Which DC motor type has current that increases with torque?

Which DC motor type has current that increases with torque?

• A. Shunt Motor
• B. Compound Motor
• C. Series Motor
• D. PM Motor

Answer: (A)

In DC machines, torque is produced by the interaction of the armature current with the magnetic field, and the relationship is T ∝ Φ Ia. In a shunt motor, the field flux Φ is essentially constant because the shunt field is connected in parallel to the supply. That means the torque is predominantly controlled by the armature current: T ∝ Ia. When the load torque increases, the motor slows, back-EMF drops, and the armature current rises to generate more torque. So, as torque demand grows, the current through the armature increases, which makes this motor type the one where current increases with torque. Understanding the others helps: a series motor has a field that follows the armature current, so Φ changes with Ia and the torque–current relationship is more interdependent (roughly T ∝ Ia^2). A compound motor combines both behaviors, and a permanent-magnet motor has a fixed flux like a shunt motor but with different practical characteristics. The key point here is that with a constant flux, increasing torque requires more armature current, which is why the shunt motor fits this behavior.

In DC machines, torque is produced by the interaction of the armature current with the magnetic field, and the relationship is T ∝ Φ Ia. In a shunt motor, the field flux Φ is essentially constant because the shunt field is connected in parallel to the supply. That means the torque is predominantly controlled by the armature current: T ∝ Ia. When the load torque increases, the motor slows, back-EMF drops, and the armature current rises to generate more torque. So, as torque demand grows, the current through the armature increases, which makes this motor type the one where current increases with torque.

Understanding the others helps: a series motor has a field that follows the armature current, so Φ changes with Ia and the torque–current relationship is more interdependent (roughly T ∝ Ia^2). A compound motor combines both behaviors, and a permanent-magnet motor has a fixed flux like a shunt motor but with different practical characteristics. The key point here is that with a constant flux, increasing torque requires more armature current, which is why the shunt motor fits this behavior.