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VFD Drive Technology Can Save Over 30% In Energy Costs
by Matt O'Kane, Control Technique Drives, Division of Emerson Electric Company
     The evolution of AC drive technology has seen many changes in a relatively small time frame. If we look back just 15 years ago, the reputation of AC drives as a reliable and cost effective method of variable speed control was poor at best. The technology offered consisted mainly of analog controls and SCR (Silicon Controlled Rectifier) power devices. The AC products on the market at that time, for the most part, were physically large, expensive and unreliable.
     But in recent years, the technology of AC variable frequency drives (VFD) has evolved into highly sophisticated digital microprocessor control, along with high switching frequency IGBTs (Insulated Gate Bi Polar Transistors) power devices. This has led to significantly advanced capabilities from the ease of programmability to expanded diagnostics. The two most significant benefits from the evolution in technology have been that of cost and reliability, in addition to the significant reduction in physical size. 
     Now that AC variable speed drives have become so accepted into the industrial market, the potential for retrofits and new installations remains very high. The growth and demand for AC drives in North America continues at a double digit pace, and is expected to continue well into the 21st century. 
     The requirement to reduce the need for additional electrical generating stations has created a benefit to the consumer in the form of demand side management. Utility rebates provide an excellent opportunity to upgrade and maximize efficiencies at minimal, or, in some cases, virtually no cost to the industrial user.
     The applications with the greatest amount of energy savings are still centrifugal pumps and fans. It is estimated that the total market for AC drives in North America in 1995 is slightly over $600 million. Of that, 36%, or approximately $225 million, was spent on fan and pump applications.
     Experience has shown that premium efficiency motors can deliver 2-8% energy savings over standard efficiency motors. However, the potential for energy savings by applying VFD's can exceed 30 percent based upon several variables. These variables are the original design philosophy of the pump or fan system, flow modulation method, duty cycle and electricity cost.
     When looking to apply VFD control to an existing pump, a basic overview of the application should be investigated. If the original design philosophy was set for the worst case maximum flow condition in a future requirement, or if the original designer used a typical 20% oversizing criteria, there is a great potential for energy savings. However, if there have been expansions, and near full flow requirements are already in use, the potential savings may be limited. Proper evaluation is critical to accessing and correctly applying VFDs.
     Energy savings also depends on the pump system's curve. A centrifugal pump operates at the intersection of its Head-Capacity Curve and the Systems Head Curve. The Total System Head is comprised of three components: 1) Static Head (Hs), 2) Pressure Head (Hp), and 3) Friction Head (Hf); refer to Figure 1. 
     The Static Head component is the elevation difference between the liquid level on the suction side of the pump and the point of liquid discharge. It is fixed in a system and does not vary with flow. The Pressure Head is established by the application or process and is also independent of flow.
     The Friction Head in a system is compromised of all losses in the system (i.e. friction through piping, valves, fittings and pump entry and exit losses). It is flow dependent, and will vary as the flow through the system increases or decreases, or as additional restrictions occur.
     Typically, when selecting a pump for a VFD retrofit, a design point is chosen (Refer to Figure 1). If valve restriction is the method of flow control, operation moves from D to P2 as the valve is closed. If the speed of the motor is varied, the pump curve shifts with each new speed, intersecting the system curve at the corresponding flow point.
Figure 1
     The power savings for operating point A1 over P1 is 17.8 KW (Refer to figure 2). If the cost of electricity is $.06 per KWH, and the system operates for 1,000 hours at 4,000 GPM, the savings utilizing an VFD, is $1,068. If the flow is 3,000 GPM for 1,000 hours, the savings is $2,376. As you can see, the savings can be significant over the life of the installation.
Figure 2
     Additional benefits which are readily seen include: the reduction and/or elimination of motor starters, less stress on the AC motor windings and bearings, and a decrease in stress and wear on the pump or fan itself. This all equates to a smoother, longer lasting and more efficient operation process.
     Equal to the advancement of VFD technology is the role today's power transmission/motion control distributors play in providing not only the VFD, but proper application knowledge as well. Great strides have been made by local power transmission/motion control distributors to provide application start-up and troubleshooting assistance. All of these make distributors your primary source for investigating and providing cost effective energy saving solutions.
This article is provided courtesy of PTDA.
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