Electrical – Operating a 60 hz Induction motor at 50 hz

induction motormathvfd

If i had an Induction motor that runs on 460 volts ,35 amps , develops a power of 35 hp at a speed of ~1800 RPM ( data acquired from a nameplate )

How is it possible to calculate the same parameters if i wanted to run this motor at 50 hz instead ? what equations should i use ? , all i know is that power would decrease as well as RPM (obviously).

Best Answer

Motor torque is a direct function of the design ratio of voltage and frequency, or the V/Hz ratio. Anything within +-10% of the design V/Hz ratio will allow the motor to develop design torque without significant added heating.

480V is the DISTRIBUTION voltage in North America, the actual NEMA motor DESIGN voltage is 460V to allow for voltage drop. A motor labeled as 460V 60Hz is designed for a V/Hz ratio of 7.67:1 (460/60) and if you multiply that back into 50Hz, that's where you get the 383V. So giving it anywhere between 345 and 420V at 50Hz should be absolutely fine for that motor. It will develop full torque, but because of the frequency being 5/6ths, will run 17% slower. because the mechanical power (HP or kW) is a function of speed and torque and your torque remains the same, the power will also drop by 17%, so if it was rated for 35HP, that will be roughly 30HP or 22kW.

Given that 35HP was never a "standard" NEMA HP rating for an AC induction motor (standards are 30HP and 40HP), my guess is that your motor STARTED OUT life as a 22kW 380V 50Hz motor (which IS a standard size) and was re-rated for use in North America as "35HP" at 460V 60Hz. So if you want to go back to running it on a 380-415V 50Hz system again as a 22kW motor, have at it.