Fig. 218.
This is a two-pole machine, direct current; the figure is introduced to show the “parts” only—as this dynamo has been largely superseded by others of the four pole type.
Note.—A, Magnet yoke; B, Magnet and field piece; C, Pole piece; D, Zinc field piece; E, Armature; F, Commutator; I, Quadrant; JJ, Brushes; K, Adjusting handle for the brushes; L, Switch pivot; M, Pilot lamp receptacle; N, Negative lug; O, Switch lever; P, Positive lug; Q, Positive terminal; R, Negative terminal; S, Negative rod; T, Pole piece; UU, Bearings; X, Slides for belt tightener; VVV, Driving pulley; Y, Connecting blocks, one on each side of machine.
An electric motor is a machine for converting electrical energy into mechanical energy; in other words it produces mechanical power when supplied with an electric current; a certain amount of energy must be expended in driving it; the intake of the machine is the term used in defining the energy expended in driving it; the amount of power it delivers to the machinery is denominated its out-put.
The difference between the out-put to the intake is the real efficiency of the machine; it is well known that the total efficiency of an electric distribution system, which may include several machines, usually ranges from 75 to 80 per cent., at full load, and should not under ordinary circumstances fall off more than say 5 per cent. at one-third to half load; the efficiency of motors varies with their size, while a one horse-power motor will, perhaps, have an efficiency of 60 per cent., a 100 horse-power may easily have an efficiency of 90 per cent. and the larger sizes even more.
The general and growing application of electric power to the driving of all kinds of machinery including pumps makes the question of motor driving one of the most important in the power field. For many purposes, a single speed is sufficient, but for others, it is imperative that the speed should be variable; and for still others, though not absolutely necessary, a speed adjustment is very desirable.
While the direct-current motor has been in this field so long that its properties are well known and its possibilities fully developed, in the operation of motors located in the immediate neighborhood of the generator the alternating-current motor has marked advantages where a large area of territory has to be covered and the conditions are nearly uniform, that is to say—
Where the current has to be transmitted a long distance and the load is approximately constant, the alternating system is preferred, as it can be operated with small main lines or conductors. This effects a saving in copper, over the direct system which requires larger conductors.