Energy saving using AC drive is always vary application to application. Following points you need to consider before using AC drives for energy saving.
1. How much process flow and pressure requirement through AC motors
2. Existing control methodology like control valve in pumps, Damper or guide vane for Fans & Blowers etc. and position of the valve or dampers
3. If you have process flow and pressure data and pump or fan design data, you can calculate energy saving using affinity law
4. Loading and Unloading cycle for compressor application. If the unloading time is higher for compressor application, you will get better energy saving
5. Using affinity law, you can calculate the energy saving with consideration of AC drive losses. With this you can calculate the Pay back of AC drives.
Simply put, an AC drive can save energy COMPARED to OTHER means of VARYING flow and/or pressure in centrifugal machines, like pumps and fans. If you are not VARYING the speed, you will not save energy, regardless of whether is full speed or not. There are more cost effective means of permanently CHANGING the output if it will run continuously at that output. But if VARYING the speed is a part of your process (the work that the machine does), then compared to OTHER methods of accomplishing that, a AC drive will save significant energy. This is a very commonly misunderstood and overstated part of the picture.