In centrifugal fans and pumps operating at part load, an inverter usually reduces consumption. The affinity laws apply to these machines: flow varies with speed, pressure with the square of speed, and the power drawn roughly with the cube of speed. A small reduction in speed gives a much larger drop in shaft power than its proportion.

In systems where flow is throttled by a valve or damper, the motor keeps running at full speed and excess energy is wasted in the throttling element. When speed is reduced with an inverter, this loss disappears.

For a realistic assessment:

  • Duty profile: If the system runs at full flow most of the year, the gain is limited; with long part-load operation the difference grows.
  • Static head: In pumps with a large geodetic head the cube relationship does not fully apply; below a certain speed the pump cannot deliver.
  • Inverter losses: The inverter itself has losses; in a system running permanently at full speed there is no gain.
  • Measurement: The most reliable way is to measure the flow and power profile of the existing system.

A numerical gain can only be given by a system-specific calculation. For suitable motors see the industrial fan motors and pump electric motors pages, and for the general approach the article electric motor energy savings: where to start.