Hitachi RAS-10HRNM, Service Manual, Page: 453 / 482

11.2 Thermistor
Thermistors for the outdoor unit
1. Thermistor for discharge gas temperature (THM9).
2. Thermistor for outdoor ambient temperature (THM7).
3. Thermistor for evaporating temperature (THM8).
Thermistor for upper part temperature of compressor
(For prevention of discharge gas overheating)
1 A thermistor for the upper part temperature of the compressor
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.
2 If discharge gas temperature increases excessively,
compressor temperature increases. At the worst, compressor
motor winding will be burnt out.
3 When the upper part temperature of compressor increases
during heating operation, the unit is controlled according to the
following method.
• An electronic expansion valve of outdoor units is (are)
opened to return the liquid refrigerant to the compressor
through the accumulator, decreasing compressor
temperature.
• If the compressor upper part temperature increases
exceeding 132 ºC even if an electronic expansion valve
opens, the compressor is stopped, in order to protect the
compressor.
In cooling operation, the above function is also available.
4 If compressor upper part temperature increases excessively, the protection control is activated and the compressor is
stopped according to the following method.
Temperature (ºC)
200
200
100
0
25 30 100 132 150
R25 = 200kΩ
B = 3920k
72.3
14.2 5.3
Resistance characteristics of thermistor for
discharge gas overheating protection
Resistance value of thermistor (kΩ)
11 Electrical checks of
main parts
445
SMGB0060 rev. 1- 12/2010
11