Sequence Of Operation - Trane TTA120D300AA Service Facts

Sequence of Operation

NOTICE
Equipment Damage!
Ensure the disconnect for the indoor air handler is closed before operating the system.
Operating the outdoor unit without the indoor fan energized can cause unit trip-out on high
pressure control and/or liquid flood back to the compressor.
Electromechanical Controls
General
Operation of the system cooling (and optional heating) cycles is controlled by the position of the
system switch on the room thermostat. Once the system switch is placed in either the HEAT or
COOL position, unit operation is automatic. The optional automatic changeover thermostat, when
in the AUTO position, automatically changes to heat or cool with sufficient room temperature
change.
Evaporator Fan (Indoor Supply Air)
The evaporator fan is controlled by an ON/AUTO switch on the room thermostat. With the switch
positioned at AUTO and the system operating in the cooling mode, fan operation coincides with
the cooling run cycles. If the system is equipped with heat and is operating in the heating mode
while the fan switch is at AUTO, fan operation coincides with the heating run cycles. When the fan
switch is positioned at ON, fan operation is continuous.
Cooling Mode
With the disconnect switch in the ON position, current is supplied to the compressor sump
heater(s), phase monitor and control transformer. The sump heater(s) supplies heat to the
compressor(s) during the "Off" cycle. The phase monitor looks at the incoming power to verify
that there is no reversed phase, no phase imbalance, and no loss of phase. If the phase monitor
detects any of these three conditions, it will shut off control voltage. The transformer steps down
the line voltage to 24V for the low voltage control circuit. When the room thermostat system switch
is positioned at COOL and the fan switch is at AUTO, the compressor contactor energizes on a call
for cooling. When the contacts of the compressor contactor close, operation of the compressor and
condenser fan begins. The evaporator fan contactor also energizes on a call for cooling and initiates
evaporator fan operation.
On units with dual circuits, the second stage of cooling is initiated as a result of the 2-stage
thermostat calling for additional cooling.
ReliaTel™ Controls
The ReliaTel™ Controls is a microelectronic control feature, which provides operating functions
that are significantly different than conventional Electromechanical units. The ReliaTel™
Refrigeration Module (RTRM) uses Proportional/Integral control algorithms to perform specific
unit functions that govern the unit operation in response to application conditions.
The RTRM provides compressor anti-short cycle timing functions through minimum "Off" and
"On" timing to increase reliability, performance and to maximize unit efficiency. Upon power
initialization, the RTRM performs self-diagnostic checks to ensure that all internal controls are
functioning. It checks the configuration parameters against the components connected to the
system. The system LED located on the RTRM module is turned "On" within one second after
power-up if all internal operations are okay.
SS-SVF15A-EN 7