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VFD Drive Technology Can Save Over 30% In
Energy Costs
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by Matt O'Kane, Control Technique Drives, Division of
Emerson Electric Company
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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Figure 1
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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. |
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Figure 2
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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. |
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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. |
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This article is provided
courtesy of PTDA.
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