Yes, the torque available at 1800 rpm (90hz) would be right back where you were with the 1800rpm motor (@60hz). Still just 5hp.
I'm not sure of this but motor impedance/resistance will ask for more voltage at 90hz but you will be topped at max voltage when reaching 60hz so current will decrease and Torque will decrease too untill you reach the point where conveyor ask too much torque to run. You need a safety factor to do that and here is doesn't seem to have much room...with a 5hp....
the gearbox handle the nominal torque of a 5hp 1800rpm so theoretically the gear box won't suffer of using a 7.5hp as long as you aren't asking for more torque at 60hz...the 7.5 will just have more remaining torque a 15hz but probably still less than the 5hp at 1800rpm.
With the yaskawa, can you boost the output voltage a little bit at low rpm? You could get more power with the same current....Not sure how far you can go this way but i use to do this when i have to move a sticking belt...
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