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Energy savings with variable frequency drives

As nowadays there is a trend in green energy production and energy efficiency, what solutions/applications would you recommend in reduction consumed energy of a motor driven by variable frequency drive. Almost all respectful VFD manufacturers, has a energy saving calculator mainly for ventilation and pump applications (variable torque).

A VFD is already energy efficient, as opposed to changing from DC Drives to VFD. However I have found one key element to energy conservation - operational speed and installed speed. A 4 pole motor running at 600 to 900 rpm, changed to a 6 pole motor will make considerable savings. A motor running slowly does not produce the kw's for the job, a motor running closer to rated speed does. The key lies in the application / production speed, and the installed motor speed.

Energy saving using VFDs is always vary application to application. Following points you need to consider before using VFDs 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 VFD losses. With this you can calculate the Pay back of VFDs.

The power factor standard was not set up correctly to begin with. The standard the way it has currently been set up, rewards consumers for increasing their efficiency of energy consumption and penalizes consumers for decreasing their efficiency of energy production. What is wrong with this picture? Shouldn't an energy use efficiency standard reward consumers for conservation and shower the consumer with accolades for participating in production to meet our growing demands? (the exact opposite of what we are seeing the utility companies advocate today) Something is rotten in Denmark. A consumption vs production energy standard would be a much better standard than an increase in efficiency of use vs a decrease in efficiency of energy use standard. Who initiated this standard anyways? If I were a customer paying demand charge penalties for being more of a producer than a consumer, I would not stand for it!

Simply put a VFD 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 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 VFD will save significant energy. This is a very commonly misunderstood and overstated part of the picture.

The other time I have seen VFDs save energy in non-centrifugal machines is when mechanical vari-drives are used, such as variable pulley systems, hydraulic drives, particle clutches etc., but only IF it is only speed control you are after (as opposed to torque multiplication when using a vari-drive for instance). The losses in a VFD are slightly lower than the losses in most other non-electronic speed control methods.

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