Viconics VBZS Application Guide - Manual (Page 24)

24
Variable Frequency Drive Operation:
In this case the G fan output is used as an enable / disable output for the VFD.
The VFD should be configured to be reverse acting:
- Control signal = 0 VDC = VFD at high flow = Higher pressure.
- Control signal = 10 VDC = VFD at low flow = Lower pressure.
When the VFD is disabled (Terminal G is off), the static pressure control loop is off and the VFD output is at
0VDC. Please note that the fan is ALWAYS on during occupied periods and that it will cycle on demand
with the heating and cooling staged only during unoccupied periods.
When the VFD is enabled (Terminal G is on), the static pressure control loop is enabled and the VFD will
modulate to maintain the desired static pressure set point according to the static pressure input reading at
the RTU controller. The current static pressure value can be read at the RTU controller at any time by using
the manual scroll function and displaying the pressure prompt.
3H) Minimum Supply Heat Temperature Control and Lockout
The VZ7656F1000B master controller features a minimum supply air temperature PI control loop that is
used to prevent the supply air temperature from dropping below the Min SH (Minimum Supply Heat
Temperature) parameter value.
This control sequence is only available in the VZ7656F1000B modulating heat rooftop controller.
An outdoor temperature supply air lockout is also one of the VZ7656F1000B features. This lockout only
affects the minimum supply heat control sequence. Room temperature control is not affected.
The SH Lock is the parameter used to set the outdoor air temperature above which the minimum supply
heat temperature control sequence is disabled.
Example 1:
Outdoor Temperature:
60°F
SH Lock: 55°F
Minimum Supply Heat Temperature Control is disabled
Example 2:
Outdoor Temperature:
50°F
SH Lock: 55°F
Minimum Supply Heat Temperature Control is enabled
3I) Indoor Air Quality Control Sequences & Operation
The fresh air damper can be controlled through more than one sequence to achieve different control
strategies such as free cooling (economizer mode), minimum fresh air control and CO
2
level control. Here
are the control sequences available:
Note: For the sequences mentioned below, the following conditions must be met in order for the
sequences to be performed as stated:
-
Max Pos parameter value must be greater than Min Pos Parameter value.
- Mac CO
2
parameter value must be greater than Min CO
2
Parameter value.
- Max FA parameter value must be greater than Min FA Parameter value.