Moving coil instruments are of two types :
- Permanent magnet type ( for dc only) : Direct current indicating instruments.
- Dynamometer type (for ac and dc both ).
1. PMMC instruments
This type of instruments is based on the principle of permanent magnet moving coil mechanism i.e when a current carrying conductor is placed in a magnetic field , mechanical forces acts on the conductor. The coil placed in magnetic field and carrying current is attached to the moving system .With the movement of the coil the pointer moves over the scale .
It consists of a powerful magnet with soft iron piece and light rectangular coil of many turns of fine wire wound on aluminium former inside which is an iron core. The purpose of coil is to make the field uniform . The coil is mounted on spindle and acts as the moving element. The current is fed into and out of the coil by means of springs which also provides controlling torque.
- Uniform scale
- Very effective eddy current damping
- No hysteresis loss
- As working field is very strong, therefore, such instruments are not affected by stray fields.
- Cannot be used for ac measurements.
- Costlier as compared to MI (moving iron) instruments.
- Can be used for both ac and dc supply.
- Free from hysteresis and eddy current errors.
- Torque/weight ratio is small , therefore such instruments have frictional errors which reduce sensitivity.
- Scale not uniform.
- Effect of stray field is significant.
2. Dynamometer type Instruments :
These are modified form of permanent magnet type . Here operating field is produced and not by a permanent but by another fixed coil. The moving system and the control system are similar to those of permanent magnet type . Such instruments can be used for both ac and dc circuits . These instruments are based on the principle that mechanical force exists between the current carrying conductors.
A dynamometer type instruments essentially consists of a fixed coil and a moving coil. The fixed coil is split into two equal parts which are placed close together and parallel to each other. The moving coil is pivoted in between two fixed coils. The fixed and moving coil may be excited separately or may be connected in series. The controlling torque is provided by two springs.
Deflecting torque – deflection of the pointer is directly proportional to the square of the operating current . Hence, the scales of these instruments is non uniform in nature.