Hitachi RAS-E08H2A - Page 45

– 43 –
Fi
g
. 3-3 Volta
g
e wave
f
orm at each point
U
transistor c
h
oppe
d
C
hoppin
g
period (208
μ
sec.
)
Chopping perio
d
T
transistor ON
U
2
8
0
V
-300
V
V
oltage at
VV
Upper arm transistor
Current a
t
ON
+
-
A
B
V
T
i
m
e
W
T
V
U
V
U
pper ar
m
t
ran
sis
tor
L
o
w
e
r arm
t
ran
s
i
s
tor
W
W
U
EF
++
W
+
+
--
V
-
Fi
g
. 3-2
S
witchin
g
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)