THIRD LECTURE LIGHT AND POWER DISTRIBUTION 1"^ N A DIRECT current distribution system, the motor load is connected to the outside mains at 220 volts, "^"^ and only very small motors, as fan motors, between outside mains and neutral ; since the latter connection, with a large motor, would locally unbalance a system. The effect of a motor on the system depends upon its size and starting current, and with the large mains and feeders, which are gener- ally used, even the starting of large elevator motors has no appreciable effect, and the supply of power to electric elevators represents a very important use of direct current distribution. In alternating current distribution systems, the effect on the voltage regulation, when starting a motor, is far more severe; since alternating current motors in starting usually take a larger current than direct current motors starting with the same torque on the same voltage; and the current of the alternating current motor is lagging, the voltage drop caused by it in the reactance is therefore far greater than would be caused by the same current taken by a non-inductive load, as lamps. Furthermore, alternating current supply mains usually are of far smaller capacity, and therefore more affected in voltage. Large motors are therefore rarely connected to the lighting mains of an alternating current system, but separate transformers and frequently separate feeders are used for the motors, and very large motors commonly built for the primary distribution voltage of 2200, are connected to these mains. For use in an alternating current distribution system, the synchronous motor hardly comes into consideration, since the synchronous type is suitable mainly for large powers, where it is operated on a separate circuit. 38 GENERAL LECTURES The alternating current motor mostly used in small and moderate sizes — such as come into consideration for power distribution from a general supply system — is the induction motor. The single-phase induction motor, however, is so inferior to the polyphase induction motor, that single-phase motors are used only in small sizes; for medium and larger sizes the three-phase or two-phase motor is preferred. This however, introduces a complication in the distribution system, and the three-wire single-phase system therefore is less suited for motor supply, but additional conductors have to be added to give a polyphase power supply to the motor. As the result thereof, motors are not used in alternating current systems to the same extent as in direct current systems. In the alternat- ing current system, however, the motor load is, if anything, more important than in the direct current system, to increase the load factor of the system ; since the efficiency of the alter- nating current system decreases with decrease of load, while that of a direct current system increases. Compared with the direct current motor, the polyphase induction motor has the disadvantage of being less flexible: its speed cannot be varied economically, as that of a direct current motor by varying the field excitation. Speed variation of the induction motor produced by a rheostat in the armature or secondary circuit, in the so-called form "M" motor is accomplished by wasting power : the power input of an induc- tion motor always corresponds to full speed; if the speed is reduced by running on the rheostat, the difference in power between that which the motor actually gives, and that which it would give, with the same torque, at full speed, is consumed in the rheostat. Where therefore different motor speeds are required, pro- visions are made in the induction motor to change the number LIGHT AND POWER DISTRIBUTION 39 of poles; thereby a number of different definite speeds are available, at which the motor operates economically as "multi- speed" motor. The starting torque of the polyphase induction motor with starting rheostat in the armature (Form L motor) is the same as the running torque at the same current input, just as in the case of the direct current shunt motor with constant field excitation. In the squirrel cage induction motor, how- ever, (Form K motor) the starting torque is far less than the running torque at the same current input; or inversely, to produce the same starting torque, a greater starting current is required. In starting torque or current, the squirrel cage induction motor has the disadvantage against the direct current motor. It has, however, an enormous advantage over it in its greater simplicity and reliability, due to the absence of commutator and brushes, and the use of a squirrel cage armature. The advantage of simplicity and reliability of the squir- rel cage induction motor sufficiently compensates for the disadvantage of the large starting current, to make the motor most commonly used. In an alternating current distribution system, however, great care has to be taken to avoid the use of such larger motors at places where their heavy lagging starting currents may affect the voltage regulation; in such places, separate transformers and even separate primary feeders are desirable. The single-phase induction motor is not desirable in larger sizes in a distribution system, since its starting current is still larger; in small sizes, however, it is extensively used, since it requires no special conductors, but can be operated from a single-phase lighting main. 40 GENERAL LECTURES The alternating current commutator motor is a single- phase motor which has all the advantages of the different types of direct current motors; it can be built as constant speed motor of the shunt type, or as motor with the charac- teristics of the direct current series motor : very high starting torque with moderate starting current. It has, however, also the disadvantages of the direct current motor: commutator and brushes; and so requires more attention than the squirrel cage induction motor. Alternating current generators now are almost always used as polyphase machines, three-phase or two-phase, and transmission lines are always three-phase, though in transform- ing down, the system can be changed to two-phase. The power supply in an alternating current system therefore is practically always polyphase ; and since a motor load, which is very desir- able for economical operation, also requires polyphase currents, alternating current distribution systems always start from poly- phase power. The problem of alternating current distribution therefore is to supply, from a polyphase generating system, single-phase current to the incandescent lamps, and polyphase current to the induction motors. PRIMARY DISTRIBUTION SYSTEMS I. Two conductors of the three-phase generating or transmission system are used to supply a 2200 single-phase system for lighting by step-down transformers and three-wire secondary mains ; the third conductor is carried to those places where motors are used and three-phase motors are operated by separate step-down transformers. In the lighting feeders, the voltage is then controlled by feeder regulators, or, in a smaller system, the generator excitation is varied so as to main- LIGHT AND POWER DISTRIBUTION 41 tain the proper voltage on the lighting phase. At load, the three-phase triangle then more or less unbalances, but induction motors are very little sensitive to unbalancing of the voltage, and by their regulation — ^by taking more current from the phase of higher, less from the phase of lower voltage — tend to restore the balance. For smaller motors, frequently two transformers are used, arranged in "open delta" connection. 2. Two-phase generators are used, or in the step-down transformers of a three-phase transmission line, the voltage is changed from three-phase to two-phase; the liehting feeders are distributed between the two phases and controlled by poten- tial regulators so that the distribution for lighting is single- phase, by three-wire secondary mains. For motors, both phases are brought together, and the voltage stepped down for use on two-phase motors. This requires four, or at least three, prim- ary wires to motor loads. 3. From three-phase generators or transmission lines, three separate single-phase systems are operated for lighting; that is the lighting feeders are distributed between the three phases, and all three primary wires are brought to the step- down transformers for motors. This arrangement, by dis- tributing the lighting feeders between the three phases, would require more care in exactly balancing the load between all three phases than two, but a much greater unbalancing can be allowed without affecting the voltage. 4. Four-wire three-phase primary distribution with grounded neutral, and 2200 volts between outside conductors and neutral. The lighting feeders are distributed between (the three circuits between outside conductors and neutral, and motors supplied by three of such transformers. This system is becoming of increasing importance, since it allows economical distribution to distances beyond those which can be reached 42 GENERAL LECTURES with 2200 volts : with 2600 volts on the transformers — as the upper limit of primary distribution voltage — the voltage be- tween outside conductors is 4500, and the copper economy of the system therefore is that of a 45CX) volt three-phase system. 5. Polyphase primary and polyphase secondary distri- bution, with the motor connected to the same secondary mains as the lights. SYSTEMS OF LOW TENSION DISTRIBUTION FOR LIGHTING AND POWER. I. Two- Wire Direct Current or Singi.e-Phase ho Volts. Fig 6. This can can be used only for very short distances, since its copper economy is very low, that is, the amount of conduc- tor material is very high for a given power. Cu. I. //Orv O 6 I i Fife. 6. Two-Wire System. LIGHT AND POWER DISTRIBUTION 43 2. Three- WiRS Direct Current or Singi^e-Phase iio- 220 Volts. Fig. 7. Neutral one-half size of the two outside conductors. The two outside conductors require one-quarter the copper of the two wires of a no volt system; since at twice the voltage and one-half the current, four times the resistance or one-quarter dfO/r JUO^ M>^ "1 f I Fig. 7. Three-Wire System. the copper is sufficient for the same loss (the amount of con- ductor material varying with the square of the voltage). Adding then one-quarter for the neutral of half-size, gives 4 X ; = jg or altogether ^ + ^ = jg of the conductor material required by the two-wire no volt system. That is, the copper economy is ^. This is the most commonly used system, since it is very economical, and requires only three conductors. It is, however, a single-phase system, and therefore not suitable for operating polyphase in- duction motors. Co A 44 GENERAL LECTURES 3. Four-Wire Quarter-Phase (Two-Phase). Fig. 8. Two separate two-wire single-phase circuits, therefore no saving in copper over two-wire systems. That is, the cop- per economy is: Cu. i. Fl§. 8. Four- Wire Two-Phase System. 4. Three-Wire Quarter-Phase. Fig. 9. Common return of both phases, therefore saves one wire or one-quarter of the copper; hence has the copper economy: Cu. 4 In this case however, the middle or common return wire carries V2 , or 1.41 times as much current as the other two wires, and when making all three wires of the same size, the copper is not used most economically. A small further saving is therefore made by increasing the middle wire and decreasing the 9 ^^^-o Fig. 9. Three- Wire Two-Phase System. outside wires so .that the middle wire has 1.41 times the section of each outside wire. This improves the copper economy to: Cu. 0.73 LIGHT AND POWER DISTRIBUTION 45 5. Three- Wire Three-Phase. Fig. 10. A three-phase system is best considered as a combination of three single-phase systems, of the voltage from line to neutral, and with zero return (because the three currents neutralize each other in the neutral). Compared thereto the two-wire single-phase system can be considered as a combination of two single-phase circuits from wire to neutral with zero return. Fl^. 10. Three- Wire Three-Phase System. In a 1 10 volt single-phase system the voltage from line to neutral equals -j-, in a three-phase system equals — 7^. The ratio of voltages is ~2r~TT-z, or ^ ^ no ~ ~* 3 and the square of the ratio of voltages equals ^; and as the copper economy varies with the square of the voltage, the copper economy for the three-wire three-phase system is : Cu. I 6. Five- Wire Quarter-Phase. Fig. 11. Neglecting the neutral conductor, the five-wire quarter- phase system can be considered as four single-phase circuits without return, from line to neutral, of voltage no. Com- pared with the two-wire circuit, which consists of two single- phase circuits without return, of -^ volts. No. 6 therefore has twice the voltage of No. i ; therefore one-quarter the copper. GENERAL LECTURES Making the neutral half the size of the main conductor adds one-half of the copper of one conductor, or ^ of i = ^ so giving a total of | + 32* that is, a copper economy of : Cu. = 32' 32 ce / //o A a //o \ p tr f r JIJIO^ //Car Fife. 11. Five- Wire Two-Phase System. 7. Four- Wire Three-Phase. Fig. 12. Lamps connected between line and neutral. Neglecting the neutral, the system consists of three single- phase circuits without return, of 1 10 volts, and compared with T ■T T T I //O/K _i_ i_l Fife. 12. Four- Wire Three-Phase System. the two-wire circuit of -^ between wire and neutral without return, it therefore requires one-quarter the copper. Making the neutral one-half size adds g of the copper, or glof 4 = 24 » ^^'^ so gives a total copper economy of 24] • 4 ~" 24* LIGHT AND POWER DISTRIBUTION 47 8. Three-Wire Single-Phase Lighting with Three- Phase Power. Fig. 13. Lighting: Half size neutral, same as No. 2, therefore copper economy : Cu. = ^ Power: Three-wire three-phase 220 volts; that is, the same as No. 5, but twice the voltage, thus one-quarter the 13 3 3 copper of No. 5, or j of ;j = ^: Cu. = ^ ^