Medium Voltage Motors

No, of course - you should reduce voltage applied proportionally to the frequency reduction.

For your motor to work properly, you must maintain the same V/Hz ratio of the original design. In your case, that motor was designed for 4160/60 = 69.3 V/Hz ratio. So if you have 50Hz, the voltage must be 50 x 69.3 = 3466V maximum. If you do not reduce the voltage to that level (or within 10%) the motor will saturate and over heat quickly, which will damage the winding insulation. The motor will also turn 20% slower (50/60), so the overall mechanical power capacity will be lower as well.

Thanks JRaef for your reply.