38
FORM 145.32-IOM5
ISSUE DATE: 04/08/2019
SECTION 1 – INSTALLATION
The power (and optional low voltage wiring for trans-
ducer) wiring can be found inside the VFD enclosure.
No extra power wiring is required; sufficient length is
provided. The VFD option does not include an evapo-
rator fan bypass circuit in case of microdrive failure.
Microdrive must be replaced to re-activate the unit. In
case of a VFD failure, the evaporator fan stops run-
ning. However, unit compressors continue to run until
a low pressure safety trip activates.
For detailed installation and operation instructions, refer
to Variable Frequency Drive manual (
Form 145.13-NO1
).
Do not run evaporator fan motor below
30Hz. Otherwise, coil freeze-up and nui-
sance lock outs can occur.
Building excessive ductwork static pressure
can cause damage to unit or personnel.
Outdoor Fan VFD (208-230V and 460V Only)
If the unit must operate at an outdoor ambient tempera-
ture below 50.0°F, install the optional VFD condenser
fan option to maintain an acceptable condensing pres-
sure. The optional VFD controller for the condenser
fan is available on 8 and 10 ton models (208-230/3/60
and 460/3/60 power supply only).
The purpose of this option is to maintain a proper re-
frigerant head pressure during low ambient conditions.
Using a VFD for controlling the condenser fan speed
is an energy and cost effective solution for low ambi-
ent control. The VFD for the condenser fan is available
as either a factory built option or field installed kit. The
field installed kit comes complete with a manual bypass
electrical circuit. In the unlikely case of VFD failure, it
can be used to revert condenser fan control back to stan-
dard starter mode.
Mounted in the condenser corner post (or the special
enclosure for a field kit VFD), the VFD allows the
operator to set the high head pressure level (the fac-
tory default is 320 psig). To meet the desired refriger-
ant pressure setpoint, the VFD controls the condenser
fan motor's frequency (speed). The pressure transducer
must be only connected to circuit #1.
The VFD option does not include a condenser fan by-
pass circuit in the case of a microdrive failure. The mi-
crodrive needs to be replaced if a failure occurs to the
drive to put the unit back to normal operation. In the
case of a VFD failure, the condenser fan stops running.
However, the unit compressors continue to run until a
low pressure safety trip is activated.
For detailed installation and operation instructions, refer
to Variable Frequency Drive manual (
Form 145.13-NO1
).
HOT GAS BYPASS (HGBP)
To allow for low cooling load operation, a direct-acting
pressure-modulating bypass control valve is installed
on the system #1 discharge line. To maintain a desired
minimum evaporator pressure, use this valve to divert
high temperature, high pressure refrigerant around the
TXV. HGBP is standard on all units with variable air
volume (VAV) and optional on units with constant vol-
ume (CV).
Adjusting HGBP Setpoint
When the HGBP valve opens, select the coil suction
pressure or coil temperature by adjusting the screw
on the HGBP valve. To set the load, run the unit and
cool down the evaporator coil. To do this, either shut
the fans or block the airflow until the suction pressure
drops at least 5 psi below the chosen evaporator coil
setpoint. Next, allow the bypassed gas to raise the pres-
sure. The screw spring adjustment can be set until the
HGBP valve closes at the chosen setpoint.
The pressure of the evaporator coil is set to maintain an
evaporator coil temperature above the point that frost
and coil freeze-up can form.
