Electrical Measuring Instruments

  1. The instruments which are used to measure electrical quantities are called electrical instruments.

Classification of Electrical Instruments

  1. Absolute instruments: these instruments give the value of the quantity to be measured in terms of the constants of the instrument. Example: tangent galvanometer.
  2. Secondary Instruments: these types of instruments determine the electrical quantity to be measured directly in terms of deflection.
Sr. No. Type Effect Produced Suitable for Instrument
1 Moving-Iron Magnet effect DC and AC Ammeter, Voltmeter
2 Permanent Magnet Moving Coil (PMMC) Electrodynamic effect DC only Ammeter, Voltmeter
3 Dynamometer Type Electrodynamic effect DC and AC Ammeter, Voltmeter, Wattmeter
4 Induction type Electro-magnetic induction effect AC only Ammeter, Voltmeter, Wattmeter, Energy-meter
5 Electrothermic type Thermal effect DC and AC Ammeter, Voltmeter
6 Electrolytic meter Chemical effect DC only Ampere-hour meter
7 Electrostatic Type Electrostatic effect DC and AC Voltmeter only

Types of secondary instruments

  1. Indicating instruments: these instruments indicate the magnitude of electrical quantity being measured. Examples Ammeter, voltmeter, wattmeter etc.
  2. Integrating instruments: measure the total energy (in KWh) in a given period. Example, Energy meter.
  3. Recording Instruments: the instruments which give a continuous record of the variations of the electrical quantity being measured are called recording instruments. Example, ECG, EEG, etc.

Essentials of Indicating Instruments

Deflecting torque:

Controlling Torque

  1. Spring control method
  2. Gravity Control Method

Damping Torque

  1. Air friction damping
  2. Fluid Friction Damping
  3. Eddy Current Damping