AC METERING CIRCUITS

Chapter 12 AC METERING CIRCUITS

Lessons In Electric Circuits Volume II – AC Book
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Summary of Contents

Lessons In Electric Circuits Volume II – AC Book

  • Chapter 12AC METERING CIRCUITSContents 368,12.1 368,AC 368,voltmeters 368,and 368,ammeters . . . . . . . . . . . . . . . . . . . . . . . . 359 375,12.2 375,Frequency 375,and 375,phase 375,measurement . . . . . . . . . . . . . . . . . . . . 366 383,12.3 383,Power 383,measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374 385,12.4 385,Power 385,quality 385,measurement . . . . . . . . . . . . . . . . . . . . . . . . . 376 387,12.5 387,AC 387,bridge 387,circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378 397,12.6 397,AC 397,instrumentation 397,transducers . . . . . . . . . . . . . . . . . . . . . . 388 406,12.7 406,Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397 406,Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39712.1AC voltmeters and ammetersAC electromechanical meter movements come in two basic arrangements: those based on DC move-ment designs, and those engineered specifically for AC use. Permanent-magnet moving coil (PMMC)meter movements will not work correctly if directly connected to alternating current, because thedirection of needle movement will change with each half-cycle of the AC. (Figure 369,12.1) Permanent-magnet meter movements, like permanent-magnet motors, are devices whose motion depends on thepolarity of the applied voltage (or, you can think of it in terms of the direction of the current).In order to use a DC-style meter movement such as the D’Arsonval design, the alternating currentmust be rectified into DC. This is most easily accomplished through the use of devices called diodes.We saw diodes used in an example circuit demonstrating the creation of harmonic frequencies froma distorted (or rectified) sine wave. Without going into elaborate detail over how and why diodeswork as they do, just remember that they each act like a one-way valve for electrons to flow: actingas a conductor for one polarity and an insulator for another. Oddly enough, the arrowhead in eachdiode symbol points against the permitted direction of electron flow rather than with it as one mightexpect. Arranged in a bridge, four diodes will serve to steer AC through the meter movement in aconstant direction throughout all portions of the AC cycle: (Figure 369,12.2)359