What is a characteristic of a Series DC Motor?

• A. Low starting torque
• B. High starting torque; used in traction (train motors)
• C. High speed regulation
• D. Constant speed operation

Answer: (B)

In a Series DC motor, the field winding is in series with the armature, so the magnetic flux grows with the operating current. At startup, there is little back electromotive force to oppose the supply, so current surges and the motor develops a strong magnetic field. Since torque depends on both current and flux (roughly T ∝ Φ Ia, and Φ grows with Ia), the starting torque is very high—much larger than in other types of DC motors. This makes it ideal for traction applications, such as train motors, where a strong initial push is needed to start moving heavy loads. A key trade-off, though, is that speed regulation is poor: as load changes, the speed can vary widely because back EMF is tied to speed and flux to current. So, this characteristic—high starting torque with suitability for traction—is the defining feature.