I require urgently 75 hp motor which should be compactible for both 50Hz/60Hz, I'm very much confused as frequency changes speed will change but I'm wondering how the power rating of the motor remain same in practical power must vary but manufacturer claims power remains constant? Is it possible? 
A: It is possible for a motor to deliver the same power output at 50Hz or 60Hz. The torque delivered at 50Hz would be 20% higher than 60Hz for 75hp motor. More important is what is the load rpm which you want to drive; for example a 4 pole motor would run at 1470rpm on 50Hz supply and 1770rpm at 60Hz. What is your supply frequency, Voltage limits and load rpm? What is the motor data for extreme Voltage operation for your supply frequency?
What are the loads and rpm? It is important because the standard motors would be from USA for 480V 60Hz power supply at source. Such a motor if driven at 400V 50Hz would draw 20% higher current to deliver the same load, with armature conductors overheating due to 44% higher Copper losses. Basically it means a 100hp 480V 60Hz motor will be derated to 75hp output at same rated torque and current when connected to 400V 50Hz, at 20% lower speed (nominal 1800rpm would drop to 1500rpm for 4 pole). What is the power required by the loads at 400V 50Hz and 480V 60Hz at corresponding speeds? It is crucial for selection of motor.
This motor should be designed to operate at 60Hz (1800rpm for 4 pole) for mechanical components life, and temperature rise limits to be tested for 400V 75hp 50Hz for electrical loading. Insulation test voltage would be usually 3kV for a minute. As such makers like ABB, Siemens (named just a few) may have representation in your area and may be able to help you.
You can buy a GoHz frequency converter to convert between 50Hz and 60Hz.
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