Electrical Measuring Instruments and Instrumentations

ELECTRICAL MEASURING INSTRUMENTS AND INSTRUMENTATIONS

LEARNING OUTCOMES-

1- Connect and repair different indicating and recording instruments in electric circuit. 2- Use instruments for measuring different electrical quantities. 3- Use sensors for measuring non electrical quantities.

UNIT-1 Introduction to electrical measuring instruments- 1.1 Concept of measurement and instruments. Measurement is the process of systematically assigning numbers to objects and their properties, to facilitate the use of mathematics in studying and describing objects and their relationships. Some types of measurement are fairly concrete: for instance, measuring a person’s weight in pounds or kilograms, or their height in feet and inches or in meters. Note that the particular system of measurement used is not as important as a consistent set of rules: we can easily convert measurement in kilograms to pounds, for instance. Although any system of units may seem arbitrary (try defending feet and inches to someone who grew up with the metric system!), as long as the system has a consistent relationship with the property being measured, we can use the results in calculations. Measurement is not limited to physical qualities like height and weight. Tests to measure abstractions like intelligence and scholastic aptitude are commonly used in education and psychology, for instance: the field of psychometrics is largely concerned with the development and refinement of methods to test just such abstract qualities. Establishing that a particular measurement is meaningful is more difficult when it can’t be observed directly: while you can test the accuracy of a scale by comparing the results with those obtained from another scale known to be accurate, there is no simple way to know if a test of intelligence is accurate because there is no commonly agreed-upon way to measure the abstraction “intelligence.” To put it another way, we don’t know what someone’s actual intelligence is because there is no certain way to measure it, and in fact we may not even be sure what “intelligence” really is, a situation quite different from that of measuring a person’s height or weight. These issues are particularly relevant to the social sciences and education, where a great deal of research focuses on just such abstract concepts.

1.2 Types of electrical measuring instruments- 1.2.1 Indicating instruments: Indicating instruments indicate, generally the quantity to be measured by means of a pointer which moves on a scale. Examples are ammeter, voltmeter, wattmeter etc. Recording instruments: These instrumentsrecord continuously the variation of any electrical quantity with respect to time.

1.2.2 Indicating instruments- Integrating instrument are electrical measuring instruments, which measures the electrical quantity and sum up with previous value and provides a cumulative result of electrical quantity under measurement. Eg: House hold energy meter. The best example for integrating instrument is energy meter.

1.2.3 Recording instruments- The recording instrument is defined as the instrument which records the continuous variation of the magnitude of an electrical quantity for a particular period. It is used in a placed where the continuous reading of circuit conditions is required. The record is used for future reference or computational work.

1.3 Essential of indicating instruments- 1.3.1 Deflecting torque-The deflecting torque is produced by the electromagnetic action of the current in the coil and the magnetic field. When the torques are balanced the moving coil will stop and its angular deflectionrepresents the amount of electrical current to be measured against a fixed reference, called a scale

1.3.2 Controlling Torque.- This torque is produced by the spring action and opposes the deflection torque so as the pointer can come to rest at the point where these two torques are equal (Electromagnetic torque = control spring torque)

1.3.3 Damping torque-The damping torque is necessary to bring the moving system to rest to indicate steady reflection in a reasonable short time.It exists only as long as the pointer is in motion.Under the absence of damping torque the pointer oscillates for a short period of time and comes to steady position

UNIT -2

2.1 AMMETER An ammeter (from Ampere Meter) is a measuring instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems. It is generally represented by letter 'A' in a circle. Ammeters have very low resistance and are always connected in series in any circuit.

2.2 VOLTMETER A voltmeter is an instrument used for measuring In a measuring instrument, the damping torque is necessary to bring the moving system to rest to indicate steady reflection in a reasonable short time.It exists only as long as the pointer is in motion.Under the absence of damping torque the pointer oscillates for a short period of time and comes to steady position difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an In a measuring instrument, the damping torque is necessary to bring the moving system to rest to indicate steady reflection in a reasonable short time.It exists only as long as the pointer is in motion.Under the absence of damping torque the pointer oscillates for a short period of time and comes to steady position

Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Portable instruments, usually equipped to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.

2.3 MOVING IRON ISNTRUMENTS-

Moving-iron instruments are generally used to measure alternating voltages and currents. In moving-iron instruments the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil

2.4 MOVING COIL INSTRUMENTS-

The current is fed to the coil through spiral springs which are two in numbers. The coil which carries a current, which is to be measured,moves in a strong magnetic field produced by a permanent magnet and a pointer is attached to the spindle which shows the measured value.

Multiple choice question

Q.1 The use of ______instruments is merely confined within laboratories as standard instruments.

a- absolute b- indicating c-recording d- intrigrating

Q.2 ______instruments are those which measure the total quantity of electricity in a particular time.

a- absolute b- indicating c-recording d- integrating

Q.3 Which of the following are integrating instruments?

a- voltmeter

b- ammeter

c- wattmeter

d- energy meter

Short Q/A

Q.1 Define indicating instruments.

Q.2 Define integrating instruments.

Q.3 Define damping torque

Q.4 Explain voltmeter

Long Q/A

Q.1 What are essentials of indicating instruments?

Q.2 Explain moving iron instruments

Q.3 Explain moving coil instruments.