Danfoss VLT Series Design Manual page 22

2. Introduction to the VLT HVAC Drive
2.6.5. Better control
If an adjustable frequency drive is used for controlling the flow or pressure of a system, improved
control is obtained.
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An adjustable frequency drive can vary the speed of the fan or pump, thereby obtaining variable
control of flow and pressure.
Furthermore, an adjustable frequency drive can quickly adapt the speed of the fan or pump to
new flow or pressure conditions in the system.
Simple control of process (flow, level or pressure) utilizing the built-in PID control.
2.6.6. Cos φ compensation
Generally speaking, an adjustable frequency drive with a cos φ of 1 provides power factor cor-
rection 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.
2.6.7. Star/delta starter or soft-starter not required
When larger motors are started, it is necessary 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 required if an adjustable frequency drive is used.
As illustrated in the figure below, an adjustable frequency drive does not consume more than rated
current.
2.6.8. The cost of using an adjustable frequency drive is not signifi-
cantly high.
The example on the following page shows that a lot of extra equipment is not required when an
adjustable frequency drive is used. It is possible to calculate the cost of installing the two different
systems. In the example, the two systems can be established at roughly the same price.
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1 = VLT HVAC Drive
2 = Star/delta starter
3 = Soft-starter
4 = Start directly on line power
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide