Inverter with pre-charging capacitor to reduce inrush current

Details Inverter with pre-charging capacitor to reduce inrush current Inverter with pre-charging capacitor to reduce inrush current

2002-07-24

2003-11-25

Riley, Shawn (Department: 2838)

Electric power conversion systems

Current conversion

With starting arrangement

C388S903000

Reexamination Certificate

active

06654262

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a power conversion device; in particular, relates to an improvement thereof to limit an inrush current while reducing overall weight and volume of the device even under a high rated power.
BACKGROUND ART
FIG. 16
is a circuit diagram showing a structure of a conventional power conversion device (which will be hereinafter referred to as “first prior art”) having an inrush current limiting circuit. This power conversion device
451
comprises an inverter
5
, a capacitor
12
, a main power source
10
, switches
11
,
11
a,
and a resistor
11
b
. The power conversion device
451
is utilized by connecting a three-phase inductive load
50
, such as a motor, to output terminals U, V and W of the power conversion device
451
.
The main power source
10
supplies source input terminals P, N of the inverter
5
with a DC (i.e. direct current) main power source voltage V
DC1
. The capacitor
12
is connected to the source input terminals P, N, and thereby functions to hold a DC voltage supplied to the inverter
5
at a constant level by means of its capacitance C
DC
. The switch
11
, structured as a relay, functions as a main switch that turns ON when power supply to the inductive load
5
is started and turns OFF when stopped.
The switch
11
a
and the resistor
11
b
are interposed between the main power source
10
and the capacitor
12
so as to limit an inrush current which flows to charge the capacitor
12
immediately after the switch
11
turns on. After the switch
11
turns ON to start the inverter
5
, the switch
11
a
remains OFF until charging of the capacitor
12
is almost completed. Accordingly, the inrush current is limited by the resistor
11
b.
When the charged voltage across the capacitor
12
almost reaches the main power source voltage V
DC1
, the switch
11
a
turns on. Thereafter, the inverter
5
starts its normal operation. Thus, a DC current is supplied to the inverter
5
by the main power source
10
with little loss, during the normal operation of the inverter
5
.
However, since a large current supplied by the main power source
10
flows through the switch
11
a
and the resistor
11
b
, the switch
11
a
and the resistor
11
b
are required to have a high rated power, as well as the switch
11
as the main switch. In the power conversion device
451
, therefore, it has been a problem that overall weight and volume of the device are large and a manufacturing cost thereof is high.
Further, since the switch
11
and the switch
11
a
(in particular, the switch
11
) have to operate mechanically while a high voltage being applied, the reliability and durability thereof have been problematic. In particular, arcing is influential to the reliability and durability. Moreover, conduction losses of both the switch
11
and the switch
11
a
during the normal operation of the inverter
5
have also been not negligible concerns.
FIG. 17
is a circuit diagram showing a structure of another conventional power conversion device (which will be hereinafter referred to as “second prior art”) having an inrush current limiting circuit. This power conversion device
452
is characteristically different from the power conversion device
451
according to the first prior art in that an NTC (negative, coefficient thermistor)
11
c
is interposed between the main power source
10
and the capacitor
12
in place of the switch
11
a
and the resistor
11
b.
Usually when starting the inverter
5
, since the NTC
11
c
is low in temperature, it has high resistance. The inrush current flowing after the switch
11
turns ON is, therefore, limited by high resistance of the NTC
11
c
. The temperature of the NTC
11
c
rapidly rises up due to loss heat generated in the NTC
11
c
. Accordingly, the resistance of the NTC
11
c
rapidly decreases, which decreases loss in the NTC
11
c.
The loss generated in the NTC
11
c
during the operation of the inverter
5
is, however, not negligible, and therefore, the power conversion device
452
has been disadvantageously applicable only to devices having a low rated power (e.g. few kW or less). Further, if the inverter
5
starts operation before the NTC
11
c
has sufficiently been cooled down, e.g. the inverter
5
restarts immediately after it stopped, the NTC
11
c
does not sufficiently function as an inrush current limiter, which has also degraded the reliability of the device.
FIG. 18
is a circuit diagram showing a structure of still another conventional power conversion device (which will be hereinafter referred to as “third prior art”) having an inrush current limiting circuit. This power conversion device
453
is disclosed in Japanese Patent Application Laid-Open No. 6-115836 (1994), and is characteristically different from any one of the power conversion devices
451
and
452
in that an initial charging circuit for charging the capacitor
12
is connected to the capacitor
12
in parallel.
The initial charging circuit comprises a reactor
11
d,
a switching element
11
e
, a DC power source
11
f
, a controller unit
11
g
, a resistor
11
h
, a base drive circuit
11
j
and a diode
11
k
. When the inverter
5
is started, the switch
11
is initially set OFF. During this period, the switching element
11
e
repeatedly turns ON and OFF due to the function of the controller unit
11
g
. As a result, a current is repeatedly charged into and discharged from the reactor
11
d
, and the discharged current is repeatedly supplied to the capacitor
12
. Thus, the initial charging circuit functions as such a charge-pumping circuit as to charge up the capacitor
12
. After the capacitor
12
almost completes charging, the initial charging circuit stops its operation and the switch
11
turns on. Thereafter, the inverter
5
starts the normal operation.
The power conversion device
453
is advantageously applicable to devices having a high rated power, and does advantageously not require any of the switch
11
a
and the resistor
11
b
for limiting the inrush current (see FIG.
16
). However, the power conversion device
453
needs the reactor
11
d
and the switching element
11
e
, which is a power element, and therefore, the power conversion device
453
has been posing a problem in that overall weight and volume are large and a manufacturing cost is high, similarly to the power conversion device
451
.
DISCLOSURE OF INVENTION
Accordingly, it is an object of the present invention to obtain a power conversion device which eliminates the above mentioned problems and limits an inrush current while reducing overall weight and volume of the device even under a high rated power.
In order to achieve the object, a first aspect of the present invention is directed to a power conversion device. The power conversion device comprises: a first switching element, one main electrode thereof being connected to a first source line; a first freewheeling diode connected to the first switching element in inverse-parallel; a second switching element, one main electrode thereof being connected to other main electrode of the first switching element, and other main electrode thereof being connected to a second source line; a second freewheeling diode connected to the second switching element in inverse-parallel; a third switching element, one main electrode thereof being connected to the first source line; a third freewheeling diode connected to the third switching element in inverse-parallel; a fourth switching element, one main electrode thereof being connected to other main electrode of the third switching element, and other main electrode thereof being connected to the second source line; a fourth freewheeling diode connected to the fourth switching element in inverse-parallel; a capacitor, one end and other end thereof being connected to the first source line and the second source line, respectively; a first switch, one end thereof being connected to one source line of a set of the first source line and the second source line; an initial charging circuit having a DC power source and a second switch

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