In the series motor, the field winding and armature winding are connected in series. (Fig. 1.6<2>).
A feature of this motor is that the rotating speed changes significantly with load variation. It generates a large torque at startup or low speeds. When the load is reduced, the motor runs at a high speed.
Such a feature is generally called series characteristics. Utilizing this feature, this type of motor has been used in specific applications such as cranes, electric trains, and elevators.
At present, series motors are replaced with induction motors or synchronous motors containing inverters for variable speed control.
This motor also rotates on alternating current, which will be introduced later in connection with the commutator motor. However, rotating a motor designed as a DC motor on alternating current increases core loss and other types of loss, causing abnormal heat generation.
Incidentally, is it possible to change a shunt motor to a series motor and vice versa by switching the connection of the field winding and armature winding?
To put is simply, the aforementioned change is not possible from a practical standpoint. This is because the shunt motor winds a fine wire a large number of turns to configure a field winding (that has large resistance), while the series motor has a field winding consisting of a thick wire turned a small number of times (that has small resistance).
If the field winding and armature winding of a series motor are shunt-connected, it causes the field current to surge, which can burn the field winding out. Also, series-winding to connect the field winding and armature winding of a shunt motor will reduce the flow of field current (= armature current), preventing the motor from operating at the designed level of performance.