If the new impeller is a spare of the same size and type as the old one, no calculation is usually needed. If its diameter, width, number or angle of blades, blade type or weight has changed, the motor power must definitely be reassessed; changes that look small can make a big difference to the power requirement.

DRG-180L-4 22 kW 1500 rpm cast iron frame DRG three-phase motor – motor power after a fan impeller change

Why is the effect of diameter so large?

For geometrically similar impellers turning at the same speed, the power absorbed rises roughly with the fifth power of the diameter. In other words, even a relatively small increase in diameter raises the motor load noticeably. This is an approximate relationship and the actual value depends on the fan curve and system resistance, but it shows why the "a few centimetres won't matter" approach is wrong.

What to check besides power

Starting time

A heavier or larger-diameter impeller accelerates more slowly because of its inertia. The longer the start, the longer the motor draws high current and heats up. With heavy impellers, it should be assessed whether starting stays within the limits allowed for the motor.

Speed limit and balance

Every impeller has a maximum tip speed specified by its maker. The new impeller must not exceed this limit at the motor speed. Impeller balance also directly affects motor bearing life.

Shaft and hub

On direct-driven fans, the hub bore, keyway and clamping arrangement of the new impeller must suit the motor shaft.

Practical verification

  • Ask the impeller maker for the fan curve or power value.
  • After installation, have a qualified person measure the current on all three phases and compare it with the nameplate current.
  • Observe the starting time and vibration.

For a preliminary calculation you can use the fan motor power calculator, see the notes on backward-curved fan motors for large-diameter centrifugal applications and, if necessary, consider a higher power option among the models in the shop.