Trane SCXG-SVX01B-EN manual sequence of, Operation operation, Step Control

sequence of

Operation operation

Compressor Safety Devices The compressors have motor temperature cutout switches in the motor windings. These switches are provided to take the compressors off line during high motor winding temperature conditions.

If a compressor low pressure cutout opens during compressor start-up, the UCM will not shut the compressor off during the first two to three minutes after start-up. This prevents possible nuisance trips during low ambient start conditions. See Table O-SO-2.

Each compressor’s discharge line contains a high pressure cutout. Under abnormal operating conditions, the cutout will open to stop compressor operation.

Table O-SO-2. Pressure cutouts

Unit	High Pressure	Low Pressure
Model	Cutout	Cutout
SXWF	360/270	20/35
SXRF	405/350	12/27

Step Control

Steps of mechanical cooling are control based on supply air or zone temperature. SeeTable O-SO-1 for compressor staging.

Capacity is based on an integrating control concept. The unit capacity matches the existing load and maintains an average supply air temperature within the supply air setpoint temperature control band region.

The supply air temperature control band is centered around supply air tempera- ture setpoint and is adjustable from 2 to 12°F. In a steady state, the unit will either maintain a constant level of cooling capacity with the supply air temperature within the control band, or the highest active cooling level will cycle to provide an average supply air temperature equal to the setpoint.

If the supply air temperature swings outside the limits of the control band, the mechanical cooling capacity will increase or decrease by one level accordingly.The change occurs by integrating the tem- perature offset from the control band limit.

A minimum time delay of five minutes follows each change in cooling level. This time delay promotes stability by allowing the system to respond to the change before any further control action occurs. As the supply air temperature ap- proaches setpoint, the time duration between changing levels of cooling capacity increases.

See Figure O-SO-2 for the typical unit operating curve. Figure O-SO-3 shows typical unit performance when supply air temperature swings exceed the control band limits.

Adjust the supply air temperature control band according to the desired unit performance. Increasing the control band reduces the equipment cycle rate and increases the maximum potential supply air temperature deviation from setpoint. Conversely, decreasing the control band reduces the maximum potential tem- perature deviation, but increases the compressor cycle rate.

Follow these recommendations concern- ing the supply air temperature control band settings based on expected unit sizing:

2 Cooling stage unit: 9°F

3 Cooling stage unit: 7°F

4 Cooling stage unit: 6°F

Low Ambient Compressor Lockout This function will lock out the compressor if the outdoor air temperature sensor reads an outdoor temperature below the low ambient compressor lockout temperature setpoint. This setpoint is adjustable at the human interface panel. Compressors will lock out when outdoor air temperature falls below that selected temperature and will start again when the temperature rises 5°F above the setpoint.

Evaporator Coil Frost Protection

FROSTAT™

The FROSTAT™ system eliminates the need for hot gas bypass. It utilizes an evaporator temperature sensor mounted on the suction line near the TXV bulb of each circuit to protect the evaporator from freezing.

If the evaporator temperature ap- proaches the specified setpoint (adjust- able between 25 and 35°F at the HI) the compressor(s) will cycle off. The supply fan remains on to help de-ice the coil. The compressors will restart when the evaporator temperature has risen 10°F above the specified cutout temperature and when the compressor(s) have been off a minimum of three minutes. This prevents rapid cycling of the compres- sors.

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