Hitachi RAS-25WX8 - Page 100

– 99 –
Intelligent power module switches power supply current according to position of the compressor motor
rotor.
The switching order is as shown in Fig. 4-2.
At point : U
+
is ON, is ON (circuit in Fig. 4-1)
: U
+
is chopped (OFF), V
-
is ON (circuit in Fig. 4-4)
Fig. 4-2 Switching order of power module
Upper arm transistor is controlled to ON/OFF by 3.3kHz chopper signal.Rotation speed of the compressor
is proportional to duty ratio (ON time/ ON time + OFF time) of this chopper signal.
Time T in Fig. 4-2 shows the switching period, and relation with rotation speed (N) of the compressor is
shown by formula below;
N = 60/2 X 1/T
Fig. 4-3 shows voltage waveform at each point shown in Fig. 4-1 and 4-4. First half of upper arm is
chopper, second half is ON, and first half of lower arm is chopper, second half is ON.
Fig. 4-3 Voltage waveform at each point
When power is supplied U
+
V
-
, because of that U
+
is chopped, current flows as shown below;
(1) When U
+
transistor is ON: U
+
transistor
U coil V coil V
-
transistor DC current detection
resistor
Point (Fig. 4-1)
(2) When U
+
transistor is OFF: (by inductance of motor coil) U coil
V coil V
-
transistor
Return U
-
diode Point (Fig. 4-4)
E
F
At point
B
A
transistor chopped
Chopping period (300 sec.)
Chopping period
T
transistor ON
B
A
280-300V
Voltage between
Upper arm transistor
Current at
ON
U
+
U
-
Upper arm
transistor
Lower arm
transistor
V
+
V
+
W
+
U
+
W
V
U
---
T
Time
EF
V
-