Raising speed of a dc series motor fed through a full converter A DC series motor drives a load whose torque is proportional to speed (T_load ∝ ω). It is supplied from a single-phase AC source via a full-controlled converter. At 500 rpm, the firing angle is 45°. To increase the speed, what adjustment is needed?

Introduction / Context:
Armature-controlled DC drives using single-phase full converters regulate average armature voltage by changing the firing angle α. For a series motor with a speed-proportional load torque, determining the direction of change of α to raise speed is a core control concept.

Given Data / Assumptions:

• DC series motor; T_load ∝ ω.
• Full-controlled (bridge) rectifier, average armature voltage Va ∝ cos α (for continuous conduction).
• At 500 rpm, α = 45°; desire higher speed at same load law.

Concept / Approach:

For a fixed supply, decreasing α raises the average armature voltage Va, increasing current and electromagnetic torque. Since T_load rises with speed, the operating point is where electromagnetic torque equals load torque. A higher Va shifts the motor’s speed–torque curve upward, producing a new intersection at a higher speed.

Step-by-Step Solution:

Verification / Alternative check:

Drive characteristics in standard texts show speed increases monotonically as α is reduced, barring field-weakening nuances (not in series field case here). Practical controllers decrease α to accelerate.

Why Other Options Are Wrong:

• Increase α (option b): reduces Va, lowering speed.
• Option c: ambiguous; the required change is specifically to reduce α.
• Decrease load torque (option d): not a converter control action; and the question asks about firing angle adjustment.

Common Pitfalls:

Confusing displacement factor effects with average voltage control; the dominant effect on speed is Va via α.

Final Answer:

Decrease the firing angle