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True or false: variable-speed operation always saves energy.

  • A.False, since a constant-torque load at half speed draws half the power at best
  • B.True, since reducing the speed always reduces the power drawn proportionally
  • C.True, provided the drive is fitted with a variable-frequency converter
  • D.False, since a converter always increases the power the load requires

Answer: A

The saving depends on the load's own power-speed relation, and the dramatic figures come from fluid loads with their cubic law. A hoist or a conveyor at half speed does half the work in the same time and saves nothing per unit moved.

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