Meters and measurements
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This document discusses different types of electrical meters. It covers principles of operation including electrostatic, electrothermal, and electromagnetic mechanisms. It describes indicating, integrating, and recording meter types and their various constructions as portable or switchboard. Finally, it lists common meter applications such as ammeters, voltmeters, watt meters and their uses in measuring electricity, power, demand and power factor.
Meters and measurements
- 6. APPLICATIONS Ammeter Voltmeter Watt and Watt-hour Meter Reactive volt-ampere Meter Kilovolt-ampere Meter Demand Meter Power-Factor Meter Frequently Meter Synchroscope Temperature Meters
- 8. DESIRED METER CHARACTERISTICS 1. Rugged Mechanism 2. Permanent Calibration 3. Proper Damping 4. Slight sensitivity to stray magnetic fields
- 9. • Very accurate meters can be built, but in many cases such meters are FRAGILE and are, therefore, not suitable for commercial work • There are many things that might cause a meter to become inaccurate with AGE, the most important being ageing of the iron in the magnetic circuits and friction or wear of the MOVING PARTS. • Meters should be DAMPED properly so that their readings are “DEAD BEAT” • THERE SHOULD BE A VERY LITTLE OSCILLATION OF THE POINTER WHEN A READING IS DESIRED. • A certain amount of STRAY MAGNETIC FIELD is practically always present in powerplants. • PROPER PRECAUTION IS TAKEN SUCH AS A STRAY FIELD WILL INTRODUCE A CERTAIN AMOUNT OF ERROR IN THE METER READINGS.
- 10. PRINCIPLE OF OPERATION ELECTROSTATIC - the fundamental principle that charged bodies, if of the same polarity, REPEL each other and, of different polarities, ATTRACT each other is utilized in the operation of voltmeters for very high voltages. The entire meter is immersed in oil tank, the oil serving the double purpose of insulator and proper damper, since the cylinder must revolve in an oil bath ELECTROSTATIC VOLTMETER
- 11. ELECTROTHERMAL Electrothermal - Meters built upon this principle depend upon the expansion and contraction of a wire carrying current to move a pointer over a suitable scale. Hot-wire movements use the heating effect of current flowing through a resistance to cause meter deflection. Each uses this effect in a different manner. Since their operation depends only on the heating effect of current flow, they may be used to measure both direct current and alternating current of any frequency on a single scale. The hot-wire meter movement deflection depends on the expansion of a high- resistance wire caused by the heating effect of the wire itself as current flows through it. A resistance wire is stretched taut between the two meter terminals, with a thread attached at a right angle to the center of the wire. A spring connected to the opposite end of the thread exerts a constant tension on the resistance wire. Current flow heats the wire, causing it to expand. This motion is transferred to the meter pointer through the thread and a pivot.
- 12. ELECTROMAGNETIC • The MAJORITY of meters used in commercial work fall in this class. • The OPERATION of the meters of this type depends upon the presence of magnetic field which acts upon a movable element. • ELECTROMAGNETIC may be further classified as follows: 1. PERMANENT MAGNET TYPE 2. MOVING IRON TYPE 3. DYNAMOMETER TYPE 4. INDUCTION TYPE
- 13. PERMANENT MAGNET TYPE Demonstration of the constructional features of permanent magnet moving coil type meters. Movement with spring control and magnetic damping Housed in a tough plastic frame with transparent front and back windows. Accuracy ± 2.5%. Basic sensitivity calibrated at 5mA, 100mV FSD Useful for demonstrating the working principle of AC or DC ammeters, voltmeters and galvanometers. Interchangeable plastic scales are available separately.
- 14. MOVING-IRON METER The meter is composed of an energizing coil, which is located at about 45 deg. With the horizontal, and a rotating element, which is merely a rectangular piece of thin soft iron mounted on a shaft. When the coil is energized, the soft iron vane seeks to place itself parallel with the magnetic flux of the coil; hence the shaft is caused to rotate. The torque thus produced is opposed by the action of a spiral spring,
- 15. DYNAMOMETER-TYPE METER The dynamometer type of meter is composed of two stationary coils and one movable coil upon which is mounted a pointer. When the current flows in these two windings. the movable coil tends to place itself parallel to the stationary coils. The motion of the movable coil is opposed by the action of a spiral spring.
- 16. INDUCTION TYPE METERS Induction type meters operate on the principle of the revolving magnetic field produced by out of phase currents that are flowing in separate windings. If an aluminum disk or drum is located between two such windings, there will be eddy current induced in the disk, and a resultant torque will be produced which will be proportional to the magnetic flux and the eddy currents flowing in the disk.
- 17. TYPE OF MECHANISM Indicating Meters – This class includes ammeters, voltmeters, wattmeters, power-factor meters, etc. • All such meters as are equipped with a POINTER that is caused to move along a proper scale, thereby indicating the conditions existing in the circuit at a particular instant.
- 18. INTEGRATING METERS All instruments that measure energy or its equivalent must be provided with a proper mechanism to sum up the energy consumed by the circuit to which the meter is connected. The most common meter of this type is watt- hour meter. The revolving disk or armature of such a meter causes a set of gears to rotate, thereby registering the total energy consumption.
- 19. RECORDING METERS A recording instrument is essentially an indicating meter, the pointer being provided with a suitable pen point and the scale being paper roll or a circular chart. In either case the chart is caused to move by some kind of clock mechanism, the pen point thereby tracing the complete record of conditions existing the circuit.
- 20. TYPE OF CONSTRUCTION PORTABLE & SWITCHBOARD • Portable instruments must be very rugged in order to withstand the severe usage to which they are generally subjected • Switchboard instruments are of two general types, rectangular or round for either surface or flush mounting.
- 21. APPLICATIONS • Ammeter – Alternating current ammeters are generally of the moving iron type, the dynamometer type, or the permanent-magnet type if equipped with a suitable rectifier circuit. • Voltmeter – In the case of voltmeters, an appropriate resistance must be used in series with the instrument in order to limit the meter. • Wattmeters– The electrodynamometer instrument is generally used for the measurement of power. The stationary coils are connected in series with the line, therefore they carry the current of the circuit. The circuit potential is applied to the moving coil. A suitable resistance must be used in series with the movable coil. • Reactive Volt-ampere (Var) Meter – In order to make a wattmeter read reactive volt-amperes, it is only necessary to shift the voltage that is applied to the moving coil by 90 deg.
- 22. • Watt-hour Meters and Reactive Volt-ampere Hours (Var-house) – By suitable connections either kilowatt-hours or reactive kilovolt-ampere hours can be obtained. The motion of the revolving disk is transferred to a set of gears that record kilowatt-hours or reactive kilovolt-ampere-hours. • Kilovolt-ampere Meters - Measurement of kilovolt-amperes involves two independent elements: a watt-hour meter and a reactive volt-ampere element as outlined above. The motions of the two independently moving elements are added vectorially by means of a ball-type differential gearing fitted within the register mechanism. The resultant motion of the sphere is proportional to total kilovolt-amperes, and if properly harnessed it can be used to indicate kilovolt-ampere demand, maximum kilovolt-ampere demand, or power factor.
- 23. • Three-phase Energy Meters – Three phase metering of real and reactive power and energy are accomplished by two or three element meters, depending on whether the system has three or four wires. • Power – factor Meter – The stationary coils are connected in series and placed directly in the line, while the movable element composed of two coils is connected across the line. • Frequency Meter – A disk carrying a pointer is pivoted between two poles which are excited with windings connected in such a way that the torques produced in the disk is such a way that the torques produced in the disk tend to neutralize each other. • Synchroscope - When synchronizing two generators, it is necessary that the voltages of the two machines be equal and in phase.