Motor Generators HE ventage ordinarily supplied the theatre by the central station ranges from 110 to 220 and since the voltage of the arc, where such are used, is in the neighborhood of 50 it can be seen that some means must be used to lower the voltage at the switchboard in order to project the picture. There are a number of ways of accomplishing this apparently simple act and the methods employed by the various electrical manufacturers "give rise to many types of machines. All these machines, however, have a common purpose and that is to change the voltage supplied to that suitable, for operation of the arc. It would be hopeless to even attempt to describe one machine^ or method as being superior to another in view of the different operating conditions and so no attempt will be made to do so. Rather, a description of each type will be given and the reader left to form his own conclusions, supported no doubt, by personal experience, as to the merits of each. Types of Machines The methods most commonly employed for receiving this change in voltage are as follows : 1. Resistance. 2. Transformer. 3. Mercury Arc Rectifier. 4. Motor Generator. 5. Rotary Converter. The use of a resistance alone is very wasteful and is seldom used where it can be avoided. The mercury arc rectifier has already been described and will not be mentioned further while the transformer will be described later on. This leaves only the motor generator and rotary converter for present attention. These two types of machines are usually included under the common name of motor generator, but, strictly, such is not the case, since the converter is different both in point of construction and principle. The various manufacturers have built their machines to have certain operating characteristics, and this gives rise to still further classifications. Thus we have machines classified as follows: 1. As to type of machine : Motor-generators, rotary converters. 2. Method of using two arcs : Series operation, multiple operation. 3. Kind of current supplied: Changing Figure 98 Figure 99 from A. C. to D. C, changing from D. C. to lower voltage D. C. 4. Regulation of arc intensity: Using ballast resistance, changing voltage of arc. Some of the makes of machines most commonly used which come under the above classification are General Electric, Westinghouse, Wagner, Hertner and Hallberg. Since the Hertner machine was described in detail last week, no further attention need be given it here. General Electric The General Electric, or G. E. machine, as it is commonly called, is a motor generator which operates the arcs in series. Machines are made to change from either A. C. to D. C. to D. C. Since both the A. C. to D. C. and D. C. to D. C. motor generator compensarcs employ the same specially designed generator, the larger part of the following information will apply to both sets : These machines give an open circuit voltage of approximately 180 volts, which drops to 60 volts when one lamp is burning. The two lamps are connected in series without any resistance whatever. Short circuiting switches are provided to close the circuit around the lamp that is not burning, thus allowing the lamps to be trimmed without opening the circuit. When the second arc is struck the machine automatically increases its voltage to approximately 120 volts. The machine accommodates itself to the length of arc Figure 100 drawn so that it gives practically the same current whether one very short arc or two long arcs in series are burning. The wiring scheme is very simple, since the panel containing the ammeter and field rheostat can be placed anywhere in the booth, while the individual short circuiting switches can be mounted on the stand of the picture machine. The very flexible source of current behind the lamp tends to keep the arc quiet and at the same intensity which allows the picture to be faded in or out without any change of light on the screen. This generator automatically protects itself and the line fuses from burning out; in fact, the carbons can be left together indefinitely on direct short circuit without any trouble. The projection arc circuit is completely insulated from the power circuit, forming a valuable safeguard to the operator while handling the projection lamps. The ratings for the General Electric motor generator sets are as follows: A. C. to D. C. and D. C. to D. C. 25 cycles 40 cycles 50 cycles 60 cycles 35 ampere g 110 volts 50 ampere I £ J 220 volts 70 ampere * 440 volts 100 ampere J £ 1 550 volts^ In addition to the above, the machines can be supplied for either one, two or three-phase operation. All of the motor generator sets, with the exception of the 35 ampere set, which is thrown directly on the line, are supplied with either hand or automatic starters. Fig. 98 shows the A. C. to D. C. set and Fig. 99 the D. C. to D. C. set. Westinghouse The Westinghouse machine is perhaps more commonly referred to as a constant potential machine All the arc circuits are in multiple, making each circuit independent of the others. This, of course, means that the voltage must be the same at twice full load, during changeover, as at full load. The name constant potential comes from this fact. Constant current is obtained by the use of ballast rheostats. Two or more circuits of different amperage can be used at the same time, either for projectors, spotlights or floodlighting. All sets can be used for single or double arc operation. The rheostats act as a ballast against fluctuations in the current caused by variations in the resistance of the arc circuit as described in a preceding article. A ballast rheostat is required for each arc. Starting switches are used for starting the squirrel ca?e induction motors with full line voltage. The single phase motors are started by using two poles of the three-phase switch. (Continued on page 1242)