Modern Gasworks Practice

ELECTRICAL PLANT IN GASWORKS 345 THE DYNAMO A dynamo is a machine for converting mechanical into electrical energy. The mechanical energy supplied induces current in tlie armature windings, the current being collected by brnsh.es and passing out for distribution to the switch-board. A small portion of the current is shunted through the field coils for magnetizing the field. The armature windings generate altemating current, and the commutator passes this out as a current of continuous flow (i.e. direct). Thus an altemating current dynamo, or “ alternator,” has 110 commutator. It is preferable when installing a gasworks dynamo to arrange for one having both series and shunt windings (compound), as this ensures a constant voltage for all variations of the load—this consideration being one of chief importance. The series and shunt windings of a compowid-wound dynamo assist one another, the series coils ensuring that the voltage does not fall when the load increases. Regulation and Control of the dynamo voltage are obtained by the use of a shunt regulator which consists of a resistance (tlie amount of whicli can be varied) which. is connected in series with the' shunt field coils. Once the series coils have been designed and tested by the makers to give a constant voltage they sh.ou.ld not be interfered with, and no regulator is provided in the series coils. SIZE OF THE INSTALLATION With regard to the size of the installation to be laid down it should be kept in mind, first, that it is necessary to arrange for more than a single plant, and, secondly, that it is not economical to work a set at much less th.an its normal capacity. In large works the ni glit load is much smaller than the day load, as little work, except that entirely inside the retort house, is carried out at night. In medium-sized works the difference in load may not be very great and, as already pointed out, in small works the load curve may be very nearly straight. In the last-named case it is advantageous to instal duplicate plant, each set being capable of dealing with the maximum load. In some instances, where the difference between the day and night load is great, it may be preferable to have two plants of capacities equal to each. period, with. a stand-by equal to the lower capacity. Then, if the smaller unit breaks down, there is a duplicate to take its place, and if the larger one requires repairs there will be two small sets for providing the power. If this arrangement is adopted provision, must be made on the switcliboard for the parallel running of the set; for, if this is not done, there will probably be trouble when the two sets are both put on. load together, more especially if the load happens to be of a variable nature. For larger plants where the total load is likely to be 80-100 kw. or over, a fairly common practice is to instal three duplicate sets so designed that two of tliem running together will be able to carry the maximum continuous load and the first set acts as a stand-by. The plant should always be arranged so that one, two, three or more sets of which it consists can run in parallel. There is no difficulty in doing this, and modem well-designed level compound-wound generators