How does increasing load torque affect speed in a separately excited DC motor operated at constant supply voltage?

How does increasing load torque affect speed in a separately excited DC motor operated at constant supply voltage?

• A. Speed increases as load increases.
• B. Speed remains constant regardless of load.
• C. Speed fluctuates unpredictably with load.
• D. Speed decreases as load increases until speed-torque balance is reached.

Answer: (D)

With constant supply voltage, a separately excited DC motor’s speed is set by the back-EMF and the armature current. The back-EMF E = kφn depends on speed, and the applied voltage satisfies V = E + I R. If the field current is fixed, φ stays basically constant, so increasing the load torque requires more armature current to produce the needed torque (T ∝ I). That extra current raises the I R drop, leaving less voltage for back-EMF, which lowers speed. The motor settles at a new steady state where the torque it produces matches the load torque, at a slower speed.

With constant supply voltage, a separately excited DC motor’s speed is set by the back-EMF and the armature current. The back-EMF E = kφn depends on speed, and the applied voltage satisfies V = E + I R. If the field current is fixed, φ stays basically constant, so increasing the load torque requires more armature current to produce the needed torque (T ∝ I). That extra current raises the I R drop, leaving less voltage for back-EMF, which lowers speed. The motor settles at a new steady state where the torque it produces matches the load torque, at a slower speed.