The operation of moving coil meters depends on the electromagnetic effect of the current (a force is exerted on the current carrying conductor when placed in a magnetic field).
Moving coil instruments are of two types
- Permanent magnet type
- Dynamometer type
Permanent magnet type moving coil instruments can be used only in dc circuits whereas Dynamometer type moving coil meters can be used to measure both ac and dc circuits.
Some of the advantages and disadvantages of moving coil instruments are given below
Merits or Advantages of moving coil instruments
- Moving coil instruments consume quite less power. This is because the resistance of moving coil is small.
- In Moving coil meters the ratio of torque to weight is quite high and this reduces the errors due to friction.
- The scale of moving coil instrument is uniform.
- A low range ammeter can be used as both ammeter and voltmeter to measure high currents and voltages.
- PMMC meters are free from errors which would be caused by hysteresis and stray magnetic fields.
- Damping is very simple and effective in moving coil meters.
Demerits or Disadvantages of moving coil instruments
- Due to friction at the bearings and temperature variations, errors are likely to be present and the observed readings are may not be accurate and fresh calibration of the meters may become necessary.
- With long usage permanent magnet gets weakened and this likely may introduce errors in readings.