To reduce the speed of the direct current motor it is only necessary to add resistance to the armature circuit so as to limit the current flowing therein, and by so doing almost any de sired speed may be obtained, from 1 per cent up to 4u11 rate of speed. There are a number of other methods by which variable speeds can be obtained, one of them being by varying the field strength. Any motor, however, operating at a lower field or armature current than nor mal conditions would require is naturally oper ating at reduced power. On account of the valuable features in relation to speed control, reversibility and the automatic speed control inherent in the shunt machine, together with the large torque of the series machine, the di rect current motor fulfils more nearly than any other the practical requirements in machine shops, textile mills and general manufacturing establishments.
For electric railway work, in which the di rect current is employed (see TRAcrioN, ELEC TRIC), the compound wound generator and series motor is the usual standard practice. Often this type of generator is overcom pounded so as to more than overcome the drop through the armature resistance and allow higher voltage at full load than at no load, so as to overcome, in a way, the drop of potential on the feeders and preserve the uniform vol tage over the system. In lighting and power work the shunt and compound dynamos are both used. (See ELECTRIC LIGHTING). And in the business centres of our large cities where the direct current is generally used, the rotary converter fed from a high tension alternating central station is very often employed, together with storage battery.
Direct current was more generally used in the earlier installations of electric distribution, in preference to alternating current, for the reason that the direct current motor was de veloped before the alternating current motor; and the earlier motors possessed many advan tages in their ability to be operated at any speed from slow speeds up to the maximum speed for which the motor was designed, and also permitted the use of storage batteries di rectly connected to the system, thus ensuring continuity and reliability of service.
The shunt and series motor each has its own field of usefulness. When a very powerful starting torque and rapid acceleration are nec essary the series motor is used, as in the case of street railway, electric locomotives, electric cranes; and on steamships where the direct current alone is used, as on the Kentucky and Kearsarge, of the United States navy, not only is electricity used for lighting, but also for operating ammunition hoists, hoisting anchors, operating boat cranes, and even the steering gear of the ship itself.
In machine-shops and manufacturing estab lishments where a more or less constant speed may be required, and in elevator work, the com pound and the shunt motor are commonly em ployed. The shunt motor is very well adapted for operating at any speed desired, and for ma chine tools it is at present without a peer for an efficient and easily regulated source of power. Unlike the series motor, where the speed varies with the load, the shunt motor is practically a constant speed machine. When thrown on the lines it rapidly comes up to normal speed, and then from no load to full load will not greatly deviate therefrom unless purposely thrown to a slower point by the controller. As a series motor would run away if left in a circuit with a load suddenly removed, the shunt motor, or sometimes the compound (which is used in order to preserve an absolutely uniform speed from no load to full load, and is necessary in a few places where absolutely constant speed is required) is the standard motor for driving textile machinery in large mills, factories and other establishments.
Direct current generally meets all of the re quirements of the consumers, as it is available for motors of any size; for lighting; for chemi cal action, such as charging storage batteries or in electroplating; or for electric heating.
With the large increase in the requirements of individual consumers, the advantages of direct current over alternating current are not as important to-day, for the reason that trans lating devices have been simplified so that alternating current may be converted, without serious difficulty, into direct current, for any special requirements.