--- title: "Suitability of DC motors for applications" subject: "Electrical Machines" chapter: 4 key: "semester_4/electrical_machines:dc-motor-suitability-applications" source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-motor-suitability-applications site: BCT Engineering --- # Suitability of DC motors for applications Electrical Machines · Chapter 4: DC Motor - Asked in: 2067 Mangsir (old course), 8 marks - Video: [Types of DC Motors](https://www.youtube.com/watch?v=v2eKOOzdgzs) — sayeekumar swaminathan - 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-suitability-applications - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4 ## Question Explain with reason, the suitability of DC series, DC shunt and DC compound motors. Identify suitable DC motor for the following application: i) Electric traction ii) Vacuum cleaner iii) Paper making iv) Shearing and punching ## Short answer DC series motors provide very high starting torque and speed falling with load, suited for traction and vacuum cleaners. Shunt motors offer nearly constant speed, ideal for paper making. Cumulative compound motors combine high starting torque with safe no-load speed, making them suitable for shearing and punching. ## Answer #### DC series motor Characteristics: very high starting torque ($T\propto I_a^2$), speed falls as load rises (automatic power adjustment), dangerous at no load. **Suitable for:** loads that need high starting torque and can be directly coupled: traction, cranes, hoists, trolleys, vacuum cleaners and similar. #### DC shunt motor Characteristics: almost constant speed from no load to full load, moderate starting torque, easy speed control by field rheostat. **Suitable for:** constant-speed loads that start with light load: lathes, drilling machines, fans, blowers, pumps and paper machines. #### DC compound motor Characteristics (cumulative): good starting torque (better than shunt) with a safe no-load speed, and a drooping speed with load. **Suitable for:** loads with sudden changes and heavy starting torque: shears, presses, punches, rolling mills, elevators and conveyors (flywheel loads). #### Application selection | Application | Suitable motor | Reason | |---|---|---| | i) Electric traction | DC series | Very high starting torque; speed falls automatically on a gradient, torque rises with load | | ii) Vacuum cleaner | DC series (universal) | High speed at light load; runs on AC or DC; compact | | iii) Paper making | DC shunt (or separately excited) | Constant, accurately controllable speed | | iv) Shearing and punching | Cumulative compound | Sudden heavy load; the flywheel supplies the energy and the motor slows, then recovers; high starting torque and safe at no load | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which motor suits electric traction and why? A. Shunt, for constant speed B. Series, for very high starting torque C. Compound, for flywheel loads D. Separately excited, for no-load safety Answer: B. Series motors give very high starting torque and speed falls on gradients. (Key point: Electric traction) 2. Which motor suits paper making? A. DC series B. Cumulative compound C. DC shunt (or separately excited), for constant controllable speed D. Universal motor Answer: C. Paper machines need constant, accurately controllable speed. (Key point: Paper making) 3. Why is a cumulative compound motor used for shears and punches? A. It has zero starting torque B. It never slows down C. It runs only at no load D. Flywheel supplies the sudden peak energy; high starting torque and safe at no load Answer: D. The compound motor slows on sudden load, the flywheel gives energy, and it is safe at no load. (Key point: Shearing and punching) 4. The DC shunt motor is suited to which loads? A. Constant-speed loads starting with light load, e.g. lathes, pumps B. Heavy-starting cranes C. Vacuum cleaners D. Sudden shock loads Answer: A. Shunt motors have nearly constant speed and only moderate starting torque. 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