Motion Picture News (Oct-Dec 1930)

102 Motion Picture News October 4, 19 3 0 \Q OHMS IO HENRYS yrmu u E^IOOV 6o CYCLES t FIG.62.D IR curve shows the voltage across the resistance R. The voltage E is that applied. The fact that the current I is not in phase with the voltage E is due to the presence of the capacity and the inductance. Now each of these produces its own reaction and if the reaction effect of the capacity is the opposite of that of the inductance, these two will tend to neutralize each other. This can best be comprehended by associating the reactance value with the phase relation. Both the capacity and the inductance possess reactance but their actions as far as the control of current is concerned is opposite. Hence if the two reactances are equal, they cancel. If they are not equal, the arithmetical difference between them is the natural resultant reactance. Which two is greater in reactance is of no consequence as far as the value of the ccrrent is concerned. The effect is a matter of the final phase relation between the current and the voltage in the circuit. The phase relations of the individual units remains unchanged but when considered collectively, the lag or lead of current depends entirely upon the relative magnitudes of the capacity of inductance reactions. As to the relation between capacity and indcctive reactance, it is customary to affix a plus sign to the inductive reactance and a minus sign to the capacity reactance. Thus when both are in one circuit, the difference is secured by subtracting the capacity reactance from the inductive reactance. Suppose that we tackle the system shown in figure 63A. R = 10 ohms 1,000,000 Xc = = 26.52 ohms 6.28x60x100 and XL = 6.28xfxL = 6.28x60xl0 = 3770 ohms The total impedance Z when resistance, capacity and inductance are in series in » circuit Z=VR2+ (Xl — Xc)2 = V 102 + (3770— 26.52) 2 = \/102 + 3743.52 = 3743.5 ohms*** ***Once again we need not consider the value of the resistance R, since it is a very small fraction of the total reactance. If considered it would make necessary two decimals in the value of the final impedance. The current 100 1 = 3743.5 = .02673 ampere Then IR = .02673 x 10 = .2673 volt and IXC = .02673 x 26.52 = .7089 volt and IXl = .02673 x 3770 = 100.77 volts The impressed voltage E is then equal to Eapp. = VIR2+ (IXl — IXc)2 = V-26732 + (100.77 — .7089) 2 = 100 volts The total power factor is R cos 6 = ■ ■ Z 10 = = 0.002673 3743.5 © = 89* 50.5' current lag because the inductance predominates. Relation Between Frequence, Capacity, Inductance and Reactance — We know that the re actance of a condenser may be varied by changing the freqeency or the capacity. Also that the reactance of an inductance may be varied by changing the frequency or the inductance. The extent of such variation and the relation between the two reactances with respect to frequency is shown in figure 64. Here we show the variation of capacity reactance and inductive reactance with respect to frequency. Note that Xc decreases with each increase in frequency and that Xl increases with frequency. At one value of frequency, capacity reactance Xc = inductive reactance Xl. Such a state is found to exist for any values of capacity and inductance when they are combined in one circuit. In other words Xl — Xc = 0. Suppose that we take a circuit such as that shown in figure 63A and maintain E at 100 volts and R at 10 ohms. C is changed to 70.4 mfd and L is .1 henry. The frequency of the line supply remains at 80 cycles. Then R = 10 ohms Xc = 37.7 ohms Xl = 37.7 ohms The total impedance Ixl Txt., Z = VR2+ (Xl— Xc)2 = V102+ (37.7 — 37.7)2 In this case the two reactances cancel since FREQUENCY FIQ.G4 they act in opposite directions and are equal and the impedance Z = V102 = 10 As is evident the impedance Z is in this case equal to the resistance R and the current E E 100 I = = = = 10 amperes Z R 10 Hence we find a state in A. C cirrji'ts wfi<-». the formula for the current is c 'dmary Ohn-.*; law as it is applied to D. C. circuits. We found that such a condition existed when the circuit was purely resistance. Here we find such a state even with capacity and inductance in the circuit. The reason for the above is that since the reactances cancel, they have no effect upon the current in the circuit, and the current is limited solely by the resistance. Such is the case at resonance and resonance exists when Xl = Xc. If we continue to solve for the voltages in the circuit we find a very peculiar condition, encountered only in series resonant circuits. Solving for the respective voltage drope we find that IR= 10x10 = 100 volts IXc = 10 x 37.7 = 377 volts IXl = 10 x 37.7 = 377 volts The fact that a voltage stepup is secured in a series circuit at resonance is very evident. With an applied voltage of 100 volts, the voltage across the capacity is 377 volts and across the inductance is likewise 377 volts. If the value of R is reduced to say 1 ohm, the current increases to 100 amperes and the voltage across Xc becomes 3770 volts and the same across the inductance. Thus it is possible to build up dangerously high voltage in series resonant circuits. The fact that such a condition occurs is utilized in practice and at the same time also causes trouble in practice, more will be said about such actions later. Now, referring again to the original ex i 1 A. \ // \ 'V v ■ft is rV-c PIG. 64 ample, the high voltages are to be found across the individual units, but since, as has been explained these voltages act in opposite directions, the total voltage across the two elements is because IXl — IXc = 0 If a voltmeter were connected across the coil only it would indicate 377 volts and the same is true of the capacity, but if a voltmeter were connected across the two outside terminals of L and C the voltage indication would be zero, assuming perfect capacity and perfect inductance. On the other hand the total voltage drop takes place across the resistance R and the power in the circuit is dissipated across this element. Application of Such Series Resotiant Circuits — The series resonant circuit finds extensive application and the paramount item which must be remembered is the impedance relation at resonance. Expressed in the simplest manner, this is as follows : Series resonant circuits possess low impedance at resonance. This becomes evident if we consider the elements L and C and imagine that R is omitted. Then the impedance Z is theoretically zero and the current would rise to an infinite value. Such is, of course, impossible because all electrical units possess some resistance. The power relations and total variation of reactance with frequency shall be discussed in lesson 20. Blend Light and Sound in Console East Pittsburgh — Blending of sound and light through the "color console" has been accomplished by engineers of the Westinghouse Electric and Manufacturing Co. One of the consoles is being installed in Severance Memorial Hall, Cleveland. When the musicians take their places beside their instruments, near them, at an instrument resembling an organ console, will sit the director of color effects. Music played will be accompanied by appropriate hues. The "color console" is provided with 36 individual controls. By means of a remote relay board, it is possible to connect one or more of the 110 circuits to any of the 36 controls. In addition to the individual controls, foot pedals vary the volume, or intensity, of color. Horizontal foot pedals are divided into nine groups so that each pedal controls the effect of four "keys" on the color console. Slanting foot pedals are arranged so that some lights can he dimmed and other lights made brighter at the same time. The intensity of all the lights may be varied by a master control. Color effects made up of combinations of colors are maintained with the same color quality through the entire range of dimming. Using More Power La Crosse, Wis. — The Rivoli here is replacing its 2,000 candlepower arc equipment with new lamphouse unit of 4,000 candlepower with automatic arc strikers. Color pictures and larger screen forced increased power.