Hitachi RAC-E08H2, Service Manual, Page: 45 / 95

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Fig. 3-3 Voltage waveform at each point
U transistor chopped
Chopping period (208
μ
sec.)
Chopping period
T
transistor ON
U
280V-300V
Voltage at V V
Upper arm transistor
Current at
ON
+
-
A
B
V
Time
W
T
V U
V
Upper arm
transistor
Lower arm
transistor
W W U
E F
+ +
W
+ +
- -
V
-
Fig. 3-2 Switching order of power module
Intelligent power module switches power supply current according to position of the compressor motor
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 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. 3-2 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. 3-3 shows voltage 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.
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. 3-3)
(2) When U+ transistor is OFF: (by inductance of motor coil) U coil
→
V coil
→
V-transistor
→
U- diode
→
Point
A
(Fig. 3-4)