3-24
to the right. Since the frequencies you will be measuring are 60 hertz or greater, the meter is incapable of
mechanically responding at this speed. The result is simply a vibration near the zero point; in addition, no
useful reading of voltage or current is obtained. This problem does not exist with the
electrodynamometer-type movement. Current flow through the stationary (fixed) coils sets up a magnetic
field. Current flow through the moving coils sets up an opposing magnetic field. With two magnetic fields
opposing, the pointer deflects to the right. If the current reverses direction, the magnetic fields of both sets
of coils will be reversed. With both fields reversed, the coils still oppose each other, and the pointer still
deflects to the right. Therefore, no rectifying devices are required to enable the electrodynamometer meter
movement to read both ac and dc. Rectifying devices are required for the D’Arsonval-type meter
movement to enable it to be used for measuring ac voltages and currents.
Q-29. What is the primary advantage of the electrodynamometer-type meter over the D’Arsonval-type
meter?
VOLTMETER
When an electrodynamometer is used as a voltmeter, no problems in construction are encountered
because the current required is not more than 0.1 ampere. This amount of current can be handled easily by
the spiral springs. When the electrodynamometer is used as a voltmeter, its internal connections and
construction are as shown in view A of figure 3-17. Fixed coils a and b are wound of fine wire since the
current flow through them will not exceed 0.1 ampere. They are connected directly in series with movable
coil c and the series current-limiting resistor.
Figure 3-17.—Circuit arrangement of electrodynamometer for use as a voltmeter and an ammeter.
