Q1Electric Drives

A well-designed closed-loop DC drive control system uses a cascaded (nested) control structure, comprising an inner, fast current control loop and an outer, slower speed control loop, together with the necessary sensing, power-conversion, and reference-generation components.

Component Selection

Power converter: a chopper (for a DC supply source) or a fully-controlled thyristor bridge rectifier (for an AC supply source), selected based on the drive's required power rating, supply type, and whether regenerative/four-quadrant operation is required (requiring a bidirectional-capable converter topology, as discussed elsewhere in this paper).

Speed feedback sensor: a tachogenerator (an analog DC generator producing an output voltage proportional to shaft speed) or, in more modern/precise implementations, a digital incremental or absolute encoder (producing a pulse train or digital position code from which speed is computed) — the encoder approach generally offers better precision, noise immunity, and integration with digital control systems, while the tachogenerator offers simplicity in purely analog control implementations.

Current feedback sensor: a current transformer/shunt resistor with isolation amplifier, or a Hall-effect current sensor, measuring the actual armature current for use in the inner current control loop and for overcurrent protection.

Controllers: PI (Proportional-Integral) controllers are standard for both the current and speed loops, chosen for their ability to eliminate steady-state error (via the integral term) while providing adequately fast, well-damped transient response (via appropriately tuned proportional and integral gains) — implemented either as dedicated analog PI controller circuits or, in modern drives, as digital control algorithms running on a microcontroller or DSP.

Control Structure and Method

The outer speed loop compares the desired reference speed against the actual measured speed (from the tachogenerator/encoder), and the resulting speed error is processed by the speed PI controller to generate a current reference command — this current reference represents the armature current needed to produce the torque required to correct the speed error. The inner current loop then compares this current reference against the actual measured armature current (from the current sensor), with the resulting current error processed by the (typically faster-responding) current PI controller to generate the final duty-cycle/firing-angle command sent to the chopper/converter power stage, which applies the corresponding armature voltage to the motor.

How the Feedback Loop Maintains Desired Performance

This cascaded structure maintains the desired speed regulation and dynamic performance through continuous, automatic error correction: any disturbance affecting the motor's actual speed (such as a sudden change in load torque, causing speed droop as discussed in earlier answers) is immediately detected by the speed feedback sensor, generating a speed error that the outer speed loop's PI controller responds to by adjusting the current reference (and hence the actual armature current/torque) to counteract the disturbance and restore the actual speed toward its commanded reference value. The inner current loop, operating with a much faster response time than the outer speed loop (a standard cascaded-control design principle, ensuring the inner loop settles quickly relative to the outer loop's dynamics), provides fast, tight regulation of armature current — both for accurate torque control and for essential current-limiting protection (preventing excessive current during transients, such as a sudden large speed-reference step change, from damaging the motor or power converter) — while the integral action within each PI controller ensures that, in steady state, both the current and speed errors are driven to exactly zero, giving precise, disturbance-rejecting closed-loop speed regulation that would not be achievable with a simple open-loop (no-feedback) control approach.