To run a 3 phase,30kwmotor at 3500RPM having torque91Nm. by an alternator ,What should be alternator specifications?

The alternator needs to be powered too, so have losses on two areas.
Which brings me to the question: Why not drive whatever you need to drive directly?

iceng19 days ago

On a simple strategy, assume the efficiency of an alternator will be about 60% and voila 30KW / 0.6 = 50KW 3 phase alternator..

I cannot specify output frequency because you do not specify how many poles your motor winding's have..

Have a rotary day :-)

rickharris19 days ago

What voltage does the motor require. OR what is the KVa from the label

30 Kw is a BIG load for an alternator (unless your talking industrial scale and an Industrial price)