Applications where load-torque is increasing with speed (pumps, fans, compressors, …)
IE1 IE2 IE3 Square-Load
Typical torque versus speed curves for 11kW, 4-pole, three-phase, cage- induction motors and load versus speed curves for -square-loads
As a general rule, high-efficiency cage-induction motors have a lower slip (see table 3), i.e. a higher speed of rotation, than motors of lower efficiency. When the torque of the application is a function of the square of the speed, like in pumps, fans, compressors etc., the increase in speed will lead to an increase in output power (torque) which could in some circumstances defeat the benefits from the improved energy efficiency (see Figure 10).
_ P out [kW]
_ P in [kW]
Example of changing of efficiency, speed and torque demand with energy efficiency class of three 11 kW, 50 Hz motors in the same application
Therefore, in such applications when a motor of lower efficiency is retrofitted by a motor of increased efficiency, the input power should not reduce as much as anticipated when comparing the efficiencies of the two motors.
In some cases the input power of the high-efficiency motor may actually increase compared to the motor of lower efficiency.