Hitachi RAF-50RPA Service Manual page 67

Intelligent power module switches power supply current according to position of the compressor motor
rotor.
The switching order is as shown in Fig. 7-2.
At point
At point
+
V
Upper arm
transistor
Lower arm
transistor
Upper arm transistor is controlled to ON/OFF by 2.5kHz-5kHz 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. 7-2 shows the switching period, and relation with rotation speed (N) of the compressor is
shown by formula below;
Fig. 7-3 shows voltage waveform at each point shown in Figs. 7-1 and 7-4. First half of upper arm is
chopper, second half is ON, and first half of lower arm is chopper, second half is ON.
+
U transistor chopped
V V Voltage at
A
280V-300V
Chopping period (208 sec.)
Upper arm transistor
Current at
B
When power is supplied U +
(1) When U + transistor is ON: U + transistor
resistor Point
(2) When U + transistor is OFF: (by inductance of motor coil) U coil
U - diode
Point
: U + is ON, V - is ON (circuit in Fig. 7-1)
: U + is chopped (OFF), V - is ON (circuit in Fig. 7-4)
E F
+
W W W U
- -
U V V
T
Fig. 7-2 Switching order of power module
N = 60/2 X 1/T
Chopping period
T
ON
Fig. 7-3 Voltage waveform at each point
V - , because of that U + is chopped, current flows as shown below;
U coil
(Fig. 7-3)
(Fig. 7-4)
67
+ +
V
-
W
-
transistor ON
U
V - transistor
V coil
V coil
Time
DC current detection
V - transistor