Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2001-12-17
2002-10-01
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S034000
Reexamination Certificate
active
06459597
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to an electric power conversion apparatus having a noise reduction device for reducing a common mode noise and a normal mode noise caused by a switching device of an electric power conversion apparatus when operating an alternate current (as referred to AC from hereon) motor.
FIG. 5
is a circuit diagram of a conventional noise reduction device applied to a system, which drives a three-phase induction motor via a three-phase converter. The noise reduction device described in
FIG. 5
is substantially the same with the noise reduction device disclosed in Japanese Unexamined Laid Open Patent Application No. H09-266677.
Shown in
FIG. 5
are a single-phase AC power supply
1
; a single-phase rectifying bridge circuit
2
as a converter; a three-phase inverter circuit
3
including semiconductor switching devices Q
1
through Q
6
, such as IGBT (Insulated Gate Bipolar Transistor), and diodes connected in opposite parallel to the respective semiconductor switching devices; a three-phase induction motor
4
as a load of the inverter circuit
3
; a noise current detector
5
, such as a zero-phase current transformer formed of a ring core
5
a
, a pair of primary windings
5
b
and
5
c
, and a secondary winding
5
d
; a noise-compensation-current supply circuit
6
connected to both ends of the secondary winding
5
d
; and a Direct Current (as referred to DC from hereon) smoothing capacitor C
0
connected to a DC intermediate circuit between the rectifying circuit
2
and the inverter circuit
3
. The polarity of each winding of the noise current detector
5
is set as shown in FIG.
5
. The rectifying circuit
2
, the DC smoothing capacitor C
0
, and the inverter circuit
3
constitute an electric power conversion apparatus.
The noise-compensation-current supply circuit
6
includes a NPN (Negative-Positive-Negative) transistor Tr
1
, a PNP (Positive-Negative-Positive) transistor Tr
2
, and a coupling capacitor C
1
for cutting the DC component. Filter capacitors C
2
and C
3
are connected to both terminals of the AC power supply
1
. Both ends of the secondary winding
5
d
are connected to the bases and the emitters of the transistors Tr
1
and Tr
2
constituting the noise-compensation-current supply circuit
6
. The collector of the transistor Tr
1
is connected to the positive terminal of the ecapacitor C
0
, and the collector of the transistor Tr
2
to the negative terminal of the capacitor C
0
.
The frame of the induction motor
4
is grounded. The emitters of the transistors Tr
1
and Tr
2
are connected to a grounding point G via a coupling capacitor C
1
. The mutual connection point of the capacitors C
2
and C
3
is also connected to the grounding point G. In
FIG. 5
, the switching devices Q
1
through Q
6
of the inverter circuit
3
are controlled by PWM (Pulse Code Modulation), which is omitted from FIG.
5
.
The operations of the conventional noise reduction device will be described below. The switching devices Q
1
through Q
6
of the inverter circuit
3
are controlled to switch on and off by the PWM pulses. The induction motor
4
is driven by the output of the inverter circuit
3
. Since electrostatic capacitance (floating capacitance) C exists between the windings of the induction motor
4
and the grounding point G as shown by the broken lines in
FIG. 5
, a leakage current pulse (common mode noise current) I
C
flows through the electrostatic capacitance C whenever a voltage pulse is applied form the inverter circuit
3
to the induction motor
4
. Since the noise current flowing straight through the grounding point G causes an electric shock and malfunction of the ground breaker, the noise current needs to be eliminated.
The noise current detector
5
detects the noise current (zero-phase current component) from the difference of the currents i
C
′ flowing through a pair of power supply lines of the DC intermediate current. When a noise current is detected, the noise current detector
5
makes a detection current i
B1
flow through the secondary winding
5
d
. The detection current i
B1
drives the transistors Tr
1
or Tr
2
.
The detection current i
B1
, which has flowed into the base of the transistor Tr
1
or Tr
2
, is amplified by the transistor Tr
1
or Tr
2
, causing a current ic
1
. The transistors Tr
1
and Tr
2
switches on and off in opposite to each other such that when the transistor Tr
1
is ON, the transistor Tr
2
is OFF, and, when the transistor Tr
1
is OFF, the transistor Tr
2
is ON.
When the noise current ic flows in the direction indicated by the arrow in
FIG. 5
, the current i
B1
caused by the current i
C
′ flowing through the primary windings
5
b
and
5
c
flows through the secondary winding
5
d
. The current i
B1
switches on the transistor Tr
2
, and the noise compensation current ic
1
circulates from the capacitor C
0
to the capacitor C
0
via any of the switching devices on the upper arm of the inverter circuit
3
, the electrostatic capacitance C of the induction motor
4
, the coupling capacitor C
1
, and the transistor Tr
2
.
Since most of the noise current i
C
flows as the current i
C1
, the current i
C
′ (=i
C
−i
C1
) flowing to the output side of the rectifying circuit
2
is reduced, and the noise voltage (the noise voltage at the AC input side terminal of the rectifying circuit
2
) is also reduced.
Since the direction of the current i
B1
flowing through the secondary winding
5
d
becomes opposite when the direction of the noise current i
C
is opposite, the transistor Tr
1
is switched on. Therefore, the noise compensation current i
C1
circulates from the capacitor C
0
to the capacitor C
0
via the transistor Tr
1
, the coupling capacitor C
1
, the electrostatic capacitance of the induction motor
4
, and anyone of the switching devices on the lower arm of the inverter circuit
3
. Since most of the noise current i
C
flows as the i
C1
in this case as well, the current i
C
′ (=i
C
−i
C1
) is reduced, and the noise voltage is also reduced.
Since the transistors Tr
1
and Tr
2
constituting the noise-compensation-current supply circuit
6
are connected via the respective collectors thereof to the smoothing capacitor C
0
, it is required that the breakdown voltages of the transistors Tr
1
and Tr
2
be high enough to endure the DC voltage of the inverter circuit
3
.
In the general purpose inverter, the AC input voltage (the effective value) thereof is 200 V, and the DC voltage (DC intermediate voltage) is around 500 V. When the AC input voltage thereof is 400 V, the DC voltage is around 1000 V. Therefore, it is necessary for the transistors Tr
1
and Tr
2
to exhibit a breakdown voltage high enough to endure these DC voltages.
However, since commercially available transistors with the breakdown voltage of 500 V operate less than half speed as that of the transistor with the breakdown voltage of 300 V, the transistor with the breakdown voltage of 500 V is not so effective to reduce the noise current.
Especially in the high frequency range, wherein the operating frequency of the inverter exceeds 1 MHz, the transistors constituting the noise-compensation-current supply circuit
6
sometimes can not catch up with such a high speed. Sometimes, the transistors constituting the noise-compensation-current supply circuit
6
operate in the opposite polarity of the noise current i
C
, resulting in amplifying noise voltage in the high frequency range.
To obviate this problem, it is necessary to employ a high frequency filter to cut the high frequency noises in addition to the filter capacitors C
2
and C
3
. The additional high frequency filter enlarges the electric power conversion apparatus and increases the cost of the apparatus.
It is an object of the invention to provide a noise reduction device, which can use elements with lower breakdown voltage than the DC voltage of the inverter, and an electric power conversion apparatus with the noise reduction device, as the current control devic
Igarashi Seiki
Tanigawa Taichi
Fuji Electric & Co., Ltd.
Kanesaka & Takeuchi
Riley Shawn
LandOfFree
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