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Electrical Instruments

The instruments that are used to measure electrical quantities (such as electric current, voltage, power, electric energy, frequency etc.) are called electrical instruments.

Classification of Electrical Instruments

The electrical instruments divided into two types

  1. Absolute instruments
  2. Secondary instruments
  3. Absolute Instruments: The instruments that give the value of the quantity to be measured in terms of the constants of the instrument are called absolute instruments. The examples of such instruments are Tangent Galvanometer and Rayleigh’s Current Balance. No previous calibration is necessary in such types of instruments. They are used only in standard laboratories.
  4. Secondary Instruments: The instruments that determine the electrical quantity to be measured directly in terms of deflection are called secondary instruments.These instruments are provided with a calibrated scale. These are the secondary instruments which are most generally used in everyday work. These instruments are most commonly used in generating stations, substations, laboratories, industries etc. Ammeters, Voltmeters, Wattmeters, Energy meters etc are the examples of secondary instruments.

Types of Secondary Instruments

The secondary instruments are further divided into following types:-

  1. Indicating Instruments:- The instruments which indicate the magnitude of electrical quantity being measured instantaneously are called indicating instruments. Examples of such instruments are ammeters, voltmeters and wattmeters.
  2. Integrating Instruments:- The instruments which measure total quantity of electrical energy in a given time are called integrating instruments. Energy meter is the example of integrating instrument.
  3. Recording Instruments:- The instruments which give a continuous record of the variations of the electrical quantity being measured are called recording instruments. EEG and ECG are the example of such instruments.

A recording instrument is merely an indicating instrument with a pen attached to its pointer. The motion of the drum is in a direction perpendicular to that of the deflection of the pen. The path traced out indicates the variation in the electrical quantity being measured during the given period.

 

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Electrical Instruments

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

  1. 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.

  1. 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

  1. 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.

  1. Integrating Instruments

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

  1. Recording Instruments

These are the instruments that give a continuous record of the variations of the electrical quantity being measured. 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

  1. 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.

  1. 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.

  • Spring Control
  • Gravity Control

      3. 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.

  • Air friction damping
  • Fluid friction damping
  • Eddy current damping

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Difference Between Ammeter and Voltmeter

Ammeter

Ammeter is an instrument which is used to measure electric current in an electric circuit.

An ammeter is connected in series with the circuit to load whose current is to measured. The operating coil of the instrument is to carry the whole of the current to be measured. When current flows through the operating coil deflecting torque is produced. The resistance of coil should be very low because coil carry the whole current of the circuit. Hence operating coil of an ammeter should have a few turns of thick wire.

Voltmeter

Voltmeter is an instrument which is used to measure voltage or potential difference between two points in an electric circuit.

A voltmeter is connected in parallel with the load or portion of the circuit whose voltage is to measured. The deflection of the meter depends upon the current flowing through the operating coil which is proportional to the voltage across the voltmeter. The resistance of coil should be very high because voltmeter is connected in parallel with the load. Hence operating coil of an voltmeter should have a large number of turns of thin wire.

 

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