Classification of Electrical Measuring Instruments

CLASSIFICATION OF ELECTRICAL MEASURING INSTRUMENTS

The various electrical measuring instruments can be broadly divided into two classes viz.

(i) Absolute instruments (ii) Secondary instruments.

(i) Absolute instruments: Those instruments which give the value of the quantity to be measured in terms of the constants of the instrument and its deflection are called absolute instruments. For example, a tangent galvanometer which is used to measure current is an absolute instrument. It is because a tangent galvanometer gives the value of current being measured in terms of the tangent of the angle of deflection, the radius and number of turns of the coil and the horizontal component of earth's magnetic field. No previous calibration or comparison is necessary in case of absolute instruments. They are used only in standard laboratories as standardising instruments.

(ii) Secondary instruments: Those instruments in which the electrical quantity being measured is given directly by the deflection of the instrument are called secondary instruments. These instruments are provided with a calibrated scale. The calibration is done with the help of an absolute instrument or another calibrated instrument. These are the secondary instruments which are most generally used in everyday work. For example, they are very commonly used in laboratories, power stations, substations, industries etc.

Types of Secondary Instruments

Secondary instruments may be classified according to their functions as

(i) indicating instruments

(ii) integrating instruments and

(iii) recording instruments.

(i) Indicating instruments Those instruments which directly indicate the value of the electrical quantity at the time when it is being measured are called indicating instruments e.g. ammeters, voltmeters and wattmeters. In such instruments, a pointer moving over a graduated scale directly gives the value of the electrical quantity being measured. For example, when an ammeter is connected in the circuit, the pointer of the meter directly indicates the value of current flowing in the circuit at that time.

(ii) Integrating instruments Those instruments which measure the total quantity of electricity (in ampere-hours) or electrical energy (in watt-hours) in a given time are called integrating instruments e.g. ampere-hour meter and watt-hour meter. In such instruments, there are sets of dials and pointers which register the total quantity 210 of electricity or electrical energy supplied to the load.

(iii) Recording instruments Those instruments which give a continuous record of the variations of the electrical quantity to be measured are called recording instruments. A recording instrument is merely an indicating instrument with a pen attached to its pointer. The pen rests lightly on a chart wrapped over a drum moving with a slow uniform speed. The motion of the drum is in a direction perpendicular to the direction of the pointer. The path traced out by the pen indicates the manner in which the bell quantity, being measured, has varied during the time of the record. Recording voltmeters are used in supply stations to record the voltage of the supply mains during the day. Recording ammeters are employed in supply stations for registering the current taken from the batteries.

Types of Indicating Instruments

The following types of ammeters and voltmeters are in common use:

(a) Moving coils (i) Permanent magnet form (ii) Dynamometer form

(b) Moving iron

(c) Hot wire

(d) Electrostatic (voltmeters only)

(e) Induction.

Here we will concentrate mainly on moving coil and moving iron types whereas a brief introduction for the others will be given. Of the above mentioned, the magnet moving coil is used for de only whereas induction type are used for ac measurement. The other types can be used both for de as well as ac a circuits.