Vlt Hvac Controls - Danfoss VLT Series Design Manual

2. Introduction to VLT HVAC Drive

2.8. VLT HVAC Controls

2.8.1. Control Principle
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A frequency converter rectifies AC voltage from mains into DC voltage, after which this DC voltage
is converted into a AC current with a variable amplitude and frequency.
The motor is supplied with variable voltage / current and frequency, which enables infinitely var-
iable speed control of three-phased, standard AC motors.
2.8.2. Control Structure
Control structure in open loop and closed loop configurations:
In the configuration shown in the illustration above, par. 1-00 is set to
reference from the reference handling system is received and fed through the ramp limitation and
speed limitation before being sent to the motor control. The output of the motor control is then
limited by the maximum frequency limit.
Select
or pressure in the controlled application. The PID parameters are located in par. group 20-**.
2.8.3. Local (Hand On) and Remote (Auto On) Control
The frequency converter can be operated manually via the local control panel (LCP) or remotely
via analog and digital inputs and serial bus.
If allowed in par. 0-40, 0-41, 0-42, and 0-43, it is possible to start and stop the frequency converter
via the LCP using the [Hand ON] and [Off] keys. Alarms can be reset via the [RESET] key. After
pressing the [Hand On] key, the frequency converter goes into Hand mode and follows (as default)
the Local reference set by using the LCP arrow keys.
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Closed loop
[3] in par. 1-00 to use the PID controller for closed loop control of e.g. speed
MG.11.B2.02 - VLT
Open loop
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is a registered Danfoss trademark
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VLT HVAC Drive Design Guide
[0]. The resulting