3-23
Q-27. What components in an electrodynamometer-type meter movement produce the magnetic field?
As shown in figure 3-16, the fixed coils are connected in series and positioned coaxially (in line)
with a space between them. The two movable coils are also positioned coaxially and are connected in
series. The two pairs of coils (fixed pair and movable pair) are also connected in series with each other.
The movable coil is pivot-mounted between the fixed coils. The main shaft on which the movable coils
are mounted is restrained by spiral springs that restore the pointer to zero when no current is flowing
through the coil. These springs also act as conductors for delivering current to the movable coils. Since
these conducting springs are very small, the meter cannot carry a high value of current.
Figure 3-16.—Internal construction of an electrodynamometer.
Q-28. What is the limiting factor as to the amount of current an electrodynamometer meter movement
can handle?
METER ACCURACY
The meter is mechanically damped by means of aluminum vanes that move in enclosed air
chambers. Although very accurate, electrodynamometer-type meters do not have the sensitivity of the
D’Arsonval-type meter movement. For this reason, you will not find them used outside of the laboratory
environment to a large extent.
METER MOVEMENT
The primary advantage of the electrodynamometer-type meter movement is that it can be used to
measure alternating as well as direct current. If you apply alternating current to the standard
galvanometer-type meter, it will not produce a usable reading. Instead, the meter will vibrate at or near
the zero reading. On one-half cycle of the ac, the meter is deflected to the left and on the other half cycle
