Hitachi RAS-50WX8 Service Manual page 82

DC 320-360V is input to power module and power module switches power supply current according to rotation position of
magnet rotor. The switching order is as shown in Fig. 3-2.
At point E: U + is ON, V - is ON (circuit in Fig. 3-1)
[
At point F: U + is chopped (OFF), V - is ON (circuit in Fig. 3-4)
Upper arm
transistor
Lower arm
transistor
Upper arm transistor is controlled to ON/OFF by 4.76kHz chopper signal. Rotation speed of the compress is proportional to
duty ratio [ON time/ (ON time + OFF time)] of this chopper signal.
Time T in Fig. 3-2 shows he switching period, and relation with rotation speed (N) of the compressor is shown by formula
below;
N=60/2
Fig. 3-3 shows voltage waveform at each point shown in Fig. 3-1 and 3-4. 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
320-360V
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
diode Point (Fig. 3-4)
+
V
-
U
Fig. 3-2 Switching order of power module
1/T
transistor chopped
Chopping period
Chopping period (313
ON
Fig. 3-3 Voltage waveform at each point
U - , because of that U + is chopped, current flows as shown below;
U coil
(Fig. 3-1)
E F
+ +
U
W
-
V
T
U -
transistor ON
T
sec.)
V - transistor
V coil
V coil
– 122 –
+
V
-
W
DC current detection
V - transistor
Return
Time