In the torque-load characteristic curve of a DC series motor, what happens as the load increases?

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RRB JE EE 22 Apr 2025 Shift 1 CBT 2 Official Paper

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Answer 1

Torque-load characteristic curve of a DC series motor

In a DC series motor, the torque (T) is approximately proportional to the square of the armature current (I_a) when the magnetic circuit is unsaturated, i.e. (T α I_a²)
As load increases, the motor slows down, increasing the armature current.
Since the field winding is in series with the armature, an increase in current also strengthens the magnetic field.
This results in a rapid increase in torque, making the motor well-suited for applications requiring high starting torque (e.g., electric trains, cranes).
At very low or no load, the motor can overspeed dangerously because the torque is low and there's insufficient counterforce — this is why DC series motors must never run without load.