To control the DC series motor this is another way called series parallel technique. This is the method normally used in traction by connecting two or more than that of the series motor are couple mechanically at the same load.
Whenever the series motors are connected in sequence (series) like shown in the figure, each and every armature of the motor receive the one-half of the rated voltage. Thus the speed will be less. If the series motors are connected in parallel, each and every armature of the motor receives the full normal voltage and hence the speed is also high. Thus we can achieve the two speeds (low or high) by connecting the motor either in series or parallel. Note for the same load on the pair of motors, the speed of the system would run nearly 4 times once motors are in parallel as while they are in series.
Series Parallel and Resistance Control Method
This series parallel method is generally employed with the resistance control. For example two series motor are mechanically coupled to drive a vehicle.
At cessation, through a starting resistance the motors are connected in series. The series motors are taking place up in series respectively and the starting resistance is gradually cut out step by step in rise the speed. When the total resistance is cutout step by step the voltage supplied to each one of the motor is about one-half of the line voltage and the speed is also increased about one-half times.
Further to increase the motor speed, then each motors are to be connected in parallel and the resistance to be connected in series at the same time. The starting resistance is again gradually reduced till full speed of the motor is achieved. At that moment field control is attained.