Rectifi er Moving Coil Meter

Chapter 6.11 Rectifi er Moving Coil Meter

Fundamental Electrical and Electronic Principles Third Edition Book
Pages 304
Views 4,515
Downloads : 23 times
PDF Size : 3.0 MiB

Summary of Contents

Fundamental Electrical and Electronic Principles Third Edition Book

  • 212Fundamental Electrical and Electronic Principles 6.11 Rectifi er Moving Coil Meter The circuit arrangement for such a meter is illustrated in Fig. 6.16 . The symbol, with the letter M in it, represents the moving coil movement. The current through the coil will therefore be a series of half-sinewave pulses, as in Fig. 6.15 .Ma.c. to bemeasured Fig. 6.16 Due to the inertia of the meter movement, the coil will respond to the average value of this current. Thus, the pointer would indicate the average value of the a.c. waveform being measured. However, the normal requirement is for the meter to indicate the r.m.s. value. Thus, some form of ‘ correction factor ’ is required. The majority of a.c. quantities to be measured are sinewaves. For this reason, the meter is calibrated to indicate the r.m.s. values of sinewaves. Now, the ratio between r.m.s. and average values is the form factor. For the sinewave, the form factor has a value of 1.11. The values chosen for shunts and multipliers, used on the a.c. ranges, are therefore modifi ed by this factor. Thus, although the pointer position corresponds to the average value, the scale indication will be the r.m.s. value. Worked Example 6.8 Q A moving coil meter has a fi gure of merit of 10 k / V. The coil has a resistance of 50 . Calculate the value of multiplier required for (a) the 10 V d.c. range, (b) the 10 V a.c. range. A R c 50 ; fi gure of merit 10 k /V; V 10 V (a) I fsd 1/10 000 100 μ A total meter resistance, R VfsdIohm1100 4 so, k ; and since , then:k RRRRRmcm110000 0005099 95.AAns