The best way to credibly determine energy savings in VFD
Visualization is the first step for energy efficiency. You can study the patterns, look for anomalies, and correlate operation of the system to energy consumed....then implement and verify savings from some of the suggestions provided.
If the VFD is being controlled by a PLC, you can check to see if the manufacturer has a Power Monitoring Module. This can give you (i.e. your PLC or HMI) amps, volts, and more importantly true power & KVA. Alternately, stand-alone power monitoring appliances are available (connection via voltage and CT's) that can visually present the power data in real-time for analysis.
If the motor is not sized properly, ie over-sized as they often are, then a VFD will offer savings even at fixed speed and load ie A motor running under loaded will be inefficient regardless of its rated efficiency and this can be corrected by use of a VFD (or a better motor sizing). But I do agree, if it is fixed speed, constant load, and properly sized then a VFD will offer very little savings - in many or most cases however a VFD is a good thing.
On some occasions and with the current technology applied to the VFD, you can actually conserve some amount of energy generated when the motor operates in braking mode and usually dissipated on braking resistors.
This is often called regenerative function and quite typical for 4 quadrants VFD applications. When the motor is on braking mode, it actually generates power. This power is usually dissipated on braking resistors. The idea is, rather than wastes the excess power, why not use it to power other VFD through common DC bus, or to self-power the corresponding VFD by restoring the power back to the supply network.
In the end the application decides if it is possible to use DC-bus, inverter, etc to obtain the best energy saving possible.
If the VFD is being controlled by a PLC, you can check to see if the manufacturer has a Power Monitoring Module. This can give you (i.e. your PLC or HMI) amps, volts, and more importantly true power & KVA. Alternately, stand-alone power monitoring appliances are available (connection via voltage and CT's) that can visually present the power data in real-time for analysis.
If the motor is not sized properly, ie over-sized as they often are, then a VFD will offer savings even at fixed speed and load ie A motor running under loaded will be inefficient regardless of its rated efficiency and this can be corrected by use of a VFD (or a better motor sizing). But I do agree, if it is fixed speed, constant load, and properly sized then a VFD will offer very little savings - in many or most cases however a VFD is a good thing.
On some occasions and with the current technology applied to the VFD, you can actually conserve some amount of energy generated when the motor operates in braking mode and usually dissipated on braking resistors.
This is often called regenerative function and quite typical for 4 quadrants VFD applications. When the motor is on braking mode, it actually generates power. This power is usually dissipated on braking resistors. The idea is, rather than wastes the excess power, why not use it to power other VFD through common DC bus, or to self-power the corresponding VFD by restoring the power back to the supply network.
In the end the application decides if it is possible to use DC-bus, inverter, etc to obtain the best energy saving possible.
