Types of instruments

Types of instruments

(1) Permanent Magnet Moving Coil (PMMC) instruments .

(2) Moving iron instruments.

(3) Electrodynamometer instruments.

(4) Thermal instruments

(5) Induction instruments.

(6) Electrostatic type instruments.

(7) Rectifier type instruments.

    The PMMC instruments can be used for direct measurement only, and the induction type for alternating current measurements only.

    The moving iron(MI) and moving coil(MC) types both depend for their action upon the magnetic effect of current. The MI instruments can be used for either direct or alternating current measurements, and is the cheapest.

   Electrodynamometer type of instruments can be used both on a.c as well as on d.c. They are useful as “transfer instruments” , as their calibration for both d.c and a.c is the same.

   The calibration for d.c and a.c is same for the thermal instruments also. They are particularly suited for a.c measurements. This is because, the deflection of the thermal instruments depends directly upon the heating effect of the alternating current. ie, upon the rms value of the current.

   For the electrostatic instruments, the electrostatic principle is only directly applicable to voltage measurements. They have the advantage that, their power consumption is extremely small.

     The induction principle is more generally used for Watt- hour meters than for ammeters and voltmeters.