--- title: "Two quadrant armature controlled DC drive; closed loop" subject: "Utilization of Electrical Energy" chapter: 3 key: "semester_7/utilization_of_electrical_energy:two-quadrant-armature-control-loop" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/two-quadrant-armature-control-loop site: BCT Engineering --- # Two quadrant armature controlled DC drive; closed loop Utilization of Electrical Energy · Chapter 3: Control of Electric Drive - Asked in: 2071 Chaitra - Video: [two quadrant dc chopper drives](https://www.youtube.com/watch?v=XAPgItajeJM) — Poorna Chandar Rao Punati - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/two-quadrant-armature-control-loop - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-3 ## Question Discuss the operation principle of two quadrant operation of armature controlled dc motor drive in open loop mode. What are the limitation of open loop operation? Draw a closed loop control scheme that over comes the limitations. ## Short answer A fully controlled converter gives positive current with positive or negative voltage, providing forward motoring and reverse regenerative braking. Open loop has poor speed regulation and no current limit. Closed loop uses an outer speed PI with limiter and an inner current PI to set firing angle, giving accurate speed and current limiting. ## Answer A **two-quadrant armature controlled drive** feeds the armature of a separately excited dc motor from a fully controlled converter (single-phase or three-phase), with the field kept constant. The converter can give positive or negative average voltage but current in one direction only, so the drive works in **quadrant I (forward motoring)** and **quadrant IV (reverse regenerative braking)**. #### Open-loop operation $$ V_a = V_{do}\cos\alpha, \quad N = \frac{V_{do}\cos\alpha - I_a R_a}{K} $$ - $0 < \alpha < 90^\circ$: $V_a > 0$, $I_a > 0$; power flows from ac supply to motor: **motoring** (quadrant I). Speed is set by $\alpha$. - $90^\circ < \alpha < 180^\circ$: $V_a < 0$. If the load drives the motor in reverse (e.g. lowering a hoist load), $E_b$ reverses, current is still positive, and the converter works as an **inverter**, returning energy to the supply: **reverse regenerative braking** (quadrant IV). - The operator simply sets $\alpha$ through a potentiometer and firing circuit. ```text ac --[full converter]--+--La--[ M ] field: const ^ alpha | [firing circuit] <-- V_c (set by potentiometer) ``` #### Limitations of open loop 1. Speed falls as load increases ($I_aR_a$ drop); poor speed regulation. 2. Speed changes with supply voltage variation and temperature (resistance change). 3. No current limit: a sudden change in $\alpha$ or a heavy load causes very high current, damaging thyristors and commutator. 4. Slow and oscillatory dynamic response; no protection against stall. 5. Discontinuous conduction at light load makes speed rise sharply. #### Closed-loop scheme (speed loop with inner current loop) ```text w* + +------+ Ia* +-----+ + +------+ Vc +-------+ +---------+ --->(O)-|speed |-|lim|-->(O)-|curr. |---->|firing |--->|converter|-> M -| | PI | +-----+ -| | PI | |circuit| +---------+ | | +------+ | +------+ | | | +------- Ia (current sensor) <-+ | +--------------------------- w (tachogenerator) <----------------+ ``` - **Outer speed loop**: the speed error is processed by a PI controller to give the current reference $I_a^*$; this corrects speed for load and supply changes (near zero steady-state error). - **Limiter**: clamps $I_a^*$ to the maximum safe value (e.g. 1.5–2 × rated), so starting, acceleration and braking take place at constant maximum current. - **Inner current loop**: a PI controller compares $I_a^*$ with the measured current and sets the control voltage $V_c$ (hence $\alpha$); it reacts fast and protects the motor and converter. - Often a cosine-wave firing scheme is used so $V_a$ is linear in $V_c$. This cascade control gives accurate speed, built-in current limit and fast response. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A fully controlled converter feeding a dc armature, with current in one direction only, gives which quadrants? A. II and III B. I and II C. I and IV (forward motoring and reverse regenerative braking) D. III and IV Answer: C. The converter can give positive or negative voltage but only positive current, so the drive works in quadrants I and IV. (Key point: Quadrants of operation) 2. What happens when the firing angle is between 90° and 180° and the load drives the motor in reverse? A. The converter works as an inverter, returning energy to the supply (regenerative braking) B. The motor stops immediately C. The converter acts as a rectifier supplying more power D. The field flux reverses Answer: A. Va becomes negative while Eb has reversed and Ia stays positive, so power flows from motor to the ac supply. (Key point: Regeneration condition) 3. Which is a key limitation of open-loop operation? A. The speed cannot be varied B. Speed regulation is excellent but cost is high C. There is no current limit, so a sudden change in α or heavy load gives very high current D. The converter cannot give negative voltage Answer: C. Without feedback nothing limits armature current, risking damage to thyristors and commutator; speed also falls with load. (Key point: Open-loop limitation) 4. In the closed-loop scheme, what does the outer speed PI controller output? A. The firing angle directly B. The armature current reference Ia*, which is clamped by a limiter C. The field current D. The tacho voltage Answer: B. The speed error gives Ia*, limited to a safe value; the inner current PI then sets Vc (hence α), giving accurate speed with built-in current limit. (Key point: Cascade control) ## Related in Utilization of Electrical Energy - [Closed-loop control of multi-motor drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclosed-loop-multi-motor-drive) — Utilization of Electrical Energy, chapter 2, asked 2× - [Four quadrant operation in DC motor control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afour-quadrant-dc-motor-control) — Utilization of Electrical Energy, chapter 3 - [Open loop and closed loop V/f inverter-fed drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclosed-loop-vf-inverter-drive) — Utilization of Electrical Energy, chapter 3 - [Single-phase fully controlled converter DC motor speed](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afull-converter-dc-motor-speed) — Utilization of Electrical Energy, chapter 3 - [Static Ward-Leonard four quadrant control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Astatic-ward-leonard-four-quadrant) — Utilization of Electrical Energy, chapter 3 - [Four quadrant operation of a hoist](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afour-quadrant-hoisting-mechanism) — Utilization of Electrical Energy, chapter 2 ## Related in other subjects - [Armature-controlled DC motor transfer function and block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atf-armature-dc-motor) — Control Systems, chapter 2 - [Dynamic model of armature-controlled DC motor](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adc-motor-de-dynamic-model) — Control Systems, chapter 2 - [Armature-controlled DC motor with gear and load](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adc-motor-gear-load) — Control Systems, chapter 2 - [Open loop versus closed loop control](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aopen-vs-closed-loop) — Embedded System, chapter 7, asked 6×