Hitachi RAF-25FX8 Service Manual page 102

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.
Upper arm
transistor
Lower arm
transistor
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;
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.
U
Voltage between
A
280-300V
Upper arm transistor
Current at
B
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
(2) When U + transistor is OFF: (by inductance of motor coil) U coil → V coil → V - transistor →
Return U - diode → Point
: U + is ON,
At point
E
: U + is chopped (OFF), V - is ON (circuit in Fig. 4-4)
At point
F
+
V
U -
Fig. 4-2 Switching order of power module
N = 60/2 X 1/T
+
transistor chopped
Chopping period
Chopping period (300 sec.)
ON
Fig. 4-3 Voltage waveform at each point
(Fig. 4-1)
B
A (Fig. 4-4)
V -
is ON (circuit in Fig. 4-1)
E F
+ +
W U
-
V
T
U -
transistor ON
T
– 120 –
+
V
Time
-
W