Electrical Measuring Instruments|Types Of Measuring Instruments

In this post, we will discuss about Electrical measuring instruments. Types of measuring instruments: Absolute Instruments & Secondary Instruments, Moving Iron & Moving Coil Instruments,(a) Indicating instruments, (b) Integrating instruments, (c) Recording instruments.

Contents

Measuring Instruments

The basic electrical quantities which often need to be measured are ;

voltage, current, energy. power, electrical power factor, frequency, resistance, inductance capacitance, speed, temperature etc.

The instruments used for all electrical measurements are called measuring instruments.

General Principles of Measurement READ HERE

They include ammeters, voltmeters, watt- meters, energy meters etc.

Electrical Instruments

The instruments which are used to measure electrical quantities are called electrical instruments. Such as ammeter, voltmeter, wattmeter, energy meter, megger etc.

Characteristics of Instruments read HERE

The electrical instruments may be classified as,

(1) Absolute instruments, (2) Secondary instruments.

Absolute instruments

The instruments which give the value of the quantity to be measured in terms of the constants of the instrument are called absolute instruments.

Such instruments do not require any previous calibration.

A common example of this type of instrument is a tangent galvanometer.

Tangent galvanometer

The tangent galvanometer is an early measuring instrument for small electric currents.

It consists of a coil of insulated copper wire wound on a circular non-magnetic frame. Its working is based on the principle of the Tangent law of magnetism.

When a bar magnet is suspended in two Magnetic fields B and B_h, it comes to rest making an angle θ with the direction of B_h.

B=B_h tanθ. This is known as the tangent law of magnetism

When a current is passed through the circular coil, a magnetic field (B) is produced at the center of the coil in a direction perpendicular to the plane of the coil.

The tangent galvanometer is arranged in such a way that the horizontal component of earth’s magnetic field (B_h) is in the direction of the plane of the coil.

The magnetic needle is then under the action of two mutually perpendicular fields

Secondary instruments

The instruments which determine the electrical quantity to be measured directly in terms of deflection are called secondary instruments.

These instruments are generally used in practical life. They are further classified as;
(a) Indicating instruments, (b) Integrating instruments, (c) Recording instruments.

(a) Indicating instruments

The instruments which indicate the magnitude of electrical quantity being measured instantaneously are called indicating instruments.

indicating-instruments-ammeter-voltmer-wattmeter

In such instruments, a pointer moves over the calibrated scale to indicate the magnitude of the electrical quantity being measured.

Examples of Indicating Instruments:Ordinary ammeters, volt meters, and watt meters fall into this category.

(b) Integrating instruments

The instruments which add up the electrical quantity, i.e., electrical energy and measure the total energy (in kWh) in a given period are called integrating instruments.

In such instruments, there are sets of dials or gears which register the total quantity of electricity or the total amount of electrical energy supplied to a circuit in a given period.

Example : Watt-hour Meter, Energy Meter

(c) Recording instruments

The instruments which give a continuous record of the variations of the electrical quantity being measured are called recording instruments.

Indicating Type Instruments

Indicating instuments are of the following types
(i) Moving iron instruments
(11) Moving coil instruments
(1i) Dynamometer type instruments
(iv) Hot wire instruments
(v) Electrostatic type instruments
(vi) Induction type instruments

Moving Iron Instruments

Moving Iron instruments are of two types-

1.Attraction type

2.Repulsion type

Attraction Type Moving Instruments

Attraction Type MI instruments are based on the principle that when an unmagnetized soft iron piece is placed in the magnetic field of a coil then the piece is attracted towards the coil.
The deflection is proportional to the square of current through the coil. Thus, the scale of such instruments in non-uniform being crowded in the beginning

Repulsion type Moving Instruments

These instruments are based on the principle of repulsion between the two iron pieces similarly magnetized.

Since deflection is proportional to the square of the current through the coil, therefore the scale of such instruments in non-uniform is crowded in the beginning.

Merits of Moving Iron Instruments

The moving iron instruments have the following advantages

1. These are cheap, robust, and simple in construction.

2. The instruments can be used for both A.C. as well as D.C. circuits.

3. These instruments have high operating torque.

4. These instruments are reasonably accurate.

disadvantages of moving iron instruments

Demerits-The following are the disadvantages of moving iron instruments

1.Such instruments have non-uniform scale.

2. These instruments are not very sensitive.

3. Errors are introduced due to change in frequency in case of A.C. measurements.

4. Higher power consumption.

Moving Coil Instruments

Moving coil instruments are of two types

Permanent magnet type (used for D.C. only)
2. Dynamometer type (used for both A.C. and D.C, only)

Moving coil instruments works on the principle that “When a current-carrying conductor is placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the field.”

Advantages of Moving coil instruments

1.Uniform scale

Very effective eddy current damping
No hysteresis loss
Power consumption low
As the working field is very strong. therefore such
instruments are not affected by stray fields
Such instruments require a small operating current
Very accurate and reliable