--- title: "Series motor for high torque; compound motor characteristics" subject: "Electrical Machines I" chapter: 4 key: "semester_4/electrical_machines_i:series-motor-high-torque-compound-characteristics" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/series-motor-high-torque-compound-characteristics site: BCT Engineering --- # Series motor for high torque; compound motor characteristics Electrical Machines I · Chapter 4: DC Motor - Asked in: 2076 Baisakh, 6 marks - Video: [Electrical Machines | Lecture - 3B | DC Motor | Torque, Armature current, Speed Characteristics](https://www.youtube.com/watch?v=M6xh7rNMC2o) — Engineering Devotion - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/series-motor-high-torque-compound-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-4 ## Question Why series motor is used for high torque applications? Draw and explain the electrical and mechanical characteristics of compound motor. ## Short answer Series motor torque ∝ I_a^2 (pre-saturation), giving very high starting torque with moderate current. Cumulative compound motor: flux increases with load, yielding high starting torque and a drooping speed characteristic without runaway; differential compound is unstable and rarely used. ## Answer #### Why a series motor is used for high torque In a series motor the field carries the full armature current, so (below saturation) $\phi \propto I_a$ and $$ T \propto \phi I_a \propto I_a^2 $$ - Doubling the current gives about **four times the torque**, so a large starting torque is obtained with moderate current. A shunt motor ($T \propto I_a$) would need far more current for the same torque. - At heavy load the speed falls automatically, so power $T\omega$ stays nearly constant and the supply is not overloaded. - Hence series motors suit **traction (electric trains, trams), cranes, hoists, elevators and conveyors**, where high starting torque is needed. They must never run without load, because speed becomes dangerously high. #### Compound motor characteristics A compound motor has both shunt and series fields. In a **cumulative** compound motor the series flux aids the shunt flux; in a **differential** one it opposes it. **1. Electrical characteristic ($T$ vs $I_a$)** - Cumulative: $\phi = \phi_{sh} + \phi_{se}$ rises with load, so torque rises faster than in a shunt motor (between shunt and series curves). Good starting torque. - Differential: $\phi$ falls with load, so torque rises slowly; starting torque is poor. **2. Speed characteristic ($N$ vs $I_a$)** - Cumulative: since flux rises with load, speed falls more than in a shunt motor, but there is a definite no-load speed (set by the shunt field), so there is no runaway. - Differential: flux falls with load, so speed stays nearly constant or rises with load; operation can be unstable. **3. Mechanical characteristic ($N$ vs $T$)**: obtained from the two above; cumulative gives a drooping curve, differential a flat/rising one. ```text T | series | / cumul. | / / shunt | / / / | / / / differential | ///_----- |/____________ Ia ``` ```text N | _.-' differential |____________________.--'' |'-.______________________ shunt | '-._ | '-.______________ cumulative | \ | '. | '-.___ | '------.______ series |_____________________________ Ia ``` | Motor | $T$ vs $I_a$ | Speed with load | Use | |---|---|---|---| | Cumulative compound | High starting torque | Falls moderately | Presses, shears, rolling mills, elevators | | Differential compound | Low torque | Nearly constant / rises | Rarely used (unstable) | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a series motor give high torque? A. T ∝ φIa ∝ Ia^2, so doubling current gives about four times torque B. Flux is constant C. Speed is constant D. Back emf is zero Answer: A. Large starting torque with moderate current. (Key point: Torque in series motor) 2. Which are typical uses? A. Fans on belts B. Traction, cranes, hoists C. Constant-speed lathes D. Clocks Answer: B. They need high starting torque. (Key point: Applications) 3. In cumulative compound, how does the series flux act? A. It opposes the shunt flux B. It is zero C. It aids the shunt flux, raising torque with load D. It removes the field Answer: C. It has a definite no-load speed. (Key point: Cumulative compound) 4. What is a feature of a differential compound motor? A. Excellent starting torque B. Speed falls steeply with load C. It runs away at no load D. Poor starting torque and possible unstable speed Answer: D. Flux falls with load. 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