Ans. Voltage above normal is a possible cause of heating, and operation below normal speed calls for an increase of field strength and reduces the effective ventilation, thus tending to cause heating.
Fig. 730.--Forced system of lubrication as applied to engine of the generating set shown in fig. 443. In engines employing the forced system of lubrication the crank pit, which is formed by the columns, is accessible through doors in the front and back of the engine. The base of the engine forms an oil tank to which is attached a small plunger pump driven by an eccentric on the shaft. The lubricant is carried under pressure to the various parts of the engine by the mechanism shown in the accompanying diagram. The oil is forced by a pump to a groove in the main bearing, and a drilled hole in the shaft connects this groove with the crank pin. From the crank pin box the oil is further forced to the wrist pin through the pipe running along the side of the connecting rod. The passage in the crosshead allows the oil to be forced from the wrist pin to the guides. As the oil is forced from one bearing to another, it is quite important that the bearing caps be set tight, otherwise the oil will escape before reaching the last bearing. After passing through the bearings, the oil is collected in the base, strained and used again. The oil should be free from foreign substances, and to guard against the introduction of any foreign matter, a strainer, which may be taken out for examination or cleaning, is attached to the suction valve of the pump. An oil pressure of from 10 to 20 lbs. should be maintained, and may be regulated by adjusting the set screw on the relief valve of the oiling system. The pressure gauge need not remain in the circuit continuously. Only mineral oils should be used for lubrication. A heavy oil gives better results and prevents knocking more effectively than thin oil. An oil which has been found to give good results, consists of two-thirds red engine oil and one-third heavy cylinder oil. As the oil passes through the bearings repeatedly, it gradually loses its lubricating properties, becoming thick and gritty, and should be occasionally run through a filter and mixed with new oil. The frequency of this change depends on the oil, as well as the number of hours the engine is in operation, and can easily be determined by observation. The oil in the reservoir should stand about 2 inches over the suction and discharge valves, and no water should be allowed to mix with it. Should any water accumulate in the base, it should be drawn off by the cock provided for the purpose before starting the engine.
Ques. How may the field magnets become heated?
Ans. By excessive field current; eddy current in pole pieces; moisture; short circuits.
Ques. What may be said with respect to excessive field current?
Ans. When heating results from this cause, all the exciting coils will be heated equally. It may be due to excessive voltage, in the case of shunt dynamos; or to an overload in the case of compound and series dynamos. In either case it may be remedied by reducing the voltage or overload. If due to the coils being incorrectly coupled up, that is, coupled up in parallel instead of in series, it will be necessary to rectify the connections or insert a resistance in series.
Ques. State the causes of eddy currents in the pole pieces?
Ans. This fault may be due to defective design or construction of the armature. Slotted armatures are particularly liable to cause this fault, if the teeth and air gap be not properly proportioned. The defect may also be occasioned by variation in the strength of the exciting current.