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.
To achieve savings this great, the designer needs to consider the relationship between the pump speed curves and the system load. Figure 2 below shows an application with nearly constant pressure over the entire range of flow. This is typical of an application where pressure is a result of elevation head. Notice that the system load line is nearly horizontal. Also, note that any portion of a pump speed plot that falls below the system load line indicates that at that speed the pump cannot meet the pressure and flow requirements, making it unusable in the system.
Any portion of a pump speed plot that lies above, or intersects, with the system load line indicates that the pump can maintain pressure at that flow rate, but because the system line is nearly parallel with the speed curves, a small change in speed yields a large change in flow. Since modern drives have the ability to control frequency to a fraction of a hertz, this may pose a minor problem, but the small range of available speeds will not capitalize as well on the laws of affinity as a variable pressure/flow system. It's worth noting that even in applications where the energy savings are not as prominent, there still may be advantages in using a VFD-such as the elimination of "water hammer"-because of the drive's ability to ramp-up speed in a controlled manner (perform a soft start).
Maximized Component Life
One of reasons that the pump should be operated at the point of highest efficiency is that its performance and service life were designed around the BEP, where fluid flow through the pump is smooth, vibration is low, heat transfer is efficient and bearing wear is lowest due to uniform loading on the pump housing. Operating the pump near the BEP also greatly reduces the possibility of suction recirculation, which can cause eddy currents in the fluid being pumped and damaging cavitation.
Improved Power Quality
Another area that a drive can produce cost savings is through the maintenance of better power quality. An area of concern in systems that employ induction motors is that of power factor. Because a motor is a device that contains both resistive and reactive (i.e., magnetic) power components, the phase relationship between the motor's current and voltage will vary. Induction motors also have high inrush current levels during start-up, and mechanical starters are incapable of controlling this inrush current. In fact, the contacts in a mechanical starter must be designed to withstand this higher amount of current during start-up.
Both power factor and inrush current create problems for the electrical utility, which increases the current carrying capacity of its distribution equipment to accommodate customer power requirements. The electrical utilities will often resort to peak demand billing, where a customer is forced to pay at a rate equal to their peak current consumption over a given period of time.
Although a drive cannot change the power factor of a motor, it is capable of acting as a buffer between the motor and the utility, by using its internal energy storage capacity to change the reflected power factor. This feature-combined with the drive's ability to limit current or perform a ramped start-make a VFD an attractive choice for motor control.
Retrofits
In existing applications that use mechanical starters, it is possible to perform a retrofit that pays for itself in a relatively short time. Care must be taken in determining what components may be retained in the existing system. It may be tempting to simply replace the starter with a drive, but a VFD is more demanding of the motor and a special class of motor is often required.
Since many of the advantages of a VFD are provided by a reduction of the motor's speed, consideration must be given to what might happen as a result. For example, at reduced speed, the thermal capacity of the motor will be reduced. This is because the motor's internal cooling fan is also turning at a reduced speed. Some applications may even require that a motor be fitted with an external cooling fan for additional cooling capacity. Because the drive is basically a high-speed switch that rapidly turns the current on and off, there are inductive effects that can lead to eddy current heating of the motor bearings. Even the insulation on the motor windings is placed under additional stress. Thankfully, all of these problems are addressed by "inverter duty-rated" motors.
Conclusion
Careful analysis of system load and pump curve characteristics will ensure that all of the advantages of a VFD are realized, and in most cases those benefits will outweigh any additional costs. Refinements in technology will make future VFD offerings even more attractive, especially as costs drop and new features become available.
Pumps & Systems, February 2008