VFD for Mixer Motor Machine

Short answer, probably not. The long answer requires background.

An AC motor uses POWER in accordance to the load it is connected to, plus a small amount of losses (you have actually already seen this you because reported that the motors are only showing 1/2 load on them). The losses stem from two groups of conditions; conversion losses and mechanical transmission losses. Conversion losses are those associated with converting electrical power to mechanical power, i.e. the losses in the motor (and the distribution system feeding it, but those can be ignored for this discussion). The mechanical transmission losses are those associated with getting the raw motor shaft power to the point of work, i.e. the gears, belts, friction, valves, guide vanes, whatever is used to mechanically couple the motor to the load and make the motor speed and/or torque match the task at hand.

A VFD saves energy by reducing the losses in the system somewhere. Changing the speed does NOT inherently reduce losses unless you are REPLACING some other method of changing the final output that had significant losses associated with it. I emphasized significant because the VFD itself has some conversion losses associated with it, so whatever losses you eliminate are partly offset by new losses the VFD introduces. So the original losses that you are removing must be greater (seems obvious, but it's often overlooked).

Just lowering the power alone does not inherently reduce the ENERGY consumed. Energy is different from POWER in that it is power over time. For example power can be expressed in watts, energy is watts in a period of time such as hours, or watt-hours. So if by reducing the speed to reduce the POWER consumed you increase the TIME it takes to do the work, the amount of ENERGY consumed in the process may not change. In fact if there are still fixed losses in the system that are not reduced with speed, making the process take longer can actually realize MORE losses over time. This means that most batch processes, i.e. those that require a certain amount of work and time, are not good candidates for energy savings with VFDs because slowing down the process just increases the batch time.

So all that said, mixers often fail on both counts, especially if they are batch mixers:

1) Where is the reduction in system losses going to come from? What "lossy" part of the power system is going to be eliminated by using a VFD?

2) Is decreasing the mixer speed going to increase the mixing time? If so, there may be no change in the ENERGY (kWh) input into the batch process.

Sometimes if they are continuous process mixers, those are often more akin to a centrifugal pump application and the load on the motor varies as the cube of the speed reduction. In those cases IF there is a valid reason to reduce the speed, i.e. there is a gear box being used now and you can eliminate the losses in the gear box, then you might save energy. But a potential problem with that is that gear boxes reduce speed but INCREASE shaft torque, whereas a VFD reduces sped and MAINTAINS shaft torque. So when eliminating a gear box, one must ALWAYS evaluate if the original designer was counting on the increased shaft torque from the gearbox because if he was, then the only way to use a VFD is to use a larger motor; larger by the gear ratio! That often introduces similar losses in the larger motor now.