Correct Answer - Option 2 : interchanging field terminals
The torque produced in DC motor is,
T ∝ ϕIa
ϕ is the main field flux
Ia is the armature current
DC series motor:
In the case of DC series motor, the field and armature winding are connected in series. If the polarity of the input voltage or supply terminals are interchanged, then it changed the direction of the field and armature current both get changed so the direction of speed will remain the same.
The direction of rotation of DC series motor may be reversed by changing the polarity of either field or the armature winding.
Separately excited DC motor:
The direction of the torque and the speed can be reversed by changing the direction of either the field current or armature current by changing the polarity of the DC voltage. However, this method is used for separately excited DC motor where it is possible to change the polarity of the field or armature.
