Final answer:
A series DC motor is commonly used for reciprocating engine starters because it provides high torque at low speeds, which is essential for initiating the engine's cycle. The series motor's ability to generate a high starting current that translates into the necessary starting torque makes it the preferred choice over other types of motors.
Step-by-step explanation:
The type of DC motor commonly used for a reciprocating engine starter is a series motor due to its ability to provide high torque at low speeds. This characteristic is vital for starting an engine as it requires a significant amount of torque to initiate the engine's cycle from a standstill. Other types of motors such as shunt motors, compound motors, or synchronous motors do not offer the same level of starting torque, making them less suitable for this application.
When discussing series-wound DC motors and their characteristics, it's also important to understand elements like back emf and resistance. For instance, in a series motor, when the speed increases, the back emf also increases, reducing the net voltage across the armature. Therefore, the current through the motor decreases accordingly. High starting current is crucial for providing the necessary starting torque, which can be clearly seen in a series motor's behavior.