Page 276 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

MODULE 9 : Electrical Meters and Calibration

LESSON 50-53 : Instruments - scales - classification -

PMMC and MI meters

Objectives

At the end of this lesson you shall be able to:
• state the instrument, classification of instruments
• classify electrical instruments
• explain the type forces required for the proper functioning of an electrical instruments
• state the prinicple and construction of PMMC, MI type instruments
• state the prinicple & construction of meggar, earth testa
• state the digital ammeter voltmeter, frequently meter, energy meter calibration of meter.

Measuring instrument
The device or instrument used for measuring an unknown quantity is called measuring instrument. These devices
compare the unknown quantity to be measured with the unit of measurement or standard quantity.
Examples are voltmeter, ammeter, measuring tape etc
The instrument which are used for measuring electrical quantities like voltage, current etc are called electrical
measuring instrument

Classification of measuring instrument
Electrical instruments may be divided into two categories, that are;
Primary instrument or Absolute instruments,
Secondary instruments.
Primary instrument

Primary or Absolute instruments gives the quantity to be measured in term of instrument constant & its deflection.
Example: Tangent Galvanometer
It gives the value of current to be measured in terms of the tangent of deflection produced by current and meter
constant (the radius and number of turns wire used and the horizontal component of earth’s field)
Secondary instrument
These instruments give the magnitude of electrical quantity to be measured directly. These instruments are
required to be calibrated by comparing with another standard instrument before putting into use.
Example: Voltmeter, Ammeter etc
Electrical measuring instruments may also be classified according to the kind of quantity, kind of current, principle
of operation of moving system.
According to principle of operation
According to the principle of operation of the moving system, electrical measuring instruments are also be
classified into;
Magnetic effect Ammeter and Voltmeter
Electrodynamic effect Ammeter, Voltmeter and wattmeter
Electromagnetic effect Ammeter and Voltmeter, wattmeter and watthour meter
Thermal effect Ammeter and Voltmeter
Chemical effect dc ampere hour meter

Magnetic effect Voltmeter

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