Modern Gasworks Practice

W5 CO MODERN GASWORKS PRACTICE it is possible that the commutator is dirty and is preyenting excitation, or some of the connections in the field coils may have become loose. The whole of the field coil connections should be carefully inspected and tightened up, and the commutator and brush gear should be thoroughly cleaned. If the machine does not then excite, it is advisable to commimicate with the makers. (1) Reversal of Polarity of Dynamos.—Dynamos driven by gas engines are sometimes liable to have their polarity reversed owing to the gas engine failing, when they are driven as a motor by the other dynamos that may be running. This will not occur if the switchboard is provided with properly acting reverse current circuit breakers. An indication of this trouble is given, by the voltmeter trying to read in the wrong direction when the machine is nm up again for starting. The polarity must be put right before any further attempt is made to run the machine, for, if the dynamos are put in parallel under thesc circumstances, a serious accident may occur. (7) The fault may be with the prime mover, which. may not be working up to speed. If the drive is by belt there may be excessive slip. (8) Breakdowns on Motor Field Coils.—The trouble may be in the field coils. A -faulty coil can nearly always be located owing to its heat. (9) If the motor lias a field coil on the bottom special care should be exercised to avoid oil collecting round it. The insulation may soon be afEected in this way. In general, it may be said that normal attention to the machinery, keeping the commütators and brush gear clean and renewing the brushes when necessary will prevent most of the troubles that may arise, and where serious breakdowns have occurred it is advisable to communicate with. the makers rather than to attempt to efiect repair without specially skilied assistance. A testing ammeter to enable the load taken by any motor to be measured is a very useful piece of apparatus to keep on the works, and a “ megger ” for measuring insulation resistances is certainly desirable on a large installation. USEFUL MEMORANDA A Volt is the unit of electrical pressure or electro-motive force. An Ampere is the unit of eurrent strength, i.e. the unit in which. the rate of flow is measured. The eurrent strength varies direetly as the pressure producing it, and inversely as the resistance opposed to its flow. Volts Amperes = —----- Ohms An Ohm is the electrical unit in which resistance is measured. Watts = volts x amperes. A Board of Trade unit — 1 kilowatt-hour. A Horse-power= 746 watts, or 1 kilowatt = 1-34 h.p. 1 Kilowatt-hour = 1-34 h.p. hours= 3,440 British thermal units.