March 2009

Energy Analysis Utilizing Variable Frequency Drives

Pump and Fan systems have traditionally been designed with across-the-line contactors for motor control to power mechanical air or water mover.  The output of these systems is controlled by mechanically constricting the flow with throttling valves or damping vanes.

While these methods reduce the flow of air or other fluids, it is very inefficient.  In effect, running a system this way is like driving a car with the accelerator pressed to the floor while controlling your speed with the brake. 

A variable frequency drive (VFD), on the other hand, allows a precise control of motor output, with significant reductions in the power required to handle the load.  This comes from the fact that most pumps and fans are variable torque loads.  Meaning, the torque required to drive the load changes according to the speed.

The effects that reduced speed has on a variable torque load are summarized by a set of rules known as the Affinity Laws.

1.   Flow produced is proportional to the motor speed.
2.   Pressure produced is proportional to the motor speed squared.
3.   Horsepower required is proportional the motor speed cubed.

As an example, a VFD running a variable torque load at 50% speed requires only 12.5% (.5x.5x.5) of the horsepower required to run it at 100% speed.  This reduction of horsepower means that it costs less to run the motor.  When these savings are applied over the annual hours of operation, significant savings can be achieved.  Pay back period for installing an AC drive are often less then 24 months.

The second affinity law shows the effect that this speed reduction has on the pressure that the pump or fan will be able to deliver.  This may be a significant consideration in pumping systems, but it is not as critical for most fan applications. In most pumping applications, the pump speed must be high enough to overcome system pressure.  This means the minimum pump speed must be considered in the energy savings calculations. If proper care is taken in engineering the application, VFD's can provide significant energy savings for many different installations.

Have you ever wondered how your facility can become more energy efficient? Visit Square D Lean Tools and Register to discover how Schneider Electric products can help you realize true energy savings.

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