Explanation of Errors in Moving Coil Instruments

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.

3.       Stray Magnetic Field Error: the errors due to stray magnetic fields may be appreciable as the operating magnetic field to weak. This may give rise to wrong reading. Such errors depends upon the direction of the stray magnetic field relative to the field of the instrument.

Remedy: This error can be reduced by using iron case for ordinary instruments (or a thin iron shield) over the working ports.

4.       Thermo-electric Error: This error pertains chiefly to ammeters using shunts and is due to uneven temperature distribution in shunt arising from unequal cooling. Position of mounting. Bad contacts at the current terminals and the peltier effect.

Remedy: The best method of eliminating such an error is by using an alloy of small thermo-electric power to copper such as maganin tarmac, therlo, but such materials require careful treatment as regards soldering and protection.

5.       Observational Error: Such errors are due to misreading of the scale, parallax in reading and errors of estimation.

·         Scale error can be reduced by re-calibrating the meter.

·         Parallax error can be reduced by placing a mirror under the scale. The shadow of the pointer and pointer itself coincided gives the correct reading without parallax error. The image of the pointer in the mirror should not be visible to the reader for zero parallax error.

·         Zero error can be eliminated by adjusting zero setting before taking the reading. Failure to adjust zero setting before making measurement causes error.

6.       Temperature: change of temperature affects the instrument resistance and stiffness of the control spring this is due to heating of the working coil and other resistance coils connected internally in the instrument causing by the operating current.

A 1^oC increase of temperature reduces the strength of springs by about 00.4 percent and reduces flux density in the air gap of the magnet by about 0.02 percent. Thus the net effect, on the average, is to increase the deflection by about 0.02 percent per ^oC.

The moving coil of measuring instrument is usually wound with copper wire having a temperature coefficient 0.004/^oC. When the instrument is used as a micro-ammeter or a milli-ammeter and the moving coil is directly connected to the output terminals of the instrument for a constant current would decrease by 0.04 per ^oC rise in temperature.

In case the moving coil instrument is used as voltmeter a large series resistance of negligible temperature coefficient (made of material like magnanin) is used. This eliminates the error due to temperature.