Ques. What advantage is obtained with series motors with respect to the connections?
Ans. A single wire only proceeds from the rheostat to the motor, so that, with the return wire, only two wires are required.
Ques. For what service are series motors specially adapted?
Ans. Series motors are used principally for electric railways, trolleys, and electric vehicles, and similar purposes where an attendant is always at hand to regulate or control the speed. They are also used on series arc light circuits in which the current is of constant strength. Very small motors are generally provided with series windings.
Fig. 419.—Speed regulation of a shunt motor. The speed of a motor depends on the voltage of the current supplied and the field strength. The motor tends to rotate so fast as to produce a reverse voltage nearly equal to that supplied to the brushes; hence, the speed varies with the voltage supplied. By decreasing this voltage then, the speed is decreased. Accordingly, the speed may be reduced by inserting, by means of a rheostat, a resistance in series with the motor. By inserting this resistance in the field circuit, the voltage at the terminals of the motor is lowered, thus giving the condition necessary to reduce the speed. The arrangement for speed regulation shown in the figure includes a starting regulator and a shunt regulator.
Shunt Motors.—A shunt motor may be defined as one in which the field coils are wound with many turns of comparatively fine wire, connected in parallel with the brushes. The current then is offered two paths: one through the armature, and one through the field coils.
Figs. 420 to 422.—Reversing the direction of rotation of a series motor. Fig. 420 shows the connections for counter clockwise rotation. The motor may be reversed: 1, by allowing the current to flow in its original direction (from D to C) in the field magnet coils, and altering the direction of the armature current by changing the two connections on the brushes A and B, thus connecting C to A and B to the return wire as in fig. 421, or 2, by leaving the direction of the current in the armature in its original direction, and reversing that of the field current, as in fig. 422. If the wires leading to the rheostat and motor directly, were reversed there would be no reversal of the motor, because by so doing, both the armature and field magnet currents would be reversed.
