Hitachi RAK-18PSPA - Page 64

Fig. 7-3 Voltage wave
f
orm at each point
U
transistor chopped
C
hopping period (208嘕 sec.
)
C
hopping perio
d
T
transistor ON
U
280
V
-300
V
V
oltage at
V
V
Upper arm transistor
C
urrent a
t
O
N
+
-
A
B
V
T
i
m
e
W
T
V
U
V
U
pper ar
m
trans
i
stor
Lower arm
trans
i
stor
W
W
U
EF
++
W
++
--
V
-
Fig. 7-2
S
witching 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. 7-2.
At point ( : U
+
is ON, V
-
is ON (circuit in Fig. 7-1)
At point ) : U
+
is chopped (OFF), V
-
is ON (circuit in Fig. 7-4)
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. 7-2 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. 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.
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 % (Fig. 7-3)
(2) When U
+
transistor is OFF: (by inductance of motor coil) U coil
V coil V
-
transistor
U
-
diode Point $ (Fig. 7-4)
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