Hitachi RAS-50YH4 Service Manual page 75

(2) 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. 8-4 shows the overload control system configuration and Fig. 8-5 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 value 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
1
. Overload external judgement circuit
Fig. 8-1. 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
compares this input with the internally set value, and if the input exceeds the set value, it enters overload
control status.
Fig. 8-7 shows the rotation speed control. When the voltage at pin
●
set value, 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. 8-4 Overload Control System Configuration
Judgement value according to the rotation speed
(internal judgement value)
Rotation speed
is then amplified and supplied to microcomputer pin
5
– 85 –
Microcomputer
OVL judgement
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
Fig. 8-5
60
Motor control
process
OVL start current data
. The microcomputer
60
of the microcomputer exceeds the