SOLUTION

Permanent Magnets Moving Coil “PMMC” Instrument 

These instruments are used either as ammeters or voltmeters and are suitable for d.c. work only. This type of instrument is based on the principle that when a current-carrying conductor is placed in a magnetic field, the mechanical force acts on the conductor. The coil placed in the magnetic field and carrying the operating current is attached to the moving system. With the movement of the coil, the pointer moves over the scale to indicate the electrical quantity being measured. This type of movement is known as the D’Arsonval movement.

Errors In PMMC Instrument

The basic sources of errors in PMMC instruments are friction, temperature, and aging of various parts. To reduce the ratio of the frictional error of torque to weight is made very high.

The most serious errors are produced by the heat generated or by changes in the temperature. This changes the resistance of the working coil, causing large errors. In the case of voltmeters, a large series resistance of a very low-temperature coefficient is used. This reduces the temperature errors.

The aging of permanent magnet and control springs also causes errors. The weakening of the magnet and springs cause opposite errors. The weakening of the magnet causes less deflection while the weakening of the control springs causes large deflection, for a particular value of current. The proper use of material and pre-aging during manufacturing can reduce tt errors demo the weakening of the control spring.