Figure 1-15.Electrodynamic meter movement.
Current flow in either direction through the three coils causes a magnetic field to exist between the
field coils. The current in the moving coil causes it to act as a magnet and exert a turning force against a
spring. If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same
time, and the turning force continues in the original direction. Since reversing the current direction does
not reverse the turning force, this type of meter can be used to measure both ac and dc if the scale is
changed. While some voltmeters and ammeters use the electrodynamic principle of operation, the most
important application is in the wattmeter. The wattmeter, along with the voltmeter and the ammeter, will
be discussed later in this topic.
MOVING-VANE METER MOVEMENTS
The moving-vane meter movement (sometimes called the moving-iron movement) is the most
commonly used movement for ac meters. The moving-vane meter operates on the principle of magnetic
repulsion between like poles (fig.1-16). The current to be measured flows through a coil, producing a
magnetic field which is proportional to the strength of the current. Suspended in this field are two iron
vanes. One is in a fixed position, the other, attached to the meter pointer, is movable. The magnetic field
magnetizes these iron vanes with the same polarity regardless of the direction of current flow in the coil.
Since like poles repel, the movable vane pulls away from the fixed vane, moving the meter pointer. This
motion exerts a turning force against the spring. The distance the vane will move against the force of the
spring depends on the strength of the magnetic field, which in turn depends on the coil current.