Types of Instrument:
Types of Instrument – Analogue ammeters and voltmeters are classified together, since there is no basic difference in their operating principles. The action of all ammeters and voltmeters, except those of the electrostatic variety, depends upon a deflecting torque produced by an electric current. In an ammeter this torque is produced by the current to be measured, or by a definite fraction of it. In a voltmeter it is produced by a current that is proportional to the voltage to be measured. Hence both voltmeters and ammeters are essentially current measuring devices.
The essential requirements of a measuring instrument are
(a) that its introduction into the circuit where measurements are to be made, should not alter the circuit conditions, and
(b) the power consumed by it be small.
Types of Instrument
The following types of instrument are mainly used as ammeters and voltmeters.
- Moving Iron
- Hot wire
- Induction type
Of these, the PMMC type can be used for dc measurements only, and the induction type for ac measurements only. The other types can be used for both. The moving coil and moving iron types depend upon the magnitude effect of current. The latter is the most commonly used form of indicating instrument, as well as the cheapest. It can be used for both ac and dc measurements and is very accurate, if properly designed.
The PMMC instrument is the most accurate type for dc measurement. Instrument of this type are frequently constructed to have substandard accuracy.
The calibration of the electrodynamometer type of instrument is the same for ac and dc. The same situation prevails for thermal instruments. These are particularly suitable for ac measurements, since their deflection depends directly upon the heating effect of the ac, i.e. upon the rms value of the current. Their readings are therefore independent of the frequency.
Electrostatic instruments used as voltmeters have the advantage that their power consumption is exceedingly small. They can be made to cover a large range of voltage and can be constructed to have sub-standard accuracy.
The induction principle is most generally used for Watt-hour meters. This principle is not preferred for use in ammeters and voltmeters because of the comparatively high cost and inaccuracy of the instrument.