Hitachi RAS-50WX8 Service Manual page 98

(2) Overload control circuit (OVL control circuit)
Oveload 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 judgement by comparing the DC current level and value.
Fig. 7-4 shows the overload control system configuration and Fig. 7-5 is a characteristic diagram of overload judgement
values. There are two judgement methods-external judgement which compares the externally set value with the DC current
values regardless of the rotation speed and internal judgement which compares the set value that varies according to the
rotation speed programmed in the microcomputer software with the DC current value.
To power module
negative - terminal
(R24)
Detection
resistor
Overload external judgement circuit
The filter consisting of R245 and C217 removes high harmonic components from the voltage generated by the current
flowing to Detection resistor; R245 and C217 average the voltage. This voltage is then input to IC4 pin
is supplied to microcomputer pin
exceeds the set value, it enters overload control status.
Fig. 7-7 shows the rotation speed control. When the voltage at pin
the microcomputer decreases the rotation speed of the compressor and reduces the load regardless of the rotation speed
commanded by the indoor microcomputer.
Voltage amp
circuit
(internal judgement)
DC voltage
Fig. 7-4 Overload Control System Configuration
Judgement value according to the rotation speed
(internal judgement value)
. The microcomputer compares this input with the internally set value, and if the input
AX-8T11 microcomputer
Judgement OVL
with respect to
externally set
value
Judgement OVL
A/D
according to the rotation
converter
speed
Selects data according
A/D
to the rotation speed and
converter
DC voltage
OVL start current data
Rotation speed
Fig. 7-5
of the microcomputer exceeds the set value,
– 138 –
Motor control
process
and amplified and