Electrical Measurements Theory of Indicating instruments

Indicating instruments: The principle, different types of control and damping arrangements in indicating instruments, Permanent Magnet Moving Coil (PMMC), Moving Iron (MI), electrostatic and dynamometer type meters, errors in indicating instruments, the extension of instrument range for ammeters and voltmeters. 

Introduction: 

Measurement: The measurement of a given quantity is the result of comparison between the quantity to be measured and a definite standard. Th instruments which are used for such measurements are called measuring instruments. 

The 3 basic quantities in the electrical measurement are 

Current (Measured by Ammeter) 

Voltage (Measured by Voltmeter 

Power (Measured by wattmeter (Power meter)) 

The necessary requirement of a measuring instrument: 1) With the introduction of the instrument in the circuit, the circuit conditions should not get altered and thus the quantity to be measured should not get affected due to the instrument used. 2) The power consumed by the instruments for their operation should be as small as possible. 

Classification of measuring instruments:

Primary instruments: These are the instruments mainly used for standardization of the secondary instruments and examples are like tangent galvanometer, bourdon tube, rayleighs galvanometer etc. 

Secondary instruments: 

These are mainly classified as 

1. Indicating instruments: These instruments make use of dial and pointer for showing or indicating the quantity for ex: Ammeters and Voltmeters. 

2. Recording instruments: These instruments give a continuous record of electrical quantity over a period of time ex: ECG, EEG, X-Y PLOTTER 

3. Integrating instruments: These instruments measure the total quantity of electricity delivered over a period of time. like a household, energy meter registers a number of revolutions made by the disc to give the total energy delivered with the help of counting mechanism. 

An essential requirement of an instrument: In case of measuring instruments the effect of an unknown quantity is converted into a mechanical force which is transmitted to the pointer which moves over a calibrated scale. for satisfactory operation of any indicating instruments following system must be present in an instrument. 

1) The deflecting system producing deflecting torque: Td 

2) The controlling system producing controlling torque: Tc 

3) The damping system producing damping torque: TD 

Flemings Left-hand rule: When current flows through a conducting wire, and an external magnetic field is applied across that flow, the conducting wire experiences a force perpendicular both to that field and to the direction of the current flow (i.e they are mutually perpendicular).

   

Flemings Right-hand rule: Fleming's Right-hand Rule (for generators) shows the direction of induced current when a conductor attached to a circuit moves in a magnetic field. The right hand is held with the thumb, index finger and middle finger mutually perpendicular to each other (at right angles), as shown in the diagram.

So basically for motoring action and to produce thrust on the conductor Flemings left-hand rule is used.