Errors in Moving Coil Instruments
The errors that usually occur in PMMC instruments are
1. The frictional error.
2. Temperature error.
3. The error owing weakening of
permanent magnet.
4. Stray magnetic field error.
5. Thermo-electric error.
6. Observational error.
1. The frictional Error: The friction of the pivot in the
jewel produces a frictional torque which affects the instrument reading. This
error is more serious for sensitive instruments designed for low operating
torque.
Remedy: The frictional error in PMMC instruments
can be produced by providing proper pivoting and balancing i.e. these errors
may be reduced by adopting a moving system of light construction and large
deflecting torque.
2. The Error Owing to Weakening of
Permanent Magnet: Weakening
of permanent magnet is due to ageing at temperature effects. The error owing to
weakening of permanent magnet occurs very slowly in good magnet but in
instruments liable to vibrations, change of position and stray magnetic fields
i.e. in portable instruments demagnetization may be considerably accelerated.
Remedy: The error owing to weakening of
permanent magnet can be reduced by proper attention to ageing and reasonable
care in use.