HP is a short hand expression of the relationship between speed and torque. HP = T * RPM / 5250. When you run a motor at its base speed at the rated voltage, you reach the maximum power that motor can develop. The torque output is dependent upon maintaining the proper ratio of voltage and frequency, called the V/Hz ratio. So for example, a 460V 60Hz 4 pole motor is designed to produce a specific amount of torque at 1800RPM (nominal). That speed is determined by the number of poles in the windings and the applied frequency.
A VFD changes the voltage and the frequency together to maintain that same V/Hz ratio so that the motor puts out the same torque regardless of speed. But once the output voltage of the VFD gets to the input voltage, you are done... the VFD cannot create voltage from thin air. That also means of course that as you lower the Hz, you are lowering the HP output of the motor. So for example on your 460V motor, at 30Hz (1/2 speed) you are at 230V output (1/2 voltage) and at 60Hz you are at 460V. If you run the motor beyond 60Hz with the VFD, you have no more voltage to increase with frequency so you are now lowering the V/Hz ratio and you are reducing torque. The HP then at that point remains fixed, so for example a 10HP motor run at 30Hz is putting out 5HP, at 60Hz it is 10HP, but at 90Hz it is still 10HP and at 180Hz it is STILL 10HP. If you apply the basic HP formula to that you will see how much torque is going down.
Some loads, especially centrifugal loads like pumps, have an increase in torque requirements as speed increases. So running the motors above base frequency can be really dangerous because now you need MORE torque and you deliver LESS. Some loads don't really need it though, the inertia is what you are working with, so at higher speeds, the lower torque is less important and/or the motor is selected to operate at that level of torque in the first place. An example might be a spindle drive on a wood shaper. The motor is designed to put out a certain amount of torque at 400Hz, so the VFD is there to provide that 400Hz output.
As to your discussion on whether the 90Hz operation will allow the motor to run cooler because of more cooling, I'm going to disagree without knowing a lot more about the application. If the motor was selected specifically to run at 90Hz and the reduction in torque was accounted for in that selection, then maybe it's fine. But if not, then running at 90Hz would likely be over loading that motor and the slight increase in cooling air (assuming there is any) will not help enough. I say "if any" because not all motor cooling fans will move more air at higher than design speed, some have fairly flat curves. In fact they may even move LESS air! So a safer bet is what your buddy said; select the motor and gear ratio to run in the "bread basket" of motor speed, which is bout 75% and above.
) of the staff engineer; he's a plastics engineer. He is saying that the motor should run at 30-45 hz to avoid overheating the motor.
