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Overload Capacity-One hears much unfavorable comment on the gas engine for its inability to maintain heavy overloads. Perhaps the turbine has spoiled us in this respect. It certainly
. appeals to the man who buys turbines "on margin," as it were, with the deliberate purpose of overloading them beyond normal limits, thinking that he will in this manner realize greater returns on his investment, even though the life be shorter and the repair bills heavier. This can hardly be termed either good engineering or good management.
Now, there is more or less justice in the general criticism, but not to the extent that some would have you believe. First, does it pay in the end to make a practice of running apparatus overloaded most of the time? Certainly not with the steamengine unit, on account of the increased steam consumption. Second, in a well-equipped lighting plant, how much overload capacity is actually called into requisition during regular operation? Does not the careful engineer put on more units when the load approaches rating? And even assuming the absence of spares, for what duration of peak is overload capacity required? At the maximum, two hours, usually less. This whole question reduces to one of load characteristics; i. e., how closely the maximum load and maximum fluctuations may be anticipated. In a lighting load the fluctuations are very small, and in a railway load they are correspondingly large.
The maximum capacity of a gas engine is evidently reached when the cylinder has taken a full charge of mixture of the highest possible heat value and density; i. e., containing the maximum B. t. u. per cubic foot. This being the case, it is evident that a considerable fall in the heat value of the gas will also reduce the maximum B. t. u, in the cylinder, although not in the same proportion, for the heat value of the mixture changes but slowly with wide variations in the heat value of gas. However, the maximum capacity of the engine is decreased somewhat with poor gas and presents a contingency that the engine builder must provide for, although the responsibility should be entirely on the producer operator.
The above point is responsible for the method of overload rating of Westinghouse gas engines. A maximum is assumed for an average quality of gas, which maximum is deliberately taken somewhat under the possible maximum in order to provide for a possible drop of some 15 or 20 per cent in the heat value of the gas. Given this fixed maximum, we now have a startingpoint for ratings. For a lighting load, it is assumed that 10 per cent overload is sufficient, hence a 550-hp' maximum engine is rated at 500 horse-power. The corresponding generator rating is now a matter of judgment. A standard generator has 25-per cent continuous overload capacity on 50-degree rise. If the unit runs, on the average, considerably below its rating, it is quite evident that a somewhat smaller generator may safely be used
and still keep within a reasonable temperature rise. But, on the other hand, where a unit will be fully loaded most of the time,
a it is evident that nothing will be gained by a smaller generator, as the heating will then exceed standard limits.
It is then a question for the purchaser to decide whether he wishes 10-per cent or 25-per cent overload capacity on a gasengine unit. He pays for it pro rata.
Automatic Load Regulator-A number of devices have been proposed for securing indirectly the desired overload capacity above-mentioned. Most of these have involved mechanical or commercial impossibilities. But the question has recently been attacked from another standpoint entailing the well-known characteristics of the storage-battery auxiliary in a novel fashion. This new system was devised particularly for direct-current work, and can be applied to alternating-current work with suitable converting apparatus. It is intended for any type of plant afflicted with extremely variable loads, and has for its object the maintenance of a constant load on the prime mover irrespective of the outside fluctuations, which are entirely carried by
the storage battery. In connection with the battery, a differentially-wound motor-driven booster is employed with directconnected exciter. Thus far the system presents nothing new. But unlike any of the numerous booster systems, the electrical control of the entire plant is vested in an automatic regulator of the relay type, illustrated in Figure 8. This regulator controls the booster exciter in such a manner that, without changing the main generator fields, outside loads many times the generator capacity are instantly transferred to the battery up to its maximum discharge rate while the generator maintains a constant output, charging the battery during light loads and assisting it during heavy loads. The novelty of the system lies in the relay regulator, which secures equilibrium of potential without manual adjustment, irrespective of generator or battery characteristics; i. e., it compensates automatically for a drooping battery characteristic without re-adjusting fields.
This result can best be shown by photograph, Figure 9, of a temporary switchboard arrangement*--regulator and generatorload meter are in the centre, battery-load meter at the right and total-load meter at the left. In the test, the load was set at 300 amperes by means of a water rheostat and the total load was suddenly varied from zero to 1200 amperes. The shadow of the needle at rest at these points indicates the range, A to B. This change threw the battery load from 300 amperes charge to goo discharge, also shown by the needle shadows C-D. But meanwhile the generator was undisturbed and held constant at 300-ampere load, E. The result of this operation is that during a sudden change in load corresponding to 300-per cent overload on the generator, the latter was automatically maintained at its full rating without the slightest attention of the operator or hand adjustment of any kind. Owing to the sluggishness of the metal in the fields, a small lag in the action of the regulator occurs, varying with the rapidity of fluctuation. In the crude apparatus on which these tests were made, absolute equilibrium was established within two seconds after the most violent change in load, but with the proper material and design of field laminations, even this small lag may be much reduced. Moreover, the generator used in the test had no flywheel, so it is a safe assumption that the lag in a large engine-type equipment under ordinary loading would be entirely negligible.
* Since these results were obtained, further tests with a complete equipment of curve-drawing instruments have been made, with the results shown in Figure 10. The upper curve represents the generator load, 'the middle the battery load and the lower the total external load. As before, the load was entirely rheostatic and varied instantaneously over a range of 1600 amperes. Over this range, which corresponds to an output of four times the generator rating, the instantaneous fluctuation was generally less than 50 amperes above and below mean, and this for only about one second. For the ordinary fluctuations which occur in the normal power plant load, the regulator produced a uniform generator output, as is shown from 6 to 7 p. m. It should be noted that this test lasted four hours, with battery discharge rates many times higher than allowable in commercial practice. During this test, no field or regulator adjustments were made, so that the generator load line truly represents the ability of the regulator to control the entire system under the most severe operating conditions.
This regulator controls not only for full generator load, but for all loads. At the lower right-hand corner, Figure 8, is a five-step resistance covering the entire range of generator output. Intermediate steps are obtained by the lever just above. By this means the average generator load may be adjusted to such a point that the hills and valleys of the load curve will just equalize and the battery be maintained at an efficient state of charge.
The apparatus for this system is quite as standard as other forms of regulating apparatus for constant voltage or constant
load. Moreover, it may be applied to existing plants without extensive changes in the 'bus structure.
As all the power required to operate it is derived from a small ammeter shunt in the 'bus, standard shunts are used for this purpose, so that with calibrated leads the regulator may be located at any desirable point.
Evidently this apparatus has a most useful field of application in gas-engine work. While avoiding the racking strains of a violently-fluctuating load, it also enables the engine to operate at its high full-load efficiency without incurring the expectation