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Group vs individual drive; constant torque vs power

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.

Differentiate between: i) Group drive and individual drive. ii) Constant torque drive and constant power drive.

(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.

 Group drive                  Individual drive
   M==line shaft==            M1-[m/c 1]
     |    |    |              M2-[m/c 2]
   [m1] [m2] [m3]             M3-[m/c 3]
   (belts, pulleys)
PointGroup driveIndividual drive
MotorsOne large motor for many machinesOne motor per machine
Initial costLower (one motor)Higher (many motors)
EfficiencyLow: shaft, belt losses; motor often lightly loadedHigh: motor matched to machine
Power factorPoor at light loadBetter
Speed controlDifficult, all machines affectedEasy for each machine
ReliabilityMotor failure stops all machinesFault affects one machine only
Flexibility of layoutPoor: machines along the shaftMachines placed anywhere
Safety and appearanceBelts and pulleys: risky, untidy, noisySafe, clean
ExpansionDifficultEasy to add machines
UseOld workshops, small units where machines work togetherModern industry, machine tools

(ii) Constant torque drive vs constant power drive

In a DC separately excited motor, T=kϕIaT = k\phi I_a and P=TωP = T\omega. Below base speed, speed is raised by armature voltage control with full field; above base speed, by field weakening at rated voltage.

 T, P
   |  T const    |
   |-------------+.
   |           / |  ' .  T ~ 1/w
   |    P    /   |-------------- P const
   |       /     |
   |     /       |
   +---/---------+--------------> speed
     armature    base   field
     voltage     speed  weakening
PointConstant torque driveConstant power drive
Torque vs speedConstant (rated) at all speedsDecreases with speed, T∝1/ωT \propto 1/\omega
Power vs speedIncreases with speed, P∝ωP \propto \omegaConstant (rated)
Speed rangeUp to base speedAbove base speed
DC motor controlArmature voltage control, rated fluxField weakening, rated voltage
Induction motor controlConstant V/f below base frequencyRated V, ff raised above base frequency (field weakening)
Armature currentRated current at all speeds, rated fluxRated current at all speeds, reduced flux
Typical loadsHoists, cranes, conveyors, positive-displacement pumpsLathes, coilers/winders, traction at high speed, milling

✓ Answer checked by Claude Opus 5.5 (high effort)

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