IE3 motors have lower rotor losses, so they slip less and run a few rpm faster than IE2 motors of the same rating. On a conveyor or compressor this does not matter. On a centrifugal pump or fan, it does: their power demand rises with the cube of speed.
10 rpm faster at 1,465 rpm → +0.68 % speed → +2.1 % shaft power
For an 11 kW pump at 75 % load, that 2.1 % is almost exactly the energy the IE3 motor saves. The pump delivers more flow than the process needs and a valve throttles it away.
How to keep the saving
- Trim the impeller in proportion to the speed increase (for 250 mm, about 1.7 mm).
- Or let the variable-speed drive hold the set point.
- Then count the saving: about 1,055 kWh per 11 kW motor per year at 6,000 hours.
Read the full worked example in forty pump motors, IE2 to IE3, and test your plant in the cost of ownership tool.