[Electric engineering.] . Meter Field po\_flfio r3 r^ra T4 nh. Fig. 45. 92 ELECTRIC TRANSMISSION. 14 106. Fig. 45 shows how to connect a Thomson record-ing wattmeter with its Y resistance so as to measure thepower on a balanced three-phase circuit. The recordingmeters are so calibrated that their reading multipliedby the meter constant, if there is one, gives the total power supplied. Fig. 45shows the connectionswhen the meter is usedon circuits where thepressure is not over500 volts. When thepressure is higher thanthis, small transform-ers t, t, Fig. 46, areused to step down thevoltage for th
[Electric engineering.] . Meter Field po\_flfio r3 r^ra T4 nh. Fig. 45. 92 ELECTRIC TRANSMISSION. 14 106. Fig. 45 shows how to connect a Thomson record-ing wattmeter with its Y resistance so as to measure thepower on a balanced three-phase circuit. The recordingmeters are so calibrated that their reading multipliedby the meter constant, if there is one, gives the total power supplied. Fig. 45shows the connectionswhen the meter is usedon circuits where thepressure is not over500 volts. When thepressure is higher thanthis, small transform-ers t, t, Fig. 46, areused to step down thevoltage for the pres-sure coil of the record-ing wattmeter, and theconnections are asshown in the figure re-ferred to. 107. If the cur-rent in each line of abalanced three-phasesystem is C, the pres-sure between theFlG- 46, lines E, and the power factor of the load/, then the watts delivered will be. W= X E X Cxf. (19.) Example.—A three-phase motor takes a current of 40 amperes froma 500-volt line. How many horsepower are supplied if the power factorof the motor is .80 ?Solution.—We have W= X ExCxf= X500 X40 X .80= 27,712. Hence, H. P. = **ft* = Ans. § 14 ELECTRIC TRANSMISSION. 93 108. The power factor of induction motors variesgreatly with the load and also with the size of the power factor of a good motor of fair size running at fullload will usually lie between .85 and .90. When running at| to f load, the power factor may drop to .75 or .80. Forthis reason, the student is cautioned against making powercalculations in connection with induction motors or any otherpiece of alternating-current apparatus by multiplying thecurrent and E. M. F. together. Such calculations may befar from correct, unless the power factor is known andtaken into account as shown above. Take, for example, thecommon case where a transformer primary is connected, say,to
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