--- title: "Group vs individual drive; constant torque vs power" subject: "Utilization of Electrical Energy" chapter: 2 key: "semester_7/utilization_of_electrical_energy:group-individual-constant-torque-power-drive" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/group-individual-constant-torque-power-drive site: BCT Engineering --- # Group vs individual drive; constant torque vs power Utilization of Electrical Energy · Chapter 2: Electric Drive System - Asked in: 2081 Baisakh, 8 marks - Video: [types of drive | electrical drive | mechanical drive | L-18 | individual drive | electric drive](https://www.youtube.com/watch?v=_oDLBxzikHU) — Mind of Engineer - 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/group-individual-constant-torque-power-drive - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-2 ## Question Differentiate between: i) Group drive and individual drive. ii) Constant torque drive and constant power drive. ## Short answer Group drive uses one motor for several machines via line shaft, giving low cost but poor efficiency, speed control and reliability; individual drive gives each machine its own motor, giving better efficiency, control and flexibility at higher cost. Constant torque drive has rated torque up to base speed with power rising; constant power drive has rated power above base speed with torque falling as 1/omega. ## Answer #### (i) Group drive vs individual drive In a **group drive** one motor drives several machines through a line shaft, belts and pulleys. In an **individual drive** each machine has its own motor. ```text Group drive Individual drive M==line shaft== M1-[m/c 1] | | | M2-[m/c 2] [m1] [m2] [m3] M3-[m/c 3] (belts, pulleys) ``` | Point | Group drive | Individual drive | |---|---|---| | Motors | One large motor for many machines | One motor per machine | | Initial cost | Lower (one motor) | Higher (many motors) | | Efficiency | Low: shaft, belt losses; motor often lightly loaded | High: motor matched to machine | | Power factor | Poor at light load | Better | | Speed control | Difficult, all machines affected | Easy for each machine | | Reliability | Motor failure stops all machines | Fault affects one machine only | | Flexibility of layout | Poor: machines along the shaft | Machines placed anywhere | | Safety and appearance | Belts and pulleys: risky, untidy, noisy | Safe, clean | | Expansion | Difficult | Easy to add machines | | Use | Old workshops, small units where machines work together | Modern industry, machine tools | #### (ii) Constant torque drive vs constant power drive In a DC separately excited motor, $T = k\phi I_a$ and $P = T\omega$. Below base speed, speed is raised by **armature voltage control** with full field; above base speed, by **field weakening** at rated voltage. ```text T, P | T const | |-------------+. | / | ' . T ~ 1/w | P / |-------------- P const | / | | / | +---/---------+--------------> speed armature base field voltage speed weakening ``` | Point | Constant torque drive | Constant power drive | |---|---|---| | Torque vs speed | Constant (rated) at all speeds | Decreases with speed, $T \propto 1/\omega$ | | Power vs speed | Increases with speed, $P \propto \omega$ | Constant (rated) | | Speed range | Up to base speed | Above base speed | | DC motor control | Armature voltage control, rated flux | Field weakening, rated voltage | | Induction motor control | Constant V/f below base frequency | Rated V, $f$ raised above base frequency (field weakening) | | Armature current | Rated current at all speeds, rated flux | Rated current at all speeds, reduced flux | | Typical loads | Hoists, cranes, conveyors, positive-displacement pumps | Lathes, coilers/winders, traction at high speed, milling | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a main drawback of a group drive compared with individual drive? A. Motor failure stops all machines and efficiency is low B. Higher initial cost C. Easy speed control of each machine D. Greater safety Answer: A. One motor serves many machines through shafts and belts, so failure stops all and losses are higher; its advantage is lower cost. (Key point: Group drive defect) 2. In a constant torque drive, how do torque and power vary with speed? A. Torque constant, power proportional to speed B. Torque proportional to speed, power constant C. Torque proportional to 1/speed, power constant D. Both constant Answer: A. With T constant, P = T w rises in proportion to speed. (Key point: Constant torque variation) 3. How is the constant-power region of a DC motor obtained? A. Armature voltage control with full field below base speed B. Field weakening at rated voltage above base speed C. Adding armature resistance D. Reducing supply frequency Answer: B. Above base speed, field weakening at rated voltage gives constant power with T proportional to 1/w. (Key point: DC control for constant power) 4. Which loads suit constant power drives? A. Hoists and conveyors B. Lathes, winders and traction at high speed C. Positive-displacement pumps D. Cranes Answer: B. Constant power suits lathes, coilers/winders and high-speed traction; hoists and conveyors are constant torque. 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