Electrical Measuring Instrument - In order to ensure the quality and efficiency of a device made by an engineer it is important it should be able to measure accurately electrical quantity. the instrument which is used to measure the electrical quantity is known as the electrical instrument. Electrical measuring instruments are those devices which are not used for measurement of electrical quantities only but these are also used to measure other quantity which is not electrical in nature. During the measuring of non-electrical quantity, the measuring quantity is converted into electrical quantity by means of using a suitable transducer.
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| Voltmeter and Ammeter |
Classification of measuring instruments
Electrical measuring instruments classified into two categories1. Primary or absolute instrument
2. secondary instrument
Primary absolute instrument
The primary or absolute instrument are those instrument which indicate the quantity to be measured in term of constants of instruments and in order to find out constant and these instruments are suitable for laboratory use only.
Example:-Tangent galvanometer which gives the measured current in terms of the tangent of the deflected angle, the radius, and a number of turns of the galvanometer.
Secondary Instruments
The instruments which measure an electrical quantity on a calibrated scale is termed as a secondary instrument. These instruments measure large values of electrical quantities. Most of the measuring instruments which are used generally are of secondary type measuring instrument. Secondary instruments are classified as follows:-
(1).Recording Instrument
The instrument which indicates the value by pen and graph paper or sheet instead of pointer and scale is called recording instruments. These instruments are mostly used in powerhouses.
(2). Indicating Instrument
The instrument which indicates the value of pointer and scale are called indicating or deflecting instruments. i.e ampere meter, voltmeter, power factor meter, etc
(3). Integrating Instrument
The instrument which records the total quantity with respect to time is called integrating instruments such as watt-meter and ampere-hour meter etc. As the mechanical force or torque will move the pointer over the calibrated scale, indicating the value of the quantity to be measured. Thus some of the torque act in indicating instruments that play an important role in measurement.The torque of An Indicating Instrument
The Analog instrument like ammeter, Voltmeter, and wattmeter consists of a moving system with a pointer attached to it. For satisfactory operation, the moving system is pivoted by jeweled bearing and is subjected to the following three different types of torque
(1). Deflecting Torque
The force or torque which bring the pointer in motion is called deflecting torque. This torque is produced due to the magnetic or heating effecting of the electric current but in deflecting torque mostly magnetic effect is utilized.
(2). Controlling Torque
The torque which is used to control the movement of pointer caused by deflecting torque is called controlling torque. This torque is used to bring the pointer at its original position when there is no current or there is no deflecting torque. The controlling torque is produced either by a spring system or gravity.
(3).Damping Torque
The force or torque which brings the pointer suddenly at its original position is called damping torque. this torque is also used to minimize the accelerating or vibration effect of pointer.there are three methods to produce damping torque in the measuring instrument.(a). Air Friction Damping System
In this system, a light aluminum piston is attached to the moving part of the instrument as shown in the figure given above. the piston moves in an air chamber which is closed at its one end.it will have a small clearance with at the air chamber. the system works on compression and suction of the air.
(b). Eddy Current Damping System
In this system, a small lightweight disc normally made of aluminum attached to the spindle is rotated in between poles of the permanent magnet and produces eddy current in the disc end.
As per Lenz's law, the direction of the induced eddy current is always opposite to the cause. this opposition of eddy current act and produce a damping torque.this is one of the best and effective methods to provide damping torque or force in the instrument.
As per Lenz's law, the direction of the induced eddy current is always opposite to the cause. this opposition of eddy current act and produce a damping torque.this is one of the best and effective methods to provide damping torque or force in the instrument.
(c). Fluid Friction Mechanism System
In this system in place of air, some liquid is used in which some small metal pieces or disc is attached to one end of the spindle is placed. In fluid, a force is produced which minimizes the vibration effect. this force or torque is called a fluid friction damping system.Different type of Controlling System
(a). Spring Control System
In this type of controlling of torque or force, a special type of spring made up of bronze is used. the spring which uses here should have some special properties like low specific resistance, non-magnetic, and low-temperature coefficient. Mostly two spring attached with the lower and upper portion of the spindle are used. The function of spring is to provide controlling either force or torque due to spring action.
(2). Gravity Control System
In this type of controlling of torque or force, a small calculated weight is attached to one end of the spindle which works on the principle of gravity, therefore it is called a gravity control system.it has been shown in the figure given above. the attached weight produces a controlling torque due to the gravitational force and this attached weight is called control weight. The direction of the controlling torque is always opposite to the direction of the deflecting torque.
Advantage and Disadvantage of Gravity Control System
Advantage
- The system is cheap.
- it is not affected by the change in temperature
- it has a long working life
- it requires very little maintenance and hence it not expensive
- This instrument requires to keep always in a vertical position.
- The scale is cramped and nonuniform.
- This instrument required to install on the ground only.
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