5565604-BIM-B-1018
36
Johnson Controls Ducted Systems
The VFD should also be grounded per the manufacturer's
specifications.
Figure 27: Simplified VFD Wiring
A terminal block located in the control box is provided for field
connection of the VFD speed reference signal (2-10 VDC) and
to the normally-open, run-permit auxiliary contact. The use of
shielded cable is recommended for the above control wiring
connections.
Optional Hot Gas Bypass (HGBP)
To allow for low cooling load operation, a direct-acting,
pressure-modulating bypass control valve installed on the
system #1 discharge line is used to divert high temperature,
high pressure refrigerant around the TXV in order to maintain a
desired minimum evaporator pressure.
The opening pressure of the bypass valve is adjustable
between 95 and 115 psig with a factory-setting of 105 psig.
HGBP is standard on all units with VAV and optional with CV
units.
Economizer Sequences
Several functions can drive the economizer, including: minimum
position, free cooling, economizer loading, and minimum
outdoor air supply.
Economizer Minimum Position
The economizer minimum position is set during occupied mode
when outside air is not suitable for free cooling. The position of
the damper is set proportionally between the "Economizer
Minimum Position and the Economizer Minimum Position Low
Speed Fan" set points, in relationship to the VFD output
percentage. On a constant volume single speed supply fan
system both set-points should be set to the same value.
Free Cooling
Four types of free cooling options are available: dry bulb
changeover, single enthalpy, dual enthalpy changeover, and
Auto.
Dry Bulb Changeover
For dry bulb economizer operation, the outside air is suitable for
free cooling if the outside air temperature is 1°F below the
Economizer OAT Enable Setpoint
and
1°F below the Return Air
Temperature.
Free cooling is no longer available if the outside air temperature
rises above
either
the Economizer OAT Enable setpoint
or
the
return air temperature.
Single Enthalpy Changeover
For single enthalpy economizer operation, the outside air is
suitable for free cooling if the outside air enthalpy is at least 1
BTU/lb below the Economizer Outside Air Enthalpy Setpoint
and
the outside air temperature is no greater than the RAT plus
9°F.
If the outside air temperature rises above the RAT plus 10°F,
free cooling is no longer available. The outside air temperature
must drop to no greater than RAT plus 9°F to enter free cooling
again.
Free cooling is no longer available if the outside air enthalpy
rises above the Economizer Outside Air Enthalpy Setpoint.
Dual Enthalpy Changeover
For dual enthalpy economizer operation, the outside air
enthalpy must be lower than the return air enthalpy by 1 btu/lb
AND the outside air temperature is no greater than the RAT
plus 9°F.
Auto
The control determines the type of free cooling changeover
based on which sensors are present and reliable. Conditions
include:
• Return and outside air dry bulb = dry bulb changeover
• Return and outside air dry bulb and outside air humidity =
single enthalpy
• Return and outside air dry bulb and return and outside air
humidity = dual enthalpy
• If either the return or outside air dry bulb sensors are
unreliable, free cooling is not available
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. Once a drive has been 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