Hitachi RAS-08PH1 - Page 50

Fi
g. 3-3
V
o
l
tage wave
f
orm at eac
h
po
i
nt
U
trans
i
stor c
h
o
pp
e
d
C
hopping period (208
μ
sec.
)
C
hopping perio
d
T
t
r
a
n
s
i
sto
r
O
N
U
2
80V
-
300V
V
oltage at
VV
Upper arm transistor
Cu
rr
e
n
t
at
ON
+
-
A
B
V
T
i
m
e
W
T
V
U
V
Upp
er ar
m
t
r
a
n
sisto
r
Lo
w
e
r
a
rm
t
r
a
n
sisto
r
W
W
U
EF
++
W
+
+
--
V
-
Fig. 3-2
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. 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)
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