Danfoss VLT Series Design Manual page 17

®
VLT HVAC Drive Design Guide
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 proportionality for such fans and pumps. For further information see the text
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 to be obtained.
When comparing with alternative control systems and technologies, a frequency converter is the
optimum energy control system for controlling fan and pump systems.
2.7.3. Example of energy savings
As can be seen from the figure (the laws of proportionality), 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, how-
ever, 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
This figure describes the dependence of flow, pres-
sure and power consumption on rpm.
Q = Flow
Q = Rated flow
1
Q = Reduced flow
2
H = Pressure
H = Rated pressure
1
H = Reduced pressure
2
.
®
MG.11.B2.02 - VLT is a registered Danfoss trademark
2. Introduction to VLT HVAC Drive
P = Power
P = Rated power
1
P = Reduced power
2
n = Speed regulation
n = Rated speed
1
n = Reduced speed
2
Q n
1 1
Flow
: =
Q n
2 2
( )
H n
1 1
Pressure
: =
H n
2 2
( )
P n
3
1 1
Power
: =
P n
2 2
2
The Laws of
2
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