Hitachi RAC-E14H2 - Page 46

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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 E: U
+
is ON, V
-
is ON (circuit in Fig. 4-1)
At point F: 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 4.8kHz 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 Figs. 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 B (Fig. 4-1)
(2) When U
+
transistor is OFF: (by inductance of motor coil) U coil → V coil → V
-
transistor →
U
-
diode → Point A (Fig. 4-4)
V
Time
W
T
V
U
V
Upper arm
transistor
Lower arm
transistor
W
U
EF
++ ++
---
U transistor chopped
Chopping period
T
transistor ON
U
280V-300V
Voltage at
Upper arm transistor
Current at
ON
+
-
Chopping period (208µ sec.)