5567611-BIM-B-1218
Johnson Controls Ducted Systems
39
Manual bypass
An optional, factory-installed manual bypass switch is available
with factory-installed VFD. The manual bypass switch is located
in the blower motor access compartment. The manual bypass
has the following three positions:
•
Drive
- routes power through the VFD for modulating
control of the indoor blower motor.
•
Line (or bypass)
- routes power directly to the motor that
provides full-speed motor operation and complete
electrical isolation of the drive.
•
Test
- routes power to the VFD but not to the motor to
allow for drive programming and diagnostics.
If a drive failure occurs, the unit does not automatically switch to
bypass mode. You must set the manual bypass switch to the
Line position. If there is a call for the fan, the indoor blower
motor runs at full-speed while in the bypass mode.
Figure 24: Simplified VFD wiring
Economizer sequences
Several functions can drive the economizer, including: minimum
position, free cooling, economizer loading, and minimum
outdoor air supply.
Economizer minimum position sequences
The six minimum position sequences are minimum position,
VAV economizer minimum position reset, fixed variable, low
ambient minimum position, air monitoring station reset, and
demand ventilation.
Minimum position
When the control is in the occupied mode and the FAN output is
energized, the economizer is positioned to the minimum
position setpoint unless another economizer function
commands it open or closed. When the control is in the
unoccupied mode, there is no minimum position.
VAV economizer minimum position reset
When the control is in the occupied mode and the FAN output
energizes and the VFD output reaches the high-fan speed
setting, the economizer damper position is the economizer
minimum position setpoint.
When the VFD output reaches then fan low-speed setting, the
economizer damper position is the economizer damper
minimum position low speed fan.
When the VFD output is between the fan high speed and fan
low speed settings, the economizer damper is position
proportionally between the economizer minimum position
setpoint and the economizer damper minimum position low
speed fan.
Note:
To disable the VAV economizer minimum position
reset, set the economizer minimum position setpoint
and the economizer damper minimum position low
speed fan to the same value.
Fixed variable
When the control is in the occupied mode and the FAN output
energizes and the VFD output reaches 100%, the economizer
damper position is the economizer minimum position setpoint.
When the VFD output reaches the lowest percent command of
the parameters above, the economizer damper position is the
economizer damper minimum position low speed fan.
When the VFD output is between 100% and the lowest percent
command, the economizer damper is positioned proportionally
between the economizer minimum position setpoint and the
economizer damper minimum position low speed fan.
Note:
To disable the fixed variable economizer minimum
position reset, set the economizer minimum position
setpoint and the economizer damper minimum position
low speed fan to the same value.
Before you begin any service, disconnect all power to
the drive. Be aware that high voltages are present in the
drive even after power is disconnected. Allow the
capacitors within the drive to discharge before you begin
service.
Do not connect AC power to the T1, T2, T3 drive
terminals to prevent damage to the VFD.
The fuses (FU3, FU4, FU5) supplied with the unit are
sized according to the electrical load of the blower
motor, but may not provide adequate protection to the
customer-installed drive, depending upon its
specifications. When a drive is selected and installed,
refer to the drive manufacturer's recommendations for
proper fuse sizing.
ELEMENTARY DIAGRAM
VFD
TB8 (IN CONTROL BOX)
RUN PERMIT
(DIGITAL)
SPEED REF
(ANALOG)
(2-10 VDC)
GND
GRN
GND
GRN
GND
1 DMTR
118 / BK
119 / BR
120 / Y
3
M
(T1)
(T2)
(T3)
( )
(L1)
(L3)
(L2)
722 / BK
725 / BR
728 / Y
720 / PR
723 / BR
726 / O
TB1
FU3
FU5
FU4
1
2
3
4
5
6
L3
L2
L1
T1
T2
T3
