Why Do Some Motors Have Both 50Hz and 60Hz on Their Nameplate, and Some Do Not?

Hz stands for hertz, which is the unit of frequency. Frequency is the number of cycles per second that a motor’s magnetic field rotates. The higher the frequency, the faster the motor spins and the more power it can deliver. The frequency of a motor depends on the input power source, which is usually 50 Hz or 60 Hz in different parts of the world. 

60Hz motor will run 20% slower on 50Hz power supply. This also results in 20% less power. Basically, running the electric machine slower usually means it will be demanding less power. That's good, as the motor also decrease 20% of its power, and the cooling fan is slow down too. But the critical factor here is the V/Hz ratio. It goes up 20%! Not good. This means that during parts of every power line cycle the magnetic structure of the motor will probably be overloaded.

50Hz motor will run 20% faster on 60Hz power supply. AC motor kilowatt is proportional to Torque times RPM. As the motor's torque is not going to change significantly with the increase of frequency, it will now output 20% more power. A 10kW 50Hz motor will be a 12kW motor on 60Hz power source.

Running a machine 20% faster is very likely going to increase its power demand by at least 20%! If the machine cyclically accelerates or decelerates in operation it will be subject to greater mechanical forces. If the motor is driving centrifugal loads their demand may even go up by the square of the speed increase.

Whenever possible a motor should be picked that is rated for the frequency of the power of the country that the motor will operate in. If the motor needs to work at 50 Hz and 60 Hz, many manufacturers make motors that are rated at both frequencies. Also, as we’ve seen above, while it is possible to run a motor at a higher or lower frequency than what it is rated for in certain situations, it is not always practical due to changes in torque, current draw and motor losses.