Danfoss VLT Design Manual page 18

Introduction
2.7 Advantages
2.7.1 Why use a Frequency Converter for
Controlling Fans and Pumps?
A frequency converter takes advantage of the fact that
centrifugal fans and pumps follow the laws of propor-
tionality for such fans and pumps. For further information
see the text and figure The Laws of Proportionality.
2.7.2 The Clear Advantage - Energy Savings
The very clear advantage of using a frequency converter
for controlling the speed of fans or pumps lies in the
electricity savings.
When comparing with alternative control systems and
technologies, a frequency converter is the optimum energy
control system for controlling fan and pump systems.
Illustration 2.4 The Graph is Showing Fan Curves (A, B and C) for
Reduced Fan Volumes.
®
VLT Refrigeration Drive Design Guide
Illustration 2.5 When Using a Frequency Converter to Reduce
Fan Capacity to 60% - More Than 50% Energy Savings May Be
Obtained in Typical Applications.
2.7.3 Example of Energy Savings
As can be seen from the figure (the laws of propor-
tionality), the flow is controlled by changing the RPM. By
reducing the speed only 20% from the rated speed, the
flow is also reduced by 20%. This is because the flow is
directly proportional to the RPM. The consumption of
electricity, however, is reduced by 50%.
If the system in question only needs to be able to supply a
flow that corresponds to 100% a few days in a year, while
the average is below 80% of the rated flow for the
remainder of the year, the amount of energy saved is even
more than 50%.
The laws of proportionality
Illustration 2.6 describes the dependence of flow, pressure and
power consumption on RPM.
Q = Flow
Q
1
Q
2
H = Pressure
H
1
H
2
Table 2.3
®
MG16G102 - VLT is a registered Danfoss trademark
P = Power
= Rated flow P = Rated power
1
= Reduced flow P = Reduced power
2
n = Speed regulation
= Rated pressure n = Rated speed
1
= Reduced pressure n = Reduced speed
2
2 2
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