Here is an example motor for the 7.5 kW motor:
7.5 kW = 10 Hp.
Next standard size = 15 HP
Select a 15 HP motor with a voltage rating of 208-230/460 Volts.
On 50 Hz this motor will develop 15 x(5/6) = 12 HP > Adequate for a 7.5 kW load.
Though not listed this motor will be suitable for 416V to 460V @ 60 Hz.
The voltage rating at 50 Hz will be 347V to 383V. But these motors are generally applied on 480 Volt circuits.
480V @ 60Hz >> 400 Volts @ 50 Hz.
The 383 volts is still a little high but we have a cushion. 90% of rated voltage will be 312 Volts. But we have a 12 HP motor with a service factor of 1.15 for an allowable short time HP of 13.8 HP and only a 10 HP load.
I am sure that this motor will be safe at 300 Volts for 5 minute intervals.
Specs.
You should look at the section of the Cowern Papers dealing with RMS HP loading of motors. There is a lot of information that is applicable. The paper also deals with torque at under voltage.
Excerpt;
The total amount of time for which RMS loading can be adequately calculated would depend somewhat
on the size of the motor but, in general, it would be safe to utilize this method for duty cycles that total
less than 5 minutes from start to finish (of one complete cycle). If the total time is beyond 5 minutes, then
the application should be referred to the motor manufacturer for more detailed analysis.
At 5 minutes at reduced voltage you are right on the edge.
See the paper here:
You may have to get past sales to application engineering to size the 110 kW motor but I am sure that there will be an off the shelf solution.
Bill
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"Why not the best?"
Jimmy Carter