--- title: "Speed control methods of DC shunt motor" subject: "Electrical Machine" chapter: 4 key: "semester_4/electrical_machine:shunt-motor-speed-control-methods" source: https://www.bctengineer.com/q/semester_4/electrical_machine/shunt-motor-speed-control-methods site: BCT Engineering --- # Speed control methods of DC shunt motor Electrical Machine · Chapter 4: DC Motor - Asked in: 2071 Bhadra, 8 marks; 2069 Bhadra (asked 2 times) - Video: [speed control of dc motor | speed control of dc shunt motor | dc motor speed control | motor control](https://www.youtube.com/watch?v=BPGt3zxUn_8) — Mind of Engineer - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/shunt-motor-speed-control-methods - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-4 ## Question Describe different methods of controlling the speed of shunt DC motor (armature control and field control methods). ## Answer The speed of a DC motor is N = (V − I_aR_a) / (KΦ), i.e. N ∝ E_b / Φ. So the speed can be changed by changing the flux Φ, the resistance in the armature circuit, or the applied voltage V. #### 1. Field control (flux control) A rheostat R_f is connected in series with the shunt field. ```text (+) o----+------------------+ | | [Rf] rheostat (M) armature, Ra | | [Shunt field] | | | (-) o----+------------------+ ``` - Increasing the field resistance decreases I_f and Φ, so the speed **rises above the base speed**. - Advantages: simple, cheap, small power loss (the field current is small), and smooth control. - Limits: only speeds above normal; the range is about 3:1; commutation gets worse and the motor may become unstable at a very weak field. #### 2. Armature control **(a) Armature resistance control**: a variable resistance R_e is inserted in series with the armature. - Speed falls as R_e increases, because E_b = V − I_a(R_a + R_e) decreases while the flux stays at full value. - Gives speeds **below normal** at constant torque. - Drawbacks: a large I_a²R_e loss, poor efficiency, speed that depends on the load, and a resistance that must carry the full armature current. Used for short-duration duty or small motors. **(b) Armature voltage control (Ward-Leonard system)** ```text AC motor -> DC generator (variable field) -> DC motor (field const.) ``` - The motor armature is fed from a separate generator whose output is varied smoothly from zero to maximum by its field rheostat. - Speed varies smoothly in both directions, even through zero. Combined with field control, it gives a wide range (up to about 30:1). - Disadvantages: high initial cost (three machines), lower efficiency and large floor space. Modern drives use thyristor or chopper control instead. | Method | Speed range | Torque | Remarks | |---|---|---|---| | Field control | above base | constant power | cheap, efficient | | Armature resistance | below base | constant torque | wasteful | | Ward-Leonard | both sides | constant torque and power | costly, smooth | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which relation shows how speed can be controlled? A. N ∝ Φ I_a B. N ∝ V x Φ C. N ∝ I_a² R_a D. N ∝ (V - I_aR_a)/Φ Answer: D. Speed can be changed via Φ, armature resistance or V. (Key point: Speed relation) 2. What does increasing the field rheostat resistance do? A. Reduces flux so speed rises above base speed B. Reduces speed below base C. Leaves speed unchanged D. Increases flux Answer: A. Lower I_f means lower Φ, so higher N. (Key point: Field control) 3. What is the main drawback of armature resistance control? A. It gives speeds only above normal B. Large I_a²R_e loss and poor efficiency C. It needs three machines D. It weakens the field Answer: B. The resistance carries full armature current. (Key point: Armature resistance) 4. What is the advantage of the Ward-Leonard system? A. Lowest cost B. Highest efficiency C. Smooth speed control in both directions, even through zero D. Only speeds above base Answer: C. Varying generator field varies armature voltage smoothly; its drawbacks are cost and size. (Key point: Ward-Leonard) ## Related in Electrical Machine - [Speed control methods of shunt DC motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-motor-speed-control-methods) — Electrical Machines, chapter 4, asked 2× - [Shunt motor speed with 1 Ω armature resistance added](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-240v-27a-1ohm-torque) — Electrical Machines I, chapter 4, asked 5× - [Shunt motor speed with 1.2 Ω armature resistance added](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-240v-30a-1p2ohm-torque) — Electrical Machines I, chapter 4, asked 5× - [Shunt motor speed with 0.1 Ω armature resistance added](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-20a-0p1ohm-armature) — Electrical Machines I, chapter 4, asked 4× - [Shunt motor speed with 0.1 Ω armature resistance, less load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-240v-5a-armature-0p1) — Electrical Machines I, chapter 4, asked 3× - [Shunt motor speed with 0.05 Ω armature resistance added](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-25a-0p05ohm-torque) — Electrical Machines I, chapter 4, asked 3× ## Related in other subjects - [DC shunt motor armature torque](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Adc-shunt-motor-torque) — Basic Electrical and Electronics Engineering, chapter 2 - [Shunt motor speed with 50 Ω field resistance (1200 rpm)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-250v-25a-half-load) — Electrical Machines I, chapter 4, asked 5× - [Shunt motor speed with field resistance added (250 V, 1400 rpm)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-250v-0p2ohm-1400rpm-50ohm) — Electrical Machines I, chapter 4, asked 5× - [Shunt motor speed with 50 Ω field resistance and lower torque](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-motor-220v-25a-50ohm-torque) — Electrical Machines I, chapter 4, asked 4×