4 The Y connection is shown in Fig. 186. The three circuits are joined at the point o, known as the neutral point, and the three wires carrying the current are connected at the points a. b. and c, respectively. If the three circuits ao, bo, and co are composed of lights they must be equaily loaded or the lights will fluctuate. lights will remain steady. α FIG. 187. с If the three circuits are perfectly balanced, the In this form of connection each wire may be considered as the return wire for the other two. If the three circuits are unbalanced, a return wire may be run from the neutral point o to the neutral point of the armature winding on the generator. The system will then be four-wire, and will work properly with unbalanced circuits. The A connection is shown in Fig. 187. Each of the three circuits ab, ac bc, receives the current due to a separate coil in the armature winding. This form of connection will work properly even if the circuits are unbalanced; and if the circuit contains lamps they will not fluctuate if the circuit be balanced or unbalanced. Measurement of Power in Polyphase Circuits.-1. Twophase Circuits.-The power of a two-phase current distributed by four wires may be measured by two wattmeters introduced into the circuit as shown in Fig. 184. The sum of the readings of the two instruments is the total power. If but one wattmeter is available, it should be introduced first in one circuit. and then in the other. If the current or e.m.f. does not vary during the operation, the result will be correct. If the circuits are perfectly balanced, twice the reading of one wattmeter will be the total power. The power of a two-phase current distributed by three wires may be measured by two wattmeters as shown in Fig. 183. The sum of the two readings is the total power. If but one wattmeter is available the coarsewire coil should be connected in series with the wire ef and one extremity of the pressure-coil should be connected to some point on ef. The other end should be connected, first, to the wire a and then to the wire d a reading being taken in each position of the wire. The sum of the readings gives the power in the circuits. 2. Three-phase Currents.-' -The power of a three-phase current may be measured by three wattmeters, connected as in Fig. 188 if the system is the Y connection and as in Fig. 189 if the system is the ▲ connection. The sum of the wattmeter readings gives the power in the system. If the circuits are perfectly balanced, three times the reading of one wattmeter is the total power. The power in a ▲ connected system may be measured by two wattmeters, as shown in Fig. 190. If the power factor of the system is greater than 0.50, the arithmetical sum of the readings is the power in the circuit. If the power factor is less than 0.50, the arithmetical difference of the readings is the power. Whether the power factor is greater or less than 0.50 may be discovered by interchanging the wattmeters without disturbing the relative connection of their coarse- and fine-wire coils. If the deflections of W2 W www the needles are reversed, the difference of the readings is the power. If the needles are deflected in the same direction as at first, the sum of the readings is the power. Alternating-current Generators.-These differ little from directcurrent generators in most respects. Any direct-current generator, if provided with collector rings instead of a commutator, could be used as a single-phase alternator. The frequency would in most cases, however, be too low to be of any practical use. The fields of alternators are always separately excited; the machines sometimes compounded by shunting some of their own current around the fields by special means. In some machines the armature is stationary and the field revolves. FIG. 190. are TRANSFORMERS, CONVERTERS, ETC. Transformers.-A transformer consists essentially of two coils of wire, one coarse and one fine, wound upon an iron core. The function of a transformer is to convert electrical energy from one potential to another. If the transformer causes a change from high to low voltage, it is known as a "step-down" transformer; if from low to high voltage, it is known as a 'step-up" "transformer. The relation of the primary and secondary voltages depends on the number of turns in the two coils. 100 Volt 100 Turns eeeeeee oooooooo 86 Turns -100 Volt 100 Turns eeeeee 00090000 100 Turns 50 V. 50 Volts-50 Volts FIG. 191. Transformers may also be used to change current of one phase to current of another phase. The windings and the arrangement of the transformers must be adapted to each particular case. In Fig. 191 an arrangement is shown whereby two-phase current may be converted into three-phase. Two transformers are required, one having its primary and secondary coils in the relation of 100 to 100, and the other having its primary and secondary in the relation of 100 to 86. The secondary of the 100-to-100 transformer is tapped at its middle point and joined to one terminal of the other secondary. Between any pair of the three remaining terminals of the secondaries there will exist a difference of potential of 50. There are two sources of loss in the transformer, viz., the copper loss and the iron loss. The copper loss is proportional to the square of the current. and is the I2R loss due to heat. If I, R1, be the current and resistance respectively of the primary, and I2, R2, the current and resistance respectively of the secondary, then the total copper loss is We = IR1 + I2R, and the perIR IR2 centage of copper loss is where Wp is the energy delivered to Wp the primary. The iron loss is constant at all loads, and is due to hysteresis and eddy currents. Р Transformers are sometimes cooled by means of forced air or water currents or by immersing them in oil, which tends to equalize the temperature in all parts of the transformer. Efficiency of Transformers.-The efficiency of a transformer is the ratio of the output in watts at the secondary terminals to the input at the primary terminals. At full load the output is equal to the input less the iron and copper losses. The full-load efficiency of transformers is usually very high, being from 96 per cent. to 98 per cent. Owing to the copper loss varying as the square of the load, and the iron loss being constant from no load to full load, the efficiency of a transformer varies considerably at different loads. Transformers on lighting circuits usually operate at full load but a It very small part of the day, though they use some current all the time to Copper loss, full load, K.W. hours.15X3=.45. = Input, K.W. hours=40+3.6+.45+.075=44.125. For the opera- The transformers heretofore discussed are constant-potential transformers and operate at a constant voltage with a variable current. tion of lamps in series a constant-current transformer is required are a number of types of this transformer. That manufactured by the General Electric Co operates by causing the primary and secondary to approach or to separate on any change in the current. Converters, etc.—In addition to static transformers, various machines are used for the purpose of changing the voltage of direct currents or the voltage, phase. or frequency of alternating currents, and also for changing alternating currents to direct or vice versa. These machines are all rotary and are known as rotary converters, motor-dynamos. and dynamotors. A rotary converter consists of a field excited by the machine itself, and an armature which is provided with both collector rings and a commutator. It receives direct current and changes it to alternating, working as a direct-current motor, or it changes alternating to direct current, working as a synchronous motor. A motor-dynamo consists of a motor and a dynamo mounted on the same base and coupled together by a shaft. A dynamotor has one field and two armature windings on the same core. One winding performs the functions of a motor armature, and the other those of a dynamo armature. A booster is a machine inserted in series in a direct current to change its voltage. It may be driven either by an electric motor or otherwise. ALTERNATING-CURRENT MOTORS. Synchronous Motors. Any alternator may be used as a motor, provided it be brought into synchronism with the generator supplying the current to it. The operation of the alternating-current motor and generator is similar to the operation of two generators in parallel. It is necessary to supply direct current to the field. The field circuit is left open until the machine is in phase with the generator If the motor has the same number of poles as the generator, it will run at the same speed; if a different number the speed will be that of the generator multiplied by the ratio of the number of poles of the motor to that of the generator. Single-phase, synchronous motors are not self-starting. Multiphase motors may be made self-starting but it is better to bring the machines to speed by independent means before supplying the current. The machines may be started by a small induction motor, the load on the synchronous motor being thrown off, or the field may be excited by a small direct-current generator belted to the motor, and this generator may be used as a motor to start the machine, current to run it being taken from a storage battery. If the field of a synchronous moter be properly regulated to the load, the motor will exercise no inductive effect on the line, and the power factor will be 1. If the load varies the current in the motor will either lead or lag behind the e.m.f. and will vary the power factor. If the motor be overloaded so that there is a diminution of speed the motor will fall out of phase with the generator and stop Synchronous motors are often put on the same circuit with induction motors. The synchronous motor in this case may, by increasing the field excitation, be made to cause the current to lead, while the induction motor will cause it to lag. The two effects will thus tend to balance each other and cause the power factor of the circuit to approach 1. Synchronous motors are best used for large units of power at high voltages, where the load is constant and the speed invariable. They are unsatis factory where the required speed is variable and the load changes. Two great disadvantages of the synchronous motor are its inability to start under load. and the necessity of direct-current excitation. Induction Motors. The distinguishing feature of an induction motor is the rotating magnetic field. It is thus explained In Fig 192 let ab, cd be two pairs of poles of a motor, a and b being wound from one leg or pair of wires of a two-phase alternating circuit, and c and d from the other leg, the two phases being 90° apart. At the instant when a and b are receiving maximum current, so as to make a a north pole and b a south pole, c and d are demagnetized, and a needle placed between the poles would stand as shown in the cut. Dur. ing the progress of the cycle of the current the magnetic flux at a decreases and that at c increases causing the FIG. 192. point of resultant maximum intensity to shift, and the needle to move clockwise toward c. A complete rotation of the resultant point is performed during each cycle of the current. An armature placed within the ring is caused to rotate simply by the shifting of the magnetic field without the use of a collector ring. The words "rotating magnetic field" refer to an area of magnetic intensity and must be distinguished from the words "revolving field" which refer to the portion of the machine carrying the pole-pieces. The field or "primary" of an induction motor is that portion of the machine to which current is supplied from the outside circuit. The armature or "secondary is that portion of the machine in which currents are induced by the rotating magnetic field. Either the primary or the secondary may revolve. In the more modern machines the secondary revolves. The revolving part is called the rotor." the stationary part the stator.' The rotor may be either of the ring or the drum type, the drum type being more common. A common type of armature is the " squirrelcage. It consists of a number of copper bars placed on the armature-core and insulated from it. A copper ring at each end connects the bars. The field windings are usually so arranged that more than one pair of poles are produced. This is necessary in order to bring the speed down to a practical limit. If but one pair of poles were produced, with a frequency of 60, the revolutions per minute would be 3600. The revolving part of an induction motor does not rotate as fast as the field, except at no load When loaded, a slip is necessary, in order that the lines of force may cut the conductors in the rotor and induce currents therein. The current required for starting an induction motor of the squirrel-cage type under full load is 7 or 8 times as great as the current for running at full-load. A type of induction motor known as Form L, built by the General Electric Co., will start with the full load current, provided the starting torque is not greater than the torque when running at full load. Induction motors should be run as near their normal primary e.m.f. as possible, as the output and torque are directly proportional to the square of the primary pressure A machine which will carry an overload of 50 per cent at normal e.m.f. will hardly carry its full load at 80 per cent of the normal e.m.f. An induction motor exercises its greatest torque when standing still, and its least when running in synchronism with the rotating field. If it be overloaded it will slow down until the induced currents in the armature are sufficient to carry the load. ALTERNATING-CURRENT CIRCUITS. Calculation of Alternating-current Circuits.-The following formule and tables are issued by the General Electric Co. They afford a convenient method of caiculating the sizes of conductors for, and determining the losses in. alternating-current circuits. They apply to circuits in which the conductors are spaced 18 inches apart, but a slight increase or decrease in this distance does not alter the figures appreciably. If the conductors are less than 18 inches apart, the loss of voltage is decreased and vice versa. No. A. W. Gauge. Let W total power delivered in watts; = D=distance of transmission (one way) in feet; E-voltage between main conductors at consumer's end of circuit; Ta variable depending on the system and nature of the load; for M-a variable, depending on the size of wire and the frequency; for continuous current= = 1; A a factor; for continuous current=6.04. Area of conductor, circular mils: DXWXK, PXE2 WXT РХЕХМ 100 D2 X W XK XA The following tables give values for the various constants: 0000 000 00 012345678 211,600 133,079 1.23 1.33 1.34 1.62 1.99 2.09 2 35 3.24 3.49 167 805 1.18 1.24 1.24 1.49 1.77 1.95 2.08 2.77 2 94 1.14 1.16 1.16 1.34 1.60 1.66 1.86 2.40 2 57 105,592 1.10 1.10 1.09 1.31 1.46 1.49 1.71 2.13 2 25 83,694 1.07 1.05 1.03 1.24 1.34 1.36 1.56 1.88 1.97 66,373 1.05 1.02 1.00 1.18 1.25 1.26 1.45 1.70 1.77 52,633 1.03 1.00 1.00 1.14 1.18 1.17 1.35 1.53 1.57 41,742 1.02 1.00 1.00 1.11 1.11 1.10 1.27 1.40 1.43 33,102 1.00 1.00 1.00 1.08 1.06 1.04 1.21 1.30 1.31 26,250 1.00 1.00 1.00 1.05 1.02 1.00 1.16 1.21 1.21 20.816 1.00 1.00 1.00 1.03 1.00 1.00 1.12 1.14 1.13 16,509 1.00 1.00 1.00 1.02 1.00 1.00 1.09 1.09 1.07 * P should be expressed as a whole number, not as a decimal; thus a 5 per cent loss should be written 5 and not .05. |