Functions of a Direct Current Meter. A direct current meter is an instrument intended for the measurement of electrical quantity in a direct current circuit. There are two main classes of direct current meters, (I) ampere-hour meters and (II) watt-hour meters. An ampere-hour meter measures the product of the current in amperes flowing in a circuit and the time in hours during which the flow is maintained. A watt-hour meter measures the product of the power in watts and the time in hours during which the flow of power is maintained.
Direct Current Ampere-hour Meters. Ampere-hour meters are used by electrical undertakings for measuring the supply of electricity to domestic and industrial consumers. These undertakings are under a statutory obligation to maintain the voltage at consumers' terminals at a declared value within close limits; assuming that the supply voltage is maintained at the declared value, an ampere-hour meter can be calibrated to register in terms of kilowatt-hours at this voltage. This principle is accepted as satisfactory in most countries where the voltage at consumers' terminals is maintained within narrow limits of the declared voltage, and since direct current ampere-hour meters are, in general, more reliable and less costly than direct current watt-hour meters the practice has much in its favour.
In addition to the foregoing, ampere-hour meters are used for measuring the current consumption in battery charging, electro-deposition and other electrolytic or industrial processes and in some instances they exercise a controlling function over these operations. Many types of ampere-hour meter have been manufactured in the past, the most important being electrolytic meters and motor meters.
Theoretically the former are capable of very accurate registration but in practice the working results are not so good as with motor meters, and the latter are preferred by most supply authorities.
Notes
current -
quantity - ,
to maintain - ;
statutory -
to assume - ,
to calibrate -
consumption -
Comprehension questions:
1.What is the intention of a direct current meter?
2. For what purpose are ampere-hour meters used?
3. What additional purposes of ampere-hour use do you know. Discuss them with your partner.
TEXT 11. AUXILIARY EQUIPMENT
Methods of Cooling
Various methods of cooling transformers are adopted in practice, depending upon the size and the local conditions. Very small transformers are cooled naturally by the atmosphere, no special cooling arrangements being necessary. Slightly larger transformers are oil-immersed, being enclosed in a tank for this purpose. The object of the oil is twofold. As an insulator, it is better than air, and it also keeps down the maximum temperature rise by setting up convection currents which tend to equalize the temperature. These convection currents carry the heat away to the walls and lid of the tank, whence it is dissipated into the atmosphere. Small tanks are made with a plain exterior, sufficient cooling surface being obtained in this manner. Rather larger tanks are made with a corrugated exterior, or are provided with fins, to increase the cooling surface. A more popular arrangement, however, is to obtain the desired increase in cooling surface by means of a number of tubes running from top to bottom on the outside of the tank. The oil immediately in contact with the transformer in the tank is heated by the transformer, and consequently rises. Convection currents cause the oil to flow outwards at the top of the tank and so it enters the tubes at the top. It is now cooled in the tubes, sinks, and re-enters the tank at the bottom. It is now heated again by the transformer, and the cycle of operations is repeated.
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The larger the tank, the greater is the number of tubes required. These are now arranged in rows, one behind the other, but very little advantage is gained by adding more tubes when they are already this stage is reached, the simple tubular design is abandoned, and external radiators are substituted for the tubes.
With this type of cooling the tank surface itself now becomes plain again. The external radiators consist of a long horizontal chamber at the top, and another at the bottom, these being joined by numbers of radiator tubes.
Other transformers designed for use with an external oil cooler have no radiators fitted to the transformer tank itself, practically the whole of the heat dissipation taking place in the oil cooler.
Water-cooling is also employed. A number of tubes are arranged in a helical coil inside the top of the transformer tank, but underneath the oil level. A stream of cold water is then passed through this cooling coil. Since the presence of even a minute percentage of water in the oil reduces its insulating properties to an enormous degree, it is extremely important that no water should escape through any leak, should one occur. In order to prevent this, the tank is made oil-tight and the oil is put under pressure. If a leak should develop, oil will leak into the tubes (which does not much matter) instead of the water leaking into the oil.
In confined spaces, and where a supply of water is not available, air-blast cooling may be adopted. The tank is now dispensed with and a blast of cold air is forced over the transformer windings. Cooling by this means is usually confined to transformers operating on the lower voltages.
The oil used for transformer immersion is a pure hydrocarbon (mineral) oil, obtained by refining crude petroleum. Its insulating properties are very adversely affected by the presence of even a minute proportion of water, and so it must be clean and practically free from moisture. Certain oils tend to form a sludge in the course of time, this being due to the slow formation of solid hydrocarbons. If this sludge should form on the windings themselves it tends to produce overheating. Certain high-grade qualities are called non-sludging oils, and these should be used in transformers in which the working temperature of the oil exceeds 80 C.
The use of the oil-expansion chamber reduces the tendency of the oil to form sludge, since the access of atmospheric oxygen is effectively prevented.
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Notes
equipment [I'kwipmant] - ,
transformer -
to immerse [I'mars] -
twofold ['tu:fould] - ,
insulator -
whence [wens] -
to dissipate [disipeit] -
corrugated ['korugeitid] -
fin [fin] -
to dispense [dis'pens] -
immersion -
adversely -
sludge - ; ;
winding -
tendency - ;
access - ; ;
Comprehension questions:
1. How are the different kinds of transformers cooled?
2. What is the principle of cooling methods for different transformers?
3. Give the characteristics of water-cooling employment. Discuss them with your partner.
TEXT 12. WATTMETERS
Instruments designed to measure the amount of power passing in a circuit are known as wattmeters.
In the case of D. C. circuits the power taken is the product of the voltage and the current flowing. Ammeter readings become almost a sufficient indication of the power taken and the installation of wattmeters an unnecessary expense.
Due to a factor which must be taken into account, namely, the power factor or phase relationship between the voltage and current, the power taken by even the simplest two-wire A. C. circuit is not so easily estimated. In some A. C. circuits, then, the installation of a wattmeter is necessary if the power is to be measured.
Common forms of wattmeter, as installed on switchboards or as portable instruments, are of the direct-indicating type with pivoted moving elements. Other forms used as standard for calibration and classed as laboratory equipment have a torsion head suspension and are "indirect" reading.
Since their modes of operation are basically the same as certain types of ammeter and voltmeter, it is only to be expected that their individual mechanical and electrical component parts are identical with or bear a close resemblance to those already considered. They will, in general, be restricted by the same errors and disadvantages, and will gain the same advantages.
Notes
to design [di'zain] -
d.c. -
voltage ['voultid3] -
flow -
ammeter ['aemita] -
reading -
installation -
expense [iks'pens] -
to estimate ['estimeit] -
switchboard ['switfbo:d] -
to pivot ['pivat] -
torsion -
suspension -
identical [ai'dentikl] -
resemblance [ri'zemblsns] -
error ['era] -
Comprehension questions:
1. Characterize the instruments designed to measure the amount of power passing in a circuit.
Work with your partner.
2. What kind of types are common forms of wattmeter?
3. What component parts of voltmeters and ammeters are identical?
