Different Types of Ammeters and Voltmeters:
The following are the different types of ammeters and voltmeters in measuring instruments are
- PMMC
- Moving Iron
- Electrodynamometer
- Hot wire
- Thermocouple
- Induction type
- Electrostatic
- Rectifier
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 the types of ammeters and voltmeters because of the comparatively high cost and inaccuracy of the instrument.