Electrical Engineering ⇒ Topic : Moving Coil Instruments

A schematic diagram of this type of instrument is shown in Fig. (a). In this instrument, the deflecting torque is proportional to the current or voltage to be measured, and therefore it is useful for measurement of dc quantities only. It cannot be used for the measurement of ac quantities since the average torque over a cycle will be zero.This type of an instrument consists of a rectangular coil of N turns wound on a soft iron core placed between the pole shoes of a U-shaped permanent magnet. The soft iron former is used not only as a former for the coil but also as a damping shell when the coil is deflected in the magnetic field. When the current passes through the coil its vertical sides experience an electromagnetic force resulting in a deflecting torque on the coil. Generally a spring control is used to provide the control torque and get a uniformly graduated scale. The control springs are used as leads to carry the current through the coil as shown in Fig(b). Damping torque is provided by the eddy currents induced in the cylinder as shown in Fig(c).

                                                                  FIGURE  Moving-coil instrument

MOVING COIL INSTRUMENTS

There are two types of moving coil instruments:

• Permanent magnet type
• Dynamometer type