Hitachi RAC-50YH7, Service Manual, Page: 63 / 103

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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)
[
]
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Upper arm transistor is controlled to ON/OFF by 3.2kHz chopper signal. Rotation speed of the compress
is proportional to duty ratio (ON time/ ON time + OFF time) of this chopper signal.
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Time T in Fig. 3-2 shows the switching period, and relation with rotation speed (N) of the compressor is
shown by formula below;
Fig. 3-2 Switching order of power module
N = 60/2 X 1/T
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Fig. 3-3 shows voltage / current waveform at each point shown in Figs. 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.
Fig. 3-3 Voltage waveform at each point
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When power is supplied U+
[
U–, 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. 3-1)
(2) When U+ transistor is OFF: (by inductance of motor coil) U coil
[
V coil
[
V– transistor
[
Return
diode
[
Point (Fig. 3-4)
A
B
B
E
F
U
V
V W
Upper arm
transistor
Lower arm
transistor
Time
+ +
T
+
+
U— V —
W
—
T
U transistor chopped
U transistor ON
ON
Voltage point
Upper arm transistor
Current at
Chopping period
320-360V
A
+
-
B