How do you calculate maximum current of 3 phase motor?

By using a simple formulae: f=np/60 where f= frequency, n is the number of pair of poles and 60 is in seconds that motor is turning. So a 4 pole motor with a frequency of 50 hertz will run at 1500 rpm.

By manupilating the formulae:

n= 50 multiplied by 60 devided by 2

therefore n= 1500rpm.

The formula 120xFrequency/poles gives synchronous speed not actual shaft RPM. Induction motors must run at a certain % slip to obtain torque, so a 4 pole motor running at 50hz would be 120x50/4 = 1500 rpm, at 4% slip (operating speed) actual shaft RPM or rated speed would be 1440 RPM.