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The following induction motor at the link has rated 50Hz stator freq. on namespace label:

http://www.vanbodegraven.nl/en/products/ac-motors/asea-mbg-200-m-60-6/

It seems it is 6 pole since it has 970 rated rpm and therefore sync. freq. is 1000 rpm. Which means this motor cannot exceed 1000 rpm unless its stator freq. more than 50Hz.

A VFD speed control reads 1070 rpm mechanical speed when at full speed. It means it is definitely applying more than 60Hz to stator windings.

My question is is that safe to apply a volateg with more than 50Hz if the rated freq. is 50Hz?

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Induction motors run on a design Volts per Hertz ratio. You didn't specify the rated voltage of the motor, but if it is 50Hz, I'll make the assumption that it is a 380VAC three phase motor. 380 VAC at 50Hz is a ratio of 7.6. Almost all 380VAC rated motors have an insulation system that can safely handle 460 VAC, which is another motor standard voltage. 460 at 60 Hz is a ratio of 7.666, so yes, the speed can be safely increased.

Now, you need to consider some other factors about induction motors. Almost all induction motors under 75KW or 100HP can be safely run up to twice rated speed, but in order to do that, you lose torque. When running over base speed, the stator voltage is kept at the maximum (380 or 460 for example), but the frequency increases. If you were to compare the operation of an induction motor from 0 RPM up to base speed, and then to the extended speed, it would be almost identical to running a DC motor from 0 to rated speed with full field, and then decreasing the field current to increase the motor speed. From 0 to base speed, both operate in the constant torque region. Past base speed, they operate in the constant power (KW or HP) region.

To answer what I think is your actual question, no, do not run a 50Hz motor at an increased voltage unless you increase the frequency to 60Hz to keep the same V/Hz ratio. Increasing the voltage without increasing the frequency leads too excessive winding heating and also reduced cooling capacity from the internal fan (or just the rotor turning).

Other considerations can come into play depending on the mode your VFD is running in, but the above are the basics.

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