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Adjustable Speed Drives As Applied To Centrifugal Pumps

Foreword
This article is a revised edition of the original D-7108 application
note as written by Dennis P. Connors, John D. Robechek, and
Dennis A. Jarc. Centrifugal Pumps The overall content and principles of this 1982
publication are still very much valid and relevant today. The significant
changes that have occurred since this article was originally
published is in AC technology. The dominant method of AC control
is by PWM inverters, particularly with the advent of the high
speed Insulated Gate Bipolar Transistors (IGBTs). This will be the
only drives technology to be considered in this revisited analysis.

ABSTRACT
Centrifugal Pumps are generally sized to operate at or near the
best efficiency point at maximum flow. The maximum flow
requirements, however, frequently occur for a very short period
during the operating cycle with the result that some method of
flow control is required. Centrifugal Pumps ,The traditional approach to flow control
has used valves; which increase system pressure, inherently

Figure 1. Affinity Laws for Centrifugal Pumps

waste energy, and generally cause the pump to operate at
reduced efficiencies.
Adjustable speed drives (ASDs) can achieve reduced flow by providing
adjustable speed pump operation. This results in reduced
system pressure and operation near the pump’s Best Efficiency
Point (BEP). Centrifugal Pumps In addition, maintenance costs might be reduced.
This paper will discuss the energy savings potential of AC ASDs
followed by a brief description of the operation and relative benefits
of PWM AC drives.
CENTRIFUGAL PUMP APPLICATIONS AND ENERGY
SAVINGS POTENTIAL
Centrifugal Pumps  are used on many industrial and commercial
applications. Many of these pumps are operated at fixed speeds,
but could provide energy savings through adjustable speed operation.
Reviewing the affinity laws for Centrifugal Pumps and a typical
operating cycle for a centrifugal application will show this.
Figure 1 graphically illustrates the physical laws of centrifugal
pumping applications. The flow is directly proportional to speed;
pressure is proportional to the square of the speed; and power is
proportional to the cube of the speed. These relationships can
also be expressed numerically as shown in figure 2. Theoretically,
it would be possible to operate at 50% flow with only 13% of the
power required at 100% flow. Since the power requirements
decrease much faster than the reduction in flow, the potential
exists for significant energy reduction at reduced flows.

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