Motor Current Reduction

My psychic powers are again focusing hard to try and identify this guest's motor configuration. I see that this motor is a, a, a, (its getting clearer now) a universal motor. Oh sorry about that a VFD will not... Ooops, my mistake that's only a 5 hp motor. Let me try again, wait, wait, there's a 30KW 3 phase induction motor operated by a VFD. But that motor and drive are built by the same company. They certainly know what they're doing. They can easily contact the company with any technical questions they have. But this would not be this guest, I must continue my search. Looking, looking, looking. AHA there it is. There's your answer.

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motor current depends on the load

For a DC motor yes. For an induction motor the phase angle changes with load.

GA. you are right. Face angle will change with load.

AC motor power consumption can be broken down into two elements: Load Power and Efficiency Losses

1) Load Power is the power it takes to drive a given load. That is constant because it is based upon the work you expect to accomplish. If you have a load to move on a conveyor, it takes a certain amount of power to produce the necessary force to move it. Nothing you do, short of changing the load, will affect the amount of power the motor consumes to do that work. So from that standpoint, your AC motor is going to consume the same amount of power regardless of how you are controlling it.

2) Efficiency Loss is a factor of the consumed power versus the load power. An AC motor consumes more power than it delivers to the load, because it takes a certain amount of power just to make the motor function at all, and there are losses associated with that. Not a lot, but over long periods of time that can add up. In addition to that, because AC induction motors only work at specific speeds based upon the applied frequency and the number of poles, sometimes that speed is not what you want for the task, so some external mechanical means of changing the final work speed is used. These devices also have inefficiencies associated with them, often referred to as "mechanical transmission losses". These are the only places in which the method of controlling the motor can make a difference. The best you can do is to reduce the motor inefficiencies and remove the mechanical speed control devices from the load to increase the overall throughput efficiency.

So the answer to your question is, maybe. If for example, you are running a pump and you must maintain a specific flow rate, you might be able to save energy with a VFD. If you use a throttling valve to reduce the working pressure from the maximum pressure the pump is using, that throttling valve represents a pressure drop, which is an inefficiency in the system. If instead, you reduce the output of the pump by turning down the speed, you will eliminate that pressure drop through the valve. A similar scenario holds true for most centrifugal fans as well. In addition, if you are using a hydraulic clutch to regulate speed of a conveyor, using a VFD can eliminate the losses in that clutch as well.

But if you are spinning a mass such as a saw blade or a grinding wheel, or other constant torque load such as a geared pump, using a VFD will not save you any energy and in fact the small amount of losses in the VFD itself may end up costing you money.