Figs. 423 to 425.—Reversing the direction of rotation of a shunt motor. Fig. 423 shows the connections for counter clockwise rotation. The motor may be reversed: 1, by allowing the current to flow in its original direction through the field magnet coils (from D to C), and reversing its direction through the armature (from A to B) as in fig. 424, or 2, by allowing the armature current to flow in its original direction (from B to A) and reversing the current through the field coils (from C to D) as in fig. 425.
Ques. What may be said with respect to the speed of a shunt motor?
Ans. It is practically constant with varying loads.
The variation of speed ranges from 1/10 to 5 per cent., except in the case of small motors, in which the variation may be much greater.
Ques. How should a shunt motor be started?
Ans. To properly start the machine, the field coils must be fully excited.
It is, therefore, necessary to switch the magnet coils immediately on to the voltage of supply, while a variable resistance must be provided for the armature circuit. To get both connections at the same time, rheostats for shunt motors are arranged as shown in [fig. 418].
Influence of Brush Position on Speed.—In the case of a shunt motor supplied with current at constant pressure, the speed is a minimum when the brushes are in the neutral plane, and the effect of giving the brushes either positive or negative lead is to increase the speed, especially with little or no load.
Ques. Why does the speed increase?
Ans. When the brushes are shifted from the neutral plane, the reverse voltage between the brushes is decreased, speed remaining unchanged. Accordingly, the pressure in the supply mains forces an increased current through the armature thus producing an increased armature pull which causes the speed to increase until the reverse voltage reaches a value sufficiently large to reduce the current to the value required to supply the necessary driving torque.