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Pumps & Systems, February 2008

A variable frequency drive (VFD) can be a valuable asset in reducing the life cycle costs in certain types of pumping applications. The traditional method of motor control in pumping applications is a low cost mechanical starter, which is essentially a large switch with a built-in motor protection device known as a thermal overload relay. A conscious decision to reduce upfront costs by using a mechanical starter may result in higher energy consumption, excessive component wear and poor power quality.

Alternatively, a VFD can potentially solve all of these problems and should be considered in new pumping applications. In some cases, an existing control system using mechanical starters can be retrofitted with a VFD. The cost of the retrofit is recovered quickly.

Advantages

Energy Savings with a VFD

In a pumping application using mechanical starters, the control solution is often designed for worst-case requirements, plus some safety margin of up to 20 percent extra pumping capacity. Because the speed of a motor controlled by a starter is fixed, the control of flow rate is often done using a technique known as throttling, which is accomplished by placing a restriction into the inlet of the pump. While the restriction does limit flow, it also results in losses, which decrease the efficiency of the system. In fact, it is this inefficiency that reduces the power requirement of the pump.

A designer sizes the pump using a technique that overlays a system load line on a series of plots that represent the pressure to flow relationship at various pump speeds. Within these plots, there is always a zone of highest efficiency; a single point of highest efficiency called the best efficiency point (BEP) exists in each speed plot. The designer will consider this when selecting a pump, and in his design will attempt to overlay this zone on the system load line, in the area where the pump does most of its work. The shape of the system load line is important in determining if a VFD is a logical choice for motor control.

In Figure 1 below, the blue line represents a typical system load for an application where head pressure is dictated by resistance to flow. This plot has been superimposed on a series of speed plots for a centrifugal pump. Each of the plots shows the relationship between flow and pressure at a given pump speed.

In a system using throttling, a restriction is placed in series with the pump to limit flow. This restriction will alter the system load line by shifting its intersection with the pump curves to the left. This shift results in reduced flow by moving the operating point into a less efficient zone for the pump. The reduction in load results in reduced energy consumption by the motor. The pump itself continues to run at close to the same speed, which means the moving components in the system experience the same (or worse) mechanical wear. This situation is analogous to driving a car and controlling its speed by riding the brake. Not only does that waste fuel (energy), but it also causes unnecessary wear on the brakes and nearly every other component in the drive train.

In applications with a system line like that in Figure 1, the use of a VFD is a much more efficient technique of controlling flow, like controlling a car's speed the way you should, with the gas pedal. A controlled reduction in motor speed results in an entirely different result than throttling. When the drive is commanded to reduce speed, a different pump curve is being selected. This new curve intersects the system load line at a different point, and energy is saved without sacrificing pump efficiency.

At this point, a review of a commonly referenced set of pump characteristics known as the laws of affinity for pumps is in order. The laws state that as the speed of the pump is changed:

• The volume of fluid pumped varies directly with the change in speed.
• The pressure of the fluid pumped varies as a square of the change in speed.
• The horsepower requirement of the pump varies as a cube of the change in speed.

From these laws, it can be seen that a decrease in the speed of the pump produces an even larger reduction in required horsepower, and this is where the VFD capitalizes on the situation. When a reduction in flow is required, the VFD is commanded to reduce the motor speed. A speed reduction of 50 percent often can result in an energy savings of up to 80 percent.