Motion Picture News (Mar-Apr 1923)

1710 Motion Picture News National Anti-Misframe League Forum implified Electricity for Projectionists Direct Current Circuits IN electric circuit i whereby energy, in the form of electricity, is carried from the }X)int where it is generated to another point where it is changed to 'a different form of energy. There are three essential parts to every electric circuit. These are: The Generator Circuit, which consists of the generator, together with its resistance, switches and the necessary connecting wires: the Feeder Circuit, consisting of the wires which carry the electricity from the point where generated to the point where it is used; the Local Circuit. which consists of the apparatus required to change the electricity into directly useful energy, the auxiliary apparatus, such as resistances (where motors are used) and the local connecting wires. Fig. 21 shows an electric circuit divided into these three fundamental parts. It must be understood that the circuit ft a in Circuit Feeder Circuit Figure 21 shown is a very simple one, devoid of much of the special auxiliary apparatus ordinarily used in such circuits. In the figure, G is the generator; S and Si are the switches controlling each end of the main circuit; M is a motor being supplied current by the generator, and R is its controlling resistance. Polarity of the Generator The direction in which a current flows through a circuit depends upon the direction in which the armature of the generator turns. If the armature revolves in a certain direction current will flow one way; if the direction of the armature is reversed, the current will flow the opposite way. This gives rise to what is known as polarity of generators (direct current only). For a certain given direction of revolution, one brush of the generator is called positive (marked +) and the other is called minus (marked — ). If the direction of the armature is now re versed so that the positive changes to negative and the negative changes to positive. Outside of the generator the current always flows from -4 (positive) to — (negative). The circuit wires connected to the brushes of the generator take the same polarity as the brushes. Thus, the wire connected to the positive brush would be the positive of the circuit. In order to determine the polarity of the wires of a direct current circuit put both wires into a glass of water. Do not touch them together. Little bubbles will be seen to come off of one wire. This is the negative. The polarity of the wires should always be marked on the switch or switchboard and Lesson III — Part II the means should then never be changed. It is very important, in the case of the direct current projection arc, that the positive wire be connected to the top carbon. Resistance of Feeder Circuit An electric circuit can be looked upon as being composed of a number of resistances connected up in different ways. Besides the electrical machinery connected in the circuit the resistance of the connecting wires also plays an important part. This is particularly true in the case of the wires connecting the Generator and Local Circuits ; i. e., the 1 eeder Circuit. Since these wires are sometimes very long the voltage drop due to them must be considered when planning a circuit. In last week's article we saw how to calculate what might be called the absolute resistance of a conductor. We will now apply this newly acquired knowledge to calculating the resistance of the Feeder Circuit shown in Fig. 22. In this illustration, G is the generator with the polarity of the brushes marked as shown ; Eg is the voltage at the brushes of th-e generator; S and Si are switches controlling the circuit; Et is the terminal voltage of the local circuit; i. e., the voltage across the local switch; A is a projection arc drawing 50 amperes from the generator, and R is its controlling resistance. Now the voltage Et will not be as great as Eg of the generator because of the drop in the feed wires. It is our problem to find out what this voltage drop is so that E, may be known. The first step will be to find what size of wire is necessary to carry the 50 amperes flowing through the line. Table IV (page 1712) shows that for rubber covered insulation, a No. 6 wire (extreme left hand column) is required to carry 50 amperes. The area of the wire in circular mils is shown in the third column. It is 26,250. Our problem is now to find the resistance of two copper wires, each 200 feet long and having an area of 26,250 circular mils. The resistance formula will appear as follows (assuming a temperature of 20° C.) : 2XKXL rC = a R = 2 X 10.37 X 200 26,250 R = 0.16 ohms The voltage drop through these wires, when each is carrying 50 amperes, will be E = I R E = 50 X 0-16 E = 8.0 volts ff = Z ohms fj ■ 4 ohms f£J20 WW — VWh f% '6 ohms Figure 23 so that E, will be equal to EK — 8 = 110 — 8 = 102 volts. The power loss in the wires will be P = I2 R p = 50 X 50 X 0.16 = 2,500 X 0.16 = 400 watts (0.4 kw.) Resistance of Local Circuit Wires are also required in the Local Circuit to connect the various pieces of apparatus together electrically. The resistance of these wires, however, can usually be neglected, as they are seldom long enough to cause a serious drop in voltage. We are more concerned, in the local circuit, with the resistances of the various pieces of apparatus forming it. In Fig. 23 is shown such a circuit together with the generator supplying it with power. This latter item can be forgotten for the moment and only that portion of the diagram to the right of the switch S need be considered. It is also assumed that the generator is so close at hand that the resistance of the feeder circuit can be neglected. The thing we wish to find out is, " What effect will the three resistances Fi, ra, and r* have upon the voltage of the circuit (Et = 120 volts) ? In other words, what current will flow as a result of these resistances being connected as shown ? Thanks to the efforts of early scientists a law is known which governs this relation and tells us what we wish to know. This law, known as Kirchoff s Law, says, " The algebraic sum of all the voltages acting in a closed circuit is equal to zero." By algebraic sum, as used above, is meant the following. Every voltage acting in a closed circuit has a sign placed before it which tells the direction in which the voltage is acting. Thus all the voltages acting in one direction have a plus (-)-) sign before them, while all the voltages acting in the opposite direction have a minus ( — ) sign before them. It is somewhat similar to polarity of wires in a direct current circuit. When no sign is shown it is understood that the voltage has a plus sign before it. Applying this law to the circuit shown in Fig. 23 we see that E,, the terminal voltage, acts in such a direction as to force current through the resistances. Suppose we call this voltage plus. We also know that each resistance trys to prevent current from flowing and causes a voltage drop, e = ir. Since these (Continued on page 1712) Feeder Circuit Zoo feet lono — 1 1 ^ 1 1 — r-" So &mp. Figure 22