Hitachi FS3 Series, Service Manual, Page: 181 / 240

3
MAIN PARTS
9
3.2.1.
RESISTANCE VALUE OF THERMISTOR
n
Discharge Gas Overheating Prevention
(Thermistor for Checking Upper Part of Compressor,
THM8, THM9, THM12 and THM3)
a. A thermistor checking the temperature of the compressor
upper part is installed, to prevent discharge gas from
overheating. If discharge gas temperature increases
excessively lubricating oil deterioration occurs and
lubricating properties deteriorate, resulting in short
compressor life.
b. If discharge gas temperature increases excessively,
compressor temperature increases. At the worst,
compressor motor winding will be burnt out.
c When the compressor upper part temperature increases
during heating operation, the unit is controlled according
to the following method.
1. An electronic expansion valve of liquid bypass opens
and high pressure refrigerant is returned to the
compressor through the accumulator, decreasing
compressor temperature.
2. If the compressor upper part temperature increases
exceeding 127 °C for 10 minutes even if an electronic
expansion valve opens, the compressor is stopped, in
order to protect the compressor. Thermistor
resistance characteristics are shown in Fig. 3.1.
d. If compressor upper part temperature increases
exceeding 127 °C for 10 minutes during cooling
operation, the compressor is stopped. (Refer to the block
diagram for details.)
Resist
ance V
a
lue of The
rmisto
r (k
W
)
Temperature (°C)
Fig. 3.1 Resistance Characteristics of Thermistor for
Discharge Gas Overheating protection
Fig. 3. 2 Refrigeration Cycle of Outdoor unit (RAS-10FS3)
n
Thermistor for Outdoor Ambient Temperature (THM7)
When outdoor temperature decrease to -8 °C or lower, the
compressor is stopped, during cooling operation. Thermistor
resistance characteristics are shown in Fig. 3. 3.
Resist
ance V
a
lue of The
rmisto
r (k
W
)
Ambient Temperature (°C)
Fig. 3. 3 Refrigeration Cycle of Outdoor unit (RAS-10FS3)
n
Thermistor for Defrosting (THM10, THM11)
The characteristics for the thermistor is the same with the
value of outdoor ambient temperature thermistor.