What is the principal advantage of the series wound DC motor?

Question

BlackTom AI · Accepted Answer

1. Relevant concepts and knowledge points:
- In a series-wound DC motor the field winding and the armature winding are connected in series, so the same current flows through both.
- Motor torque T is proportional to the product of armature current Ia and magnetic flux Φ: T ∝ Φ · Ia.
- In a series motor Φ ∝ If and If = Ia, so torque varies approximately with the square of current: T ∝ Ia^2.
- At start, rotor speed ω ≈ 0, back EMF E = kΦω ≈ 0, so the line current is limited only by winding resistance → very large starting current and thus large starting torque.

2. Step-by-step reasoning:
- Because field and armature are in series, the starting current Ia equals the field current If.
- At start, ω = 0 so back EMF is zero; the applied voltage appears across small winding resistances, producing a large Ia.
- The field flux Φ increases with Ia, so both factors in T ∝ Φ·Ia are large. Substituting Φ ∝ Ia gives T ∝ Ia^2, meaning torque rises steeply with current.
- Therefore the motor develops very high torque at standstill.

3. Intermediate/calculation steps (symbolic):
- T ∝ Φ · Ia
- For series connection, Φ = k1 · Ia → T ∝ (k1 · Ia) · Ia = k1 · Ia^2
- At start, E = k2 · Φ · ω = 0 → Ia ≈ V / Rwindings (large)

4. Why the correct answer is correct:
- The combination of large starting current and flux proportional to that current yields a very large starting torque, making "High starting torque" the principal advantage.

5. Why the other options are incorrect:
- "Low starting torque." contradicts the above physics; starting torque is high, not low.
- "Suitable for constant speed use." is false because series motors have poor speed regulation; speed varies widely with load and can run away at no-load, so they are unsuited for constant-speed applications (shunt or DC compound motors are better).

6. Conclusion:
All reasoning shows the principal advantage is high starting torque, which matches the provided correct answer: High starting torque.

What is the principal advantage of the series wound DC motor?单项选择题

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