How to find rated current of a three phase induction motor
You can not determine the rated current from the information given. The best you can do is to make an estimate based on typical motors available in the market. However a motor rated 15000 kW at 440V is not a motor that anyone could build. The required current would be something like 23,000 amps. A motor with that power rating would be rated something like 10,000 volts and 1000 amps.
By international standards, the power rating for a motor is the mechanical power that the motor can deliver under specified operating conditions. For a 3-phase motor, W = V X I X 1.73 X efficiency X power factor. Typical values of efficiency and power factor vary with influence due to power level, number of motor poles, type of enclosure and design choices made by the manufacturer. I found data for a 15 kW 400 V, 4-pole, 3-phase motor, at rated load, efficiency = 91.4%, pf = 0.83 current = 28.5 A.
If you solve the equivalent circuit, you can get a reasonable approximation of the rated current. Without the resistance in parallel with the magnetizing branch that represents the iron losses, you will be missing that contribution to current. There is also no way to determine mechanical losses from the equivalent circuit parameters.