VFD for Grinding Machine Motor

Is the mill capable of running at lower speeds? This could turn in to an expensive experiment if it can't.

Not only will it NOT save any energy, it will actually More likely use MORE energy because of the losses in the VFD. Changing speed does not automatically save energy. Changing speed with a VFD can save energy COMPARED to other means of changing speed, IF changing speed is necessary for the process or task.

In your case, changing speed on a grinder just means it will accomplish less grinding, which means to grind the same amount of chiles, you will need to run longer. Since energy = power x time, you have lowered the power but increased the time, no net change.

Power used = work performed, plus any losses in performing it. So even though you have a 40HP motor, if the load is only 50% that motor is consuming only 20HP, plus losses associated with a 40HP motor, a small portion of which will be slightly greater than that of a 20HP motor. A VFD will potentially save a small portion of that small portion of motor losses, but will introduce more losses in itself and actually consume slightly MORE power to do the same work.

If your motor is NEVER loaded more than 50%, your best energy saving technique is to replace it with a new energy efficient 20HP motor. But if it OCCASIONALLY needs the extra torque of the 40HP motor, leave it alone. If it is an old inefficient motor there might be a benefit in modernizing it with a new 40HP energy efficient version, but the cost-benefit analysis must work out.