RT-SVX50M-EN
27
Low Ambient Operation (Optional)
During low ambient operation, outside air temperature
below 50°F, the Low Ambient Controller will cycle the
outdoor fan motor "Off" and "On" based on discharge
pressure. The indoor fan motor (IDM) will continue to
operate and the outdoor fan will return to normal operation
once the ambient temperature is above 50°F.
Two-Speed Indoor Fan Control
For units equipped with two-speed indoor fan control,
standard unit operation for cooling and heating will
continue and all unit functions will operate normally, except
for the control of the indoor fan speed. All units configured
with any type of heating will heat the supply air and space
as normal utilizing full speed indoor fan by energizing the
appropriate outputs. The two-speed indoor fan unit will only
utilize two speeds during the cooling operation modes
described below.
Cooling Operation
For cooling operation, the unit will operate at Low Fan
Speed during Fan Only, Economizer Only, and the first
stage of compressor unit modes by Default. When the unit
receives a call for cooling, if the economizer is enabled, the
unit will energize the associated Low Speed output and
begin to modulate the OA Damper open above minimum
position up to 100% as necessary. By default, the supply
fan will remain at low speed for the duration of Economizer-
Only cooling. If the supply fan is on when the first stage
compressor is energized, it will remain energized or
energize at low speed for the duration of Cool 1.
When there is a call for Cool 2, the second compressor is
energized and the Supply Fan will switch to High Speed.
The unit will stage down compressors for cooling in reverse
order that they were staged and the supply fan will follow
the unit mode transitions. Once it is determined through
normal cooling control that the second compressor is no
longer necessary to meet cooling demand, the second
compressor will be staged back and the supply fan will
switch to Low Speed until the unit stages all cooling off or
receives a call for Cool 2 again. Once the Supply Fan is no
longer requested ON after a Cooling call is cleared, the
Supply Fan will remain energized at Low Speed for 60
seconds as on traditional units.
If the economizer is enabled and is being utilized as the
first stage of cooling (Y1 active), if the unit receives a Y2
request, the unit will energize the first compressor and will
switch the supply fan to high speed until the Y2 request is
cleared.
Economizer Minimum Position Handling
for 2–Speed Fan
Due to the low and high speeds of the indoor fan, when the
unit is in Fan Only, Cool 1, or Economizer Only and the
indoor fan is at low speed, the unit will not be bringing in as
much fresh air as normal. By setting a higher minimum
damper position this problem could be solved, but that
would mean more fresh air than may be desired when the
supply fan is at high speed. To overcome these situations,
the control will require the setting of two economizer
minimum positions to be used when the fan is at low and
high speed to compensate for the lower amount of fresh air
at lower fan speeds and the potential for more fresh air
than desired at higher fan speeds. These minimum
positions will be obtained from the Bldg Design Min
Position and DCV Min Position pots on the RTEM-EM. The
economizer minimum position that will be utilized while the
supply fan is at low speed will be set with the DCV Min
Position pot with a range of 0-100% and the minimum
position that will be utilized while the supply fan is at high
speed will be set with the Building Design Min Position pot
with a range of 0-50%. If the Building Design Min Position
is set to be higher than the DCV Min Position, the setpoints
will be capped at the DCV Min Position and the setpoint for
the min position at low and high fan speed will be the DCV
Min Position.
Phase Monitor
The phase monitor is a three-phase line monitor module
that protects against phase loss, phase reversal and phase
unbalance. It is intended to protect compressors from
reverse rotation. It has an operating input voltage range of
180-600 Vac, and LED indicators for ON and FAULT. There
are no field adjustments and the module will automatically
reset from a fault condition.
Low Pressure Cutout
The low pressure cutouts are wired in series with the high
pressure cutouts and the temperature discharge limits. If
this circuit is open before a compressor is started, the
compressor in that circuit will not be allowed to operate.
Anytime this circuit is opened, the compressor in that circuit
is turned off immediately.
High Pressure Cutout and Temperature
Discharge Limit
The high pressure controls and temperature discharge limit
are wired in series between the compressor outputs on the
LTB and the compressor contactors. If the high pressure
safety or temperature discharge limit opens, the
compressor on the affected circuit is turned off.
Locking Safety Device with Compressor
Anti Short Cycle Timer
This device monitors compressor safety switch trips to
prevent short cycling, protecting the compressor. A manual
reset is required after a fourth safety switch trip.
Coil Frost Protection
The Frostat™ control monitors the evaporator coil
temperature to prevent the evaporator from freezing due to
low operating temperatures whenever there is a demand
for cooling. When a open circuit has occurred, the Frostat
