Hitachi FSXN Series Service Manual page 343

11 Electrical checks of the
main parts
11.5 Pressure sensor
11.5.1 High pressure control
The high pressure is detected during the heating mode by means of a high pressure sensor and the compressor frequencies
are controlled by the proportional control method with operating capacity of the indoor units (or PID control for the
compressor frequency); therefore, the high pressure is controlled within an appropriate range. The output of the high
pressure sensor during the heating operation activates the protection control, the gas bypass.
Heating Operation
Proportional Control of
Compressor Frequency
with Operating Capacity
High
of Indoor Units
Pressure
Control
Protection Control (Gas
Bypass Control)
11.5.2 Low pressure control
The suction pressure is detected during the cooling operation by means of a low pressure sensor and the compressor
frequencies are controlled by the proportional control method with operating capacity of the indoor units (or PID control for
the compressor frequency); therefore, the suction pressure is controlled within an appropriate range.
If the suction pressure is excessively low, the cooling may not be sufficient and the parts of the refrigerant cycle may be
damaged. For this reason, if the output of the low pressure sensor indicates vacuum and a constant value for twelve or
more minutes, the compressor stops in order to protect itself.
Cooling Operation
Proportional Control of
Compressor Frequency
Low with Operating Capacity
of Indoor Units
Pressure
Control
Vacuum Protection
Control
340
SMGB0065 rev. 1 - 12/2010
4.5
2.5
0.5
2.5
0
Pressure MPa (kgf/cm
Output Characteristics of High Pressure Sensor
4.5
2.5
0.5
(0.3)
(-0.1) 0 1.0
Pressure MPa (kgf/cm
Output Characteristics of Low Pressure Sensor
5.0
2 G)
2.0
2 G)