Hitachi RAS-18YH4 Service Manual page 55

6. Microcpomputer's Peripheral Circuits
6-1. Overload control circuit (OVL control circuit)
Overload control is to decrease the speed of the compressor and reduce the load when the load on
the air conditioner increases to an overload state, in order to protect the compressor, electronic
components and power breaker.
Overloads are judged by comparing the DC current level and set value
Fig.6-1 shows the overload control system configuration and Fig. 6-4 is a characteristic diagram on
overload judgement values. There are two types of control which has named IS OVL and ID OVL.
IS OVL is limiting the whole input of this room air conditioner system through the current sensor CT1
in order to keep the maximum rating of components by reading total operating current.
ID OVL is watching and limits the compressor current through the detection resistor, which is built in
IPM in order to control the compressor reliability. Since the compressor reliability is related with its
speed, the ID OVL value is also linked with the compressor speed. Fig.6-2 shows an ID OVL limitation
curve.
All of OVL operation values were programmed into EEPROM memory.
Compressor
Motor current (ID)
Detection
resistor
Amplifier
Smoother
Detection
transformer CT1
Whole current (IS)
Fig. 6-1 Overload Control System
Microcomputer
A-D
converter
Motor
control
process
A-D
converter
– 58 –
EEPROM
ID OVL
value
IS OVL
value