Electrical instruments are used to measure electrical quantities such as current, voltage, power, energy, frequency, power factor, resistance etc. To measure these quantities several instruments such as ammeter, voltmeter, wattmeter, energy meter are used. In simple words we can say that the instruments which are used to measure electrical quantities are known as electrical instruments.

Classifications of Electrical Instruments

These instruments are divided into two types

Absolute Instruments

Secondary Instruments

Absolute Instruments

Absolute instruments are the instruments which give the value of the quantity to be measured in terms of the constants. Such instruments do not require any previous calibration. Tangent galvanometer is the example of absolute instruments.

Secondary Instruments

Secondary instruments are the instruments which determine the electrical quantities to be measured directly in terms defection.

Types of Secondary Instruments

The secondary instruments are further divided into three types.

Indicating Instruments

Integrating Instruments

Recording Instruments

Indicating Instruments

These are the instruments that indicate the magnitude of electrical quantity being measured instantaneously. In such instruments, a pointer moves over the calibrated scale. Ammeter, voltmeter, wattmeter etc are the example of indicating instruments.

Integrating Instruments

The instruments that add up the electrical quantity. Energy meter are the example of such instruments because these instruments measure the total energy (in KWH) in a given period.

Recording Instruments

These are the instruments that give a continuous record of the variations of the electrical quantity being measure. ECG is the example of such instruments.

Essentials of Indicating Instruments

Indicating instruments are those instruments in which pointer moves over the calibrated scale to indicate the magnitude of electrical quantity which to be measured. The torques required for operation of indicating instruments are

Deflecting Torque

Controlling torque

Damping torque

Deflecting Torque

It is a torque due to which pointer of instruments moves from its zero position in an indicating instrument is called deflecting torque.

Controlling Torque

It is a torque which allow the pointer to deflect in accordance to the magnitude of electrical quantity or which brings the pointer to zero position when the instrument is disconnected from the circuit.

The controlling torque is provided by following methods.

  1. Spring Control
  2. Gravity Control

Damping Torque

It is the torque which suppresses the under oscillations of the pointers and brings the pointer to its final position quickly. The damping torque is provided by following methods.

  1. Air friction damping
  2. Fluid friction damping
  3. Eddy current damping