The direction of rotation of a DC series motor (although the term "DC series" is not common in conventional motor classifications, we can understand it as a DC motor in which the current flows through the series circuit of the motor) can also be changed by changing the direction of the current.
In a DC motor, when current flows through the motor's coils, the coils are subjected to a force in the magnetic field that causes them to rotate.
The direction of this force depends on the direction of the current and the direction of the magnetic field. Since the direction of the magnetic field is usually fixed in a DC motor (generated by permanent magnets or electromagnets), changing the direction of the current can change the direction of the force on the coils, thereby changing the direction of the motor's rotation.
Specifically, the direction of rotation of a DC motor (including series type) can be changed in the following ways:
1. Reversing the polarity of the power supply: The most direct way is to change the positive and negative connections of the power supply, that is, reversing the polarity of the power supply. In this way, the direction of the current will change, and the direction of rotation of the motor will also change.
2. Changing the direction of the armature current: In more complex circuits, there may be a controller or commutator to control the direction of the current in the armature (i.e., the coils of the motor). By changing the setting of this controller or operating the commutator, the direction of the armature current can be changed, thereby changing the direction of rotation of the motor.
3. Adjusting the direction of the magnetic field (although uncommon): Although changing the direction of the magnetic field is not a common method of changing the direction of rotation in a DC motor (because the magnetic field is usually generated by a fixed permanent magnet or electromagnet), in some special cases, if the magnetic field can be changed by external control, then the direction of rotation of the motor can also be changed by adjusting the magnetic field direction. However, this method is uncommon in conventional DC motors.
In summary, for DC motors (including what may be called "DC series" motors), the most common and direct way to change their direction of rotation is to reverse the polarity of the power supply.