Saving Possible Using Inverters

In general, your observations are sort of correct but the examples are incorrect.

There are legitimate energy savings available with centrifugal loads, also known as "variable torque" or "quadratic" because with a VFD, you can take full advantage of the "affinity law" involved in their use (Google that term for more info). In a nutshell though, IF you can run the motor at reduced speed INSTEAD of throttling the full output mechanically, you will save on the additional losses associated with that mechanical throttling.

So for example if you have a variable flow requirement and a centrifugal pump, you can run the pump full speed and use a valve to reduce the flow from it. The Affinity Law dictates that the power required to move the liquid varies by the cube of the flow, so when you throttle the flow, you reduce the power input requirement of the motor driving the pump. At 1/2 flow then, you will consume 1/2 x 1/2 x 1/2 or 1/8th the power. BUT, that valve creates significant losses across it in the form of pressure drop, which represents lost energy. If you remove the valve and use a VFD to alter the flow instead, you take full advantage of the reduced power consumption without the losses of the valve. There ARE losses in the VFD, but they are less than the losses in the valve. So the energy savings of a VFD are not directly in reducing the speed, because you ALWAYS reduce the energy requirement with a reduction in flow. The savings come from REPLACING a mechanical flow restriction device.

But none of the loads you mention above are variable torque loads! All of those are constant torque loads. So your observation about work load and running time would be correct for those in particular. The power consumed by the load is directly reduced with speed, but the time for the machine to complete its task is also increased. So since energy = power X time, there are NO ENERGY SAVINGS involved. In fact, now the losses in the VFD WILL need to be taken into consideration.

By the way, VFDs are a lot more than 80% efficient, it's between 95 and 97% depending on speed and load. Still, if you do not gain any energy savings in those constant torque loads, that 3-5% equates to pure wasted energy.

SOMETIMES however, the use of variable speeds can end up saving energy in a more indirect fashion. In the world of rock crushers for example, Vertical Shaft Impactors tend to alter the shape and size of the output rock based on the speed of the platen. So IF you can tweak the speed to attain a better product, you may be able to reduce what is called the "recycle rate", meaning the product that comes through the crusher too large and must be fed back into it for a second pass. This is a proven technology IN THAT SPECIFIC APPLICATION, but does not automatically translate to other types of crushers or even to every VSI installation.

The same could hold true for a sorting conveyor. If you have to return product in a loop because it went by too fast for the sorters to get at it, then that represents wasted energy. If you can slow it down so that the sortation can take place in one pass, you will save energy.