Danfoss VLT Series Design Manual page 19

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VLT HVAC Drive Design Guide
2.7.5. Better control
If a frequency converter is used for controlling the flow or pressure of a system, improved control
is obtained.
A frequency converter can vary the speed of the fan or pump, thereby obtaining variable control
of flow and pressure.
Furthermore, a frequency converter can quickly adapt the speed of the fan or pump to new flow
or pressure conditions in the system.
2.7.6. Cos φ compensation
Generally speaking, a frequency converter with a cos φ of 1 provides power factor correction for
the cos φ of the motor, which means that there is no need to make allowance for the cos φ of
the motor when sizing the power factor correction unit.
2.7.7. Star/delta starter or soft-starter not required
When larger motors are started, it is necessa-
ry in many countries to use equipment that
limits the start-up current. In more traditional
systems, a star/delta starter or soft-starter is
widely used. Such motor starters are not re-
quired if a frequency converter is used.
As illustrated in the figure below, a frequency
converter does not consume more than rated
current.
2.7.8. Cost of using frequency converter not higher
The example on the following page shows that
a lot of equipment is not required when a fre-
quency converter is used. It is possible to
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MG.11.B2.02 - VLT is a registered Danfoss trademark
2. Introduction to VLT HVAC Drive
1 = VLT HVAC Drive
2 = Star/delta starter
3 = Soft-starter
4 = Start directly on mains
calculate the cost of installing the two differ-
ent systems. In the example on the following
page, the two systems can be established at
roughly the same price.
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