Torques and speeds required by the driven machine;
Any motor power derating due to the air-cooling method (self-ventilated or forced ventilation)
Motors rated for 50 Hz and 60 Hz
As the utilization and size of motors is actually related to torque rather than power the theoretical output power increases linearly with speed, i.e. by 20% from 50 Hz to 60 Hz.
I²R winding-losses are dominant especially in small and medium sized induction motors. These losses basically remain constant at both 50 Hz and 60 Hz as long as the torque is kept constant. Although windage, friction and iron losses increase with frequency, these losses typically play a minor role in motors with four or more poles. Therefore, at 60 Hz, the losses increase less than the 20% output-power increase compared to 50 Hz and the efficiency improves.
In practice, both 60 Hz and 50 Hz output power designations conform to the standard power levels according to IEC 60072. Therefore, an increased rating of motor power by 20% is not always possible. However the general advantage of 60 Hz still applies if the motor design is optimized for the respective supply frequency rather than just rerated.
The difference in efficiency between 50 Hz and 60 Hz also varies with the number of poles and the size of the motor. In general, the 60 Hz efficiency of three-phase, cage-induction motors in the output power range from 0,75 kW up to 370 kW is about 2,5 to 0,5 points greater when compared to the 50 Hz efficiency. Only large 2-pole motors may have a slightly lower efficiency at 60 Hz due to their higher share of windage and friction losses.
When motors are rated for operation on either 50 Hz or 60 Hz with nearly the same magnetic flux and nearly the same torque (i.e. 20% more power at 60 Hz), for example 400 V / 50 Hz / 3,0 kW and 460 V / 60 Hz / 3,7 kW, the efficiency at 60 Hz is generally higher than at 50 Hz (see Figure 5).
60 Hz Efficiency
50 Hz Penalty in %
Typical reduction of energy efficiency in %-points for 4-pole, low-voltage motors between 50 and 60 Hz when compared at the same torque (60 Hz power 20% increased)
Alternately, when motors are rated for operation on either 50 Hz or 60 Hz with nearly the same magnetic flux and the same power (i.e. 20% reduced torque at 60 Hz), for example 400 V / 50 Hz / 5,5 kW and 460 V / 60 Hz / 5,5 kW, the efficiency at 60 Hz is always higher because the utilization of the motor is reduced (see Figure 6).