Q3Electric Drives

As discussed in greater detail elsewhere in this subject area, armature voltage control operates a DC motor with its field flux held constant at the rated value, varying only the armature terminal voltage Va (typically via a chopper, for a DC supply, or a phase-controlled thyristor rectifier, for an AC supply) to control motor speed according to the standard DC motor speed equation N=(Va-IaRa)/(Kφ).

Advantages of Armature Voltage Control

Constant torque capability: since field flux φ remains at its rated value throughout the armature-voltage-controlled speed range, the motor's maximum available torque (T=Kφ·Ia,max) remains unchanged at any controlled speed within this range — the motor can deliver its full rated torque from very low speed (even near standstill) up to base speed, making armature voltage control ideally suited to applications requiring high torque across a wide low-to-medium speed range.

Simple, well-established implementation: armature voltage control using a chopper or phase-controlled rectifier is a mature, well-understood technique, straightforward to implement with standard power-electronic converter topologies and control techniques (including the cascaded current/speed PI control structure discussed elsewhere in this paper).

Good speed regulation with feedback: when combined with closed-loop speed feedback control, armature voltage control can achieve excellent, load-independent speed regulation accuracy, automatically compensating for the natural armature-resistance-drop-induced speed droop that would otherwise occur in open-loop operation.

Limitations of Armature Voltage Control

Limited to speeds at or below base speed: armature voltage control alone cannot increase motor speed beyond the point where armature voltage reaches its maximum rated value (base speed) — achieving higher speeds requires switching to flux-weakening control instead (as discussed in detail elsewhere in this paper), introducing additional control complexity and a corresponding reduction in available torque at these higher, flux-weakened speeds.

Reduced low-speed performance/resolution: at very low commanded armature voltages (very low speed operation), the armature resistance voltage drop (IaRa) becomes a comparatively larger fraction of the total armature voltage, making the actual achieved speed more sensitive to load-current variation and to any inaccuracy in the assumed/measured value of Ra, somewhat degrading speed control precision and regulation quality at very low speeds compared to mid-range speeds — this effect is generally compensated for in a well-designed closed-loop control system through IR-compensation techniques or, more robustly, through direct closed-loop speed feedback (rather than relying on open-loop voltage control alone) at low speed.

Converter cost and complexity: the power-electronic converter (chopper or controlled rectifier) required for armature voltage control adds cost and complexity compared to simply running a motor directly across a fixed supply, though this cost is universally accepted as necessary and justified given the substantial operational benefits (efficient, precise, continuously variable speed and torque control) that armature voltage control provides over any fixed-speed alternative.