DIRECT CURRENT; ELECTRICITY and TRICAL ALTERNATING CURRENT MACHINERY.
The small electric motor has displaced the steam engine in a great many uses, and con tinues to supplant it. In 1909 there were more than half a million small motors made for man ufacturing establishments in the United States. with a total horse power of nearly 3,000,000 and a valuation of $32,000,000. More and more power is being produced in large plants utiliz ing some waterfall to rotate dynamos, and sending out current, much of which is used for small electric motors, driving individual ma chines. Users of machinery in almost all lines now accept it as the best practice to equip all large machines with individual motors, and in this way not only make their operation inde pendent of other machines, but also save the ex pense of surplus power and of running shaft ing and belts when the machines are idle. While electric motors have cost more in some cases than steam or gas engines, and while the charge for electric power in many localities is higher than for direct steam power, yet the convenience and the doing away with cum bersome overhead shafts and belts has more than offset these. The individual motor per mits machines to be located without reference to shafting lines, and makes for greater effi ciency and output. With each added machine a new motor comes, and there is no throw ing out of central engine plants to make way for larger. The small electric motor may be built in as a part of the machine, or it may be bolted to the floor or ceiling. It may be di rect-connected if the speed of the machine is appropriate, or it may be connected by reducing gears, rendered nearly noiseless by introducing rawhide. Belt connection is often used direct or through cone pulleys, and where the power transm:tted is light, a friction connection is made satisfactory by covering the smaller of the opposed pulleys with a leather face. Where
machines are small it is common to group them for motor-driving; four or six machines will all be coupled to one motor, conveniently located in the centre of the group.
.Manufacturers wind motors in an almost in finite number of ways to suit an endless variety of conditions. By placing resistances in the shunt circuit, a considerable variety of speed is permitted. For printing presses a motor is made with a continuous current transformer of variable ratio. This permits a slow speed with out loss, as a small current at full pressure is instantly convertible to a large current at low pressure. Continuous current series motors are preferred when the work is irregular, as in starting under heavy load, as with a crane hoist, or when encountering extra load from f ric tion or dampness, as with cotton spinning ma chines on a day of high humidity. Single-phase alteniating current motors have special uses, but are not well adapted to sudden changes of load. Motors that are both shunts and series wound are in large demand. This type is used on passenger elevators. The driving drum that carries the wire rope that hoists and lowers the elevator is commonly driven by this type of motor. To secure the needed gear reduction without grinding noise, a steel worm and gun metal spur gear are run in oil. When the operator pulls the rope or lever in the elevator he turns the starting switch or controller on or off. A band-bralce on the motor shaft guards against racing, and is normally held out of operation magnetically. Thus if the current fails the bralce slows up the elevator. An au tomatic switch is placed to shut off the current at the top and bottom of the elevator's travel.
See ELECTRICAL MANUFACTURING INDUSTRY.