--- title: "DC shunt motor characteristics" subject: "Electrical Machine" chapter: 4 key: "semester_4/electrical_machine:dc-shunt-motor-characteristics" source: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-shunt-motor-characteristics site: BCT Engineering --- # DC shunt motor characteristics Electrical Machine · Chapter 4: DC Motor - Asked in: 2074 Bhadra, 6 marks - Video: [11 Characteristics of DC Shunt Motor](https://www.youtube.com/watch?v=qeLGDx4N0nM) — SP Engg. ACADEMY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machine/dc-shunt-motor-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-4 ## Question Explain the speed-current, torque-current and speed-torque characteristics of a DC shunt motor. ## Short answer With nearly constant flux, shunt motor speed drops slightly with load due to armature resistance drop, giving a flat N–I_a curve. Torque is proportional to armature current, a straight line. The N–T characteristic is similarly slight drooping; thus it is a constant-speed motor. ## Answer In a shunt motor the field winding is connected across the supply, so the flux Φ is nearly constant. The three characteristics follow from the relations T = KΦI_a and N = (V − I_aR_a)/(KΦ). ```text I_a ----->[Ra]---(Eb)---+ Ish ----->[Rsh]---------+--- V (constant) ``` #### 1. Speed-current characteristic (N–I_a) - N ∝ E_b = V − I_aR_a with constant Φ. - With load, I_a rises and the small drop I_aR_a reduces the speed slightly. So the curve is a nearly flat, slightly drooping straight line. - Armature reaction weakens the flux a little, which tends to hold the speed up. - Hence a shunt motor is a **constant-speed motor** (the speed falls only 5 to 10 % from no load to full load). #### 2. Torque-current characteristic (T–I_a) - T ∝ Φ I_a, and Φ is constant, so **T ∝ I_a**: a straight line through the origin. - At no load, a small current I_a0 is needed to supply the rotational losses, so the shaft-torque line starts a little to the right of the origin. - The starting torque is adequate at the permitted starting current, but less than that of a series motor. #### 3. Speed-torque characteristic (N–T) - Since I_a ∝ T, the curve is similar to the N–I_a curve: a slightly drooping line. - Speed changes very little with torque, so the motor is used where constant speed is needed. ```text N |----------- T | / | ----\ | / | \ | / +----------------> Ia +------/--------> Ia speed vs Ia torque vs Ia ``` | Characteristic | Shape | Reason | |---|---|---| | N–I_a | almost flat, slight droop | Φ constant, small I_aR_a drop | | T–I_a | straight line | T ∝ I_a | | N–T | slight droop | I_a ∝ T | Applications: lathes, fans, blowers, centrifugal pumps, machine tools. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a shunt motor nearly constant speed? A. Φ is proportional to I_a B. Back emf is zero C. Φ is constant and I_aR_a drop is small D. Armature resistance is large Answer: C. N ∝ V - I_aR_a, which changes only slightly. (Key point: Speed-current) 2. What is the T-I_a characteristic? A. A parabola B. A hyperbola C. A flat line D. A straight line (T ∝ I_a) as Φ is constant Answer: D. T = KΦI_a with constant Φ. (Key point: Torque-current) 3. How is the shunt motor starting torque compared with a series motor? A. Lower B. Higher C. Equal D. Infinite Answer: A. The series motor flux rises with current, giving higher starting torque. (Key point: Starting torque) ## Close variants set in other exams - [Torque and speed characteristics of DC motors](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashunt-series-motor-characteristics) — Electrical Machine, chapter 4, asked 2× ## Related in Electrical Machine - [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× - [Shunt motor pump, torque ∝ speed²](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashunt-motor-centrifugal-torque-square) — Electrical Machine, chapter 4, asked 2× - [Shunt motor driving pump: torque proportional to speed squared](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adcm-shunt-200v-pump-torque-square) — Electrical Machines, chapter 4, asked 2× - [Shunt motor speed at 30 A armature current](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Ashunt-motor-60a-230v-speed) — Electrical Machine, chapter 4 - [Starting current; speed and % drop of shunt motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adcm-high-starting-current-speed-drop) — Electrical Machines, chapter 4 - [Shunt motor speed at armature current 30 A](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adcm-shunt-60a-230v-speed-30a) — Electrical Machines, chapter 4 ## Related in other subjects - [Reverse current (plugging) braking of 20 kW DC shunt motor](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-reverse-current-braking-20kw) — Utilization of Electrical Energy, chapter 3 - [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 - [DC shunt motor speed at reduced voltage and starting resistance](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-reduced-voltage-starting-resistance) — Utilization of Electrical Energy, chapter 3 - [Plugging resistance for 30 kW DC shunt motor](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-30kw-plugging-resistance) — Utilization of Electrical Energy, chapter 3