Ans. This method of regulating the speed of a series motor is shown in [fig. 757]. The current through the armature will flow through all the field windings when the position of the switch lever S, is on contact 4, and the strength of the field will be the maximum. By moving the arm to contact 3, 2, etc., sections of the field winding are cut out, thus reducing the strength of field and varying the speed.
Fig. 756.--Speed regulation of shunt motor by variable resistance in the armature circuit.
Ques. How does the speed vary with respect to variation of field strength?
Ans. Decreasing the field strength of a motor increases its speed, while increasing the field strength decreases the speed.
Under the conditions of maximum field strength, as with switch S on point 1, the torque will be greatest for any given current strength and the reverse voltage also greatest at any given speed. The current through the armature of the motor, to perform any given work, will thus be a minimum, as well as the speed at which the motor has to run, in order to develop sufficient reverse voltage to permit this current to flow. Regulation of speed by varying the field strength is limited in range of action, since the field saturation point is soon reached, moreover, with too low a field strength, armature reaction produces excessive field distortion, sparking, etc.
Fig. 757.--Speed regulation of series motor by cutting out sections of the field winding. In this method the field winding is tapped at several points, dividing the coil into sections and the leads from these points are connected a multi-point switch of the type that would be used on a rheostat. By moving the lever S, to the left or right, the current will flow through one or more sections of the field winding, thus decreasing or increasing the ampere turns and thereby providing means of regulation.