Case: A fan motor of 315kw running on VFD. Its operating speed is 795 rpm but its winding temperature gets heated like R phase temperature is 126.3 Y phase temperature is 125 and B phase temperature is 120. The motor is surrounded by cooling fans still temperature gets raised. Is there any alternative or have to change the motor to 400kw?
Heating effect can be caused by improper load match of the motor and mechanical VFD. Look at the temperature rise of the motor on the name plate and match to the temperature rise you are getting if it's within the possible temperature rise according to name plate. Look at also the currents in each phase, injurious temperature rise can be caused by harmonic torque in the motor due to VFD. Look at the set frequency range of your VFD, also check that your machine is loaded accordingly, frequency can increasing losses such as hysteresis, also check that your motor cooling fan is run continuous check the ventilation of the motor assembly or filters.
Try reducing the pitch angle of the fan blades, also would like to know the number of poles of motor being used, because if its 8 pole then it's no problem to run it on around 50Hz and if the maximum voltages are 415VAC then around 680amps load is expected, also your normal running amps can give a better idea on that also if your motor is VFD type or not, if it's not then maximum allowable frequency may be 65Hz check your VFD frequency and also the motor end connection it must be connected in DELTA.
It is unlikely that the motor is running at reduced frequency and that the reduced ventilation is the problem. This fan load will have its driving power proportional to the cube (3rd power) of the speed. Almost all VFD drivetrains with such centrifugal loads will operate down to 50 % speed with no motor cooling problems - with the original shaft mounted fan/s.
My suspicion is that this is an 8 pole 50 Hz motor running above 50 Hz on the VFD. Let's assume its 50 Hz rated speed is 740 r/min. It is running at 795 r/min, so the speed ratio is 795/740 = 1.074. The cube of this is 1.24. So this motor will draw 24 % above its rated power (assuming fully rated) if it is an 8 pole motor running at approximately 53.7 Hz. With the same rated voltage - 24 % additional power will easily overheat the motor - even in the cooler ambient etc.