--- title: "Factors for selecting a motor" subject: "Utilization of Electrical Energy" chapter: 2 key: "semester_7/utilization_of_electrical_energy:factors-selecting-motor" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/factors-selecting-motor site: BCT Engineering --- # Factors for selecting a motor Utilization of Electrical Energy · Chapter 2: Electric Drive System - Asked in: 2081 Bhadra, 5 marks; 2073 Chaitra, 4 marks (asked 2 times) - Video: [SELECTION OF MOTOR FOR VARIOUS APPLICATION DSM CRITERIA OF MOTOR SELECTION in Agriculture & Industry](https://www.youtube.com/watch?v=Joo0jZWeGZg) — Simplified EEE Studies - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/factors-selecting-motor - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-2 ## Question Describe the key factors to be considered when selecting a motor for a specific application. ## Short answer Key factors include nature of supply (AC/DC, voltage, frequency), load torque-speed characteristics (starting torque, speed range), motor electrical characteristics (starting, speed control, braking), duty cycle (S1-S8), mechanical enclosure and mounting, environmental conditions (temperature, dust, explosive), cost and efficiency, and size/reliability. ## Answer Selecting a motor means choosing the type, rating and enclosure so that it drives the load correctly, economically and safely over its life. The main factors are: **1. Nature of supply** - AC or DC, single- or three-phase, voltage and frequency available; supply capacity for starting current. **2. Load (mechanical) characteristics** - Speed-torque characteristic of the load: constant torque (hoist, conveyor), torque proportional to speed squared (fan, pump), constant power (lathe, winder). - Starting torque needed (high for cranes, crushers, traction; low for fans). - Speed range and speed control; constant or variable speed. **3. Electrical characteristics of the motor** - Starting characteristics: starting torque and starting current (DOL, star-delta, soft starter, VFD). - Running characteristics: speed regulation, efficiency, power factor. - Speed control method and range. - Braking requirements: plugging, dynamic or regenerative braking. **4. Rating and duty cycle** - Continuous, short-time or intermittent duty (S1 to S8). - Overload capacity and peak torque; rms (equivalent) power for variable loads. **5. Mechanical considerations** - Type of enclosure (open, drip-proof, totally enclosed fan-cooled, flameproof) to suit the environment. - Mounting (foot/flange, horizontal/vertical), shaft and bearings. - Transmission: direct coupling, belt, gear; noise and vibration limits. **6. Environmental conditions** - Temperature, altitude, humidity, dust, explosive gases, corrosive atmosphere; insulation class. **7. Cost and economics** - Capital cost, running cost (efficiency, power factor), maintenance cost, life; availability of spares. **8. Size, weight and reliability** where space is limited. | Load | Requirement | Suitable motor | |---|---|---| | Fan, pump | Constant speed, low starting torque | Squirrel-cage IM | | Crane, hoist | High starting torque, speed control | Slip-ring IM, DC series, IM + VFD | | Traction | High starting torque, wide speed range | DC series, IM/PMSM with inverter | | Lathe, machine tool | Wide speed range, roughly constant power | DC shunt, IM + VFD | | Large compressor | Constant speed, PF improvement | Synchronous motor | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which load is matched to the correct torque-speed type? A. Fan - constant torque B. Hoist/conveyor - constant torque; fan/pump - torque proportional to speed squared C. Lathe - torque proportional to speed squared D. Winder - constant torque only Answer: B. Hoists and conveyors are constant torque, fans and pumps follow speed squared, lathes and winders are constant power. (Key point: Load characteristics) 2. Which loads need high starting torque? A. Fans B. Cranes, crushers and traction C. Centrifugal pumps D. Ventilators Answer: B. Cranes, crushers and traction need high starting torque, while fans need low. (Key point: Starting torque) 3. For a variable load, how is the motor rating chosen? A. By the minimum power in the cycle B. By the rms (equivalent) power over the cycle, checked against maximum torque C. By the supply voltage only D. By the enclosure type Answer: B. The equivalent rms power over the duty cycle sizes the motor, and peak torque is checked against overload capacity. (Key point: Duty and rating) 4. Which motor is suited to a fan or pump needing constant speed and low starting torque? A. DC series motor B. Slip-ring induction motor C. Squirrel-cage induction motor D. Synchronous motor Answer: C. Squirrel-cage induction motors are simple and suit constant-speed, low-starting-torque loads like fans and pumps. 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