2012 – winter Issue

Maximize Your Electrical Investment - Considerations for VFDs

Variable Frequency Drives (VFDs) are becoming common in industrial electrical distribution systems. There are many positive benefits to using VFDs, though some downsides do exist. Weighing the pros and cons (including fairly high initial capital costs), the decision to install a VFD can become complicated. Here are five considerations when evaluating whether a VFD is best for your application.

Load Profile
VFDs can provide substantial energy savings. In variable-torque load applications, replacing a valve- or damper-type flow control with a VFD can reduce energy consumption by orders of magnitude. However, depending on the load being driven, potential savings can be considerably lower. If flow is being constricted for only short periods of time, VFD savings are only realized during that time. If the speed of a fixed-torque load is reduced, the difference in energy savings for a VFD in comparison with a typical acrossthe- line starter is minimal.

Control Requirements
Payback calculations for installation of a VFD should include not only initial capital cost of the drive purchase and installation, but also necessary changes required to the control system. Additional flow meters, control hardware and programming, and flow valves can quickly increase the cost of a small VFD installation project. Additionally, consideration of the load should be made by a control systems expert prior to any application of a VFD. In some instances, control accuracy and stability is limited by the characteristics of the load (as dictated by the application's pump curves).

Power Factor
VFDs operate at near-unity power factor. Switching large horsepower induction motor loads from across-the- line starters to VFDs can substantially raise the power factor of a distribution system and yield additional cost savings on a monthly electrical bill.

Power Quality (Harmonics)
Harmonics are electrical voltages and currents that fluctuate at frequencies other than the nominal electrical grid frequency of 60 Hz. The role of a VFD is to adjust this frequency for the load being driven, which introduces harmonics to the system. High levels of harmonics reduce the power quality of a system. This can have detrimental effects such as control system errors, premature power factor capacitor failures, and overheating of transformers. Harmonic effects can be minimized by installing line- and/or load-side reactors on a VFD, by specifying higher pulse drives, by installing harmonic-mitigating transformers, and through other methods.

Regenerative Braking
Variable-torque loads being driven by a VFD may "push back" on the drive as it slows down. An example of this is when two pumps are operated in series, and only one of the pumps is being driven by a VFD. The flow speed is controlled by adjusting the VFD speed. The VFD selected for this application should have regenerative braking capabilities allowing the drive to recover the energy that would otherwise have been dissipated to heat.

The above considerations can help you determine whether a VFD is right for your project. Knowledge of the facility's entire electrical distribution system, a well as process and control systems, are often needed to weigh these factors. Where a large horsepower or quantity of VFDs are being added, a study or simulation of the distribution system may be required before installing the drives.

For individual assistance to evaluate if VFDs are right for your process, contact Interstates Engineering 712.722.1664