--- title: "Characteristics of DC shunt and series motors" subject: "Electrical Machines" chapter: 4 key: "semester_4/electrical_machines:dc-motor-characteristics-shunt-series" source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-motor-characteristics-shunt-series site: BCT Engineering --- # Characteristics of DC shunt and series motors Electrical Machines · Chapter 4: DC Motor - Asked in: 2070 Magh, 8 marks - Video: [Characteristics of DC motor/shunt motor and series motor-Electrical and mechanical characteristics](https://www.youtube.com/watch?v=FUc2klsuwa8) — Read Electric Vehicle - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-motor-characteristics-shunt-series - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4 ## Question Explain torque-armature current and speed-torque characteristics of DC shunt and DC series motor. ## Short answer In a DC shunt motor, flux is constant, so torque is directly proportional to armature current, and speed drops about 3-5% from no load to full load. In a DC series motor, flux increases with current, giving torque proportional to the square of current and very high starting torque; speed falls sharply as load increases, and the motor must not run unloaded due to dangerous racing. ## Answer Two main characteristics are drawn at rated (constant) supply voltage: **torque vs armature current** ($T$–$I_a$) and **speed vs torque** ($N$–$T$ mechanical characteristic). Basic relations: $T\propto\Phi I_a$, $\;N\propto\dfrac{E_b}{\Phi} = \dfrac{V - I_aR_a}{\Phi}$. #### DC shunt motor The field is across the supply, so $\Phi$ is practically constant. - **$T$–$I_a$:** $T\propto I_a$, a straight line through the origin (it curves down slightly at heavy load because of armature reaction). Starting torque is moderate because starting current is limited. - **$N$–$T$:** $N = \dfrac{V - I_aR_a}{K\Phi}$ falls only slightly (about 3–5%) from no load to full load. The speed is **nearly constant**. #### DC series motor The field carries the armature current, so $\Phi\propto I_a$ (before saturation). - **$T$–$I_a$:** $T\propto I_a^2$ for low current (a parabola); after saturation $T\propto I_a$ (a straight line). It gives a **very high starting torque**. - **$N$–$T$:** $N\propto\dfrac{V - I_a(R_a + R_{se})}{I_a}$, so speed falls rapidly as load torque increases. At light load the speed becomes dangerously high, so the motor must never run unloaded. ```text T N | series |\ series | _.-' | \ | ,' shunt | \____ | ,' _.-' |-----------__ shunt | ,' _.-' | +------------- Ia +------------- T ``` #### Comparison | Point | Shunt motor | Series motor | |---|---|---| | Flux | Constant | $\propto I_a$ | | $T$ vs $I_a$ | Straight line, $T\propto I_a$ | Parabola, $T\propto I_a^2$ | | Starting torque | Moderate | Very high | | Speed vs load | Nearly constant | Falls rapidly | | No-load running | Safe | Dangerous (racing) | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the T-Ia characteristic of a DC shunt motor? A. Parabola, T proportional to Ia^2 B. Constant torque C. Straight line through the origin, T proportional to Ia D. Hyperbola Answer: C. Flux is constant so T is proportional to Ia. (Key point: Shunt T-Ia) 2. For a series motor below saturation, torque varies as: A. Ia^2 (parabola) B. Ia C. 1/Ia D. constant Answer: A. Phi is proportional to Ia, so T proportional to Phi Ia is proportional to Ia^2; after saturation it becomes linear. (Key point: Series T-Ia) 3. How does speed of a shunt motor vary from no load to full load? A. Falls rapidly by 50% B. Rises sharply C. Falls only slightly (3-5%), nearly constant D. Falls to zero Answer: C. Only the small Ia Ra drop reduces Eb at constant flux. (Key point: Shunt speed) 4. What happens to a series motor speed as load torque increases? A. Speed stays constant B. Speed rises C. Speed is unaffected but current falls D. Speed falls rapidly, and at light load it becomes dangerously high Answer: D. N is proportional to (V - Ia(Ra+Rse))/Ia, which falls rapidly with Ia. 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