The energy equation of variable speed drives

It is interesting to look at figures from the Energy Information Administration about where energy gets consumed in our economy. These figures reveal that industrial uses account for 31% of all energy use. Industrial motor-driven systems also consume about 25% of all electricity in the U.S. and are the single largest category of electricity use in the country.

Clearly there should be many opportunities for energy savings in motor applications. But figures collected by the U.S. Dept. of Energy show there is a lot of room for improvement when it comes to industrial motors. In its most recent assessment of industrial motor systems, the DoE found about 40% of the companies it surveyed had made no improvements in fan or pump systems. And 52% had not added any kind of controls to more efficiently power variable loads.

A simple variable frequency drive consists of three sections: an input that functions as an ac rectifier; a dc link capacitor which effectively functions as a dc power supply; and an output section that creates an ac waveform that drives the motor. The frequency of the ac waveform varies to control the speed of the motor. Some VFD inverters are configured such that several output sections can share a common rectifier and dc link section.

Industrial firms often argue against installing energy efficient motor controls on the grounds that such systems are more expensive than their alternatives. The problem with this argument is that most costs associated with large electric motors are in the energy they use over their lives rather than in their up-front costs and installation expenses. Data collected by Siemens in Germany, for example, estimate the purchase price of a 150 hp motor accounts for less than 1% of its overall lifecycle costs. Energy costs account for the rest. The same kind of economics applies to smaller motors. The purchase price and installation costs of a 2-hp motor account for less than 4% of its life cycle cost. Again, energy costs make up the rest.

Thus it is logical on a variety of levels to reduce the energy costs associated with applications run by electrical motors. Variable speed drives are the usual way of approaching this goal. The typical way of controlling ac motor speed is with an inverter. The ac from the grid is first rectified to dc. This dc is then inverted to an ac waveform whose frequency is varied. This variable frequency signal drives the motor at a speed that is proportional to the frequency of the power signal.

VFDs vary motor speed as a means of saving energy in applications where the motor needn't run at full speed at all times. They are energy saving alternatives to mechanical controls such as throttling valves and dampers, which reduce flow rates while the motor still runs at full load speed. Typical uses in this category include ventilating air blowers that only need to run at full speed under the most severe environments, and pumps or compressors in which the volume of pumped fluid can vary.

A point to note is that VFDs can also save energy by recovering energy that would otherwise be wasted. VFDs outfitted with regenerative capabilities can take energy used to brake the motor and recycle it back to the power line.