--- title: "Electric drive and motor selection factors" subject: "Industrial Power Distribution and Illumination" chapter: 5 key: "semester_6/industrial_power_distribution_and_illumination:electric-drive-motor-selection" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/electric-drive-motor-selection site: BCT Engineering --- # Electric drive and motor selection factors Industrial Power Distribution and Illumination · Chapter 5: Electrification of Industrial Building - Asked in: 2068 Bhadra (old course), 8 marks - Video: [UEP Lecture 3 | Principle of selection of electrical drive |](https://www.youtube.com/watch?v=jiD0-hntMtE) — Harishankar Swarnkar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/electric-drive-motor-selection - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-5 ## Question What is an electric drive? Discuss various factors which affect the selection of motor for a particular electric drive. ## Short answer An electric drive uses a motor as prime mover with control. Motor selection depends on supply type (AC/DC, voltage), load torque-speed characteristic and starting torque, motor electrical characteristics (starting current, efficiency, pf), mechanical enclosure (IP rating, mounting), duty cycle rating, service conditions, cost, and standardization. ## Answer An **electric drive** is a system that uses an electric motor as the prime mover to control the speed, torque and direction of a mechanical load. It consists of the power source, a power modulator/controller (starter, converter), the motor, the transmission (shaft, gears, belt) and the driven load, with sensing and control units. ```text Supply → Power modulator → Motor → Transmission → Load ↑ | Control unit ←── sensors ──────┘ ``` #### Factors affecting the selection of motor 1. **Nature of supply:** AC or DC, single or three phase, voltage and frequency available. Most industries use 3-phase AC induction motors because the supply is AC. 2. **Load characteristics:** - *Speed–torque characteristic* of the load (constant torque like conveyors, torque ∝ speed² like fans and pumps, constant power like winders). - *Starting torque* required (cranes and compressors need high starting torque; fans need little). - Speed range and need for speed control (wide control → DC motor or VFD-fed induction motor). 3. **Electrical characteristics of the motor:** starting current and its effect on supply voltage, running characteristics, speed regulation, braking (plugging, rheostatic, regenerative), power factor and efficiency. 4. **Mechanical considerations:** type of enclosure (open, drip-proof, totally enclosed fan-cooled, flameproof) suited to dust, moisture or explosive atmosphere; type of bearings; mounting (foot, flange); transmission (direct, belt, gear); noise and vibration. 5. **Size and rating:** rating chosen from the duty cycle: continuous, short-time or intermittent periodic duty. Under-rating causes overheating; over-rating gives poor pf and efficiency. 6. **Service conditions:** ambient temperature, altitude, humidity, frequency of starting and reversal. 7. **Cost:** initial cost of motor and control gear, running cost (efficiency, pf), and maintenance cost. Squirrel-cage motors are cheapest and almost maintenance-free. 8. **Availability and standardisation:** standard frame sizes and ratings make spares and replacement easy. **Example:** a lift needs high starting torque, frequent starts, reversal and smooth speed control, so a VFD-fed induction motor (or DC motor) is chosen; a ceiling fan works well with a simple single-phase capacitor motor. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is an electric drive? A. A motor only, without any controller B. A system using an electric motor as prime mover to control speed, torque and direction of a load, with modulator, transmission and control C. A vehicle powered by a battery D. A fuel engine coupled to a generator Answer: B. An electric drive comprises source, power modulator/controller, motor, transmission and load with sensing and control units. (Key point: Electric drive definition) 2. Which load has torque proportional to speed squared? A. Conveyors B. Fans and pumps C. Winders D. Cranes at standstill Answer: B. Fans and pumps have T proportional to N^2; conveyors are constant torque and winders constant power. (Key point: Load characteristics) 3. What happens if a motor is under-rated or over-rated for its duty? A. Under-rating causes overheating; over-rating gives poor pf and efficiency B. Under-rating improves efficiency; over-rating overheats C. Neither has any effect D. Over-rating causes overheating; under-rating gives poor pf Answer: A. A small motor overheats under load; an oversized motor runs lightly loaded with poor pf and efficiency. (Key point: Duty-cycle rating) 4. Why is a VFD-fed induction (or DC) motor chosen for a lift? A. Lifts need little starting torque B. It needs high starting torque, frequent starts, reversal and smooth speed control C. Because squirrel-cage motors cannot be reversed D. Because lifts run at constant speed with no control Answer: B. A lift's duty demands high starting torque, frequent starting/reversal and smooth control, which VFD-fed motors provide. (Key point: Lift motor choice) ## Related in Industrial Power Distribution and Illumination - [Electric vs mechanical drive, individual vs group](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aelectric-vs-mechanical-drive) — Industrial Power Distribution and Illumination, chapter 5 - [Distribution substation site selection factors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Asubstation-site-location-factors) — Industrial Power Distribution and Illumination, chapter 4 - [Factors governing illumination level](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aillumination-factors-lighting-schemes) — Industrial Power Distribution and Illumination, chapter 7 - [Ten factors for flood, street lighting, luminaires](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aten-factors-flood-street-luminaires) — Industrial Power Distribution and Illumination, chapter 8 ## Related in other subjects - [Advantages of electric drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aadvantages-of-electric-drive) — Utilization of Electrical Energy, chapter 2 - [Block diagram of electric drive; series vs shunt starting](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adrive-block-diagram-motor-starting) — Utilization of Electrical Energy, chapter 2 - [Electric drive: parts, advantages, disadvantages](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-drive-parts-advantages) — Utilization of Electrical Energy, chapter 2 - [Weighted decision matrix for drive selection](https://www.bctengineer.com/question.md?key=semester_6%2Fmachine_design_i%3Aweighted-decision-matrix-drive) — Machine Design I, chapter 1