Reducing Life Cycle Costs in Pumping Applications with Variable Frequency Drives

Written by Greg Venhaus, Eaton Corporation

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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.

Greg Venhaus is a senior software engineer for VFDs at Eaton Corporation, 4201 N. 27^th Street, Milwaukee, WI 53216, 414-449-6000, www.eaton.com/electrical.

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