Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2001-05-10
2002-05-14
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S040000
Reexamination Certificate
active
06388903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for compensating for a voltage ripple generated in a 3-phase inverter employing four switches, and more particularly to a voltage compensating apparatus and method for a 3-phase inverter employing four switches, to compensate for severe distortion in an applied 3-phase voltage due to a voltage ripple.
2. Description of the Related Art
Generally, 3-phase inverter circuits are known which use six power elements to generate a, b, and c-phase voltages, as shown in FIG.
1
. As these phase voltages are supplied to a motor, the motor rotates.
However, such inverter circuits are expensive in that they use six power switching elements. In order to reduce the costs of such inverter circuits, an inverter circuit has been proposed which uses four switches to control a 3-phase motor. An example of such an inverter circuit is shown in FIG.
2
.
FIG. 2
is a schematic circuit diagram illustrating a conventional 3-phase inverter employing four switches. As shown in
FIG. 2
, the conventional 3-phase inverter, for example, a motor controller for controlling a 3-phase electronic motor
1
includes a pair of DC link capacitors, that is, an upper DC link capacitor C
1
and a lower DC link capacitor C
2
, connected to each other in series and adapted to receive a DC voltage rectified from an AC voltage, and to store the DC voltage therein. The 3-phase motor controller also includes a B
4
inverter
2
stage configured to turn on or off in response to a switch control signal when the DC voltage from each of the capacitors C
1
and C
2
is supplied, thereby supplying a 3-phase voltage adapted to rotate [a] the 3-phase motor
1
. The
3
phase motor
1
is coupled to a connection node between the upper and lower DC link capacitors C
1
and C
2
coupled to respective switch legs of the B
4
inverter
2
.
In the case of employing four switches, the elements associated with the c-phase are eliminated. In this case, the c-phase terminal of the motor is connected to the connection node between the upper and lower DC link capacitors C
1
and C
2
.
When a DC voltage is supplied to the upper and lower DC link capacitors C
1
and C
2
in the above mentioned configuration, these capacitors C
1
and C
2
are charged with the supplied voltage. The charged voltage from each capacitor is supplied to the B
4
inverter
2
.
The B
4
inverter
2
supplied with the charged voltage supplies phase voltages to the motor
1
as its switches are turned on or off. The B
4
inverter
2
has four switching statuses, as shown in
FIGS. 4
a
to
4
d
. The following description will be made in association with the case in which the 3-phase motor has a Y-connection scheme. In the following description, “0” means an ON state of the upper switching elements in the B
4
inverter
2
whereas “1” means an ON state of the lower switching elements. Where the B
4
inverter
2
has a status of <0, 0>, respective lower switches of legs S
1
and S
2
are turned on whereas respective upper switches of the legs S
1
and S
2
are turned off.
Where only the upper ones of the switches respectively corresponding to the four voltage vectors of the B
4
inverter
2
are switched on, that is, in a state <1, 1>, the voltage charged in the upper DC link capacitor C
1
, V
1
, is supplied to the 3-phase motor
1
. In this state, no voltage is supplied from the lower DC link capacitor C
2
to the 3-phase motor
1
.
On the other hand, when only the lower switches are switched on, that is, in a state <0, 0>, the voltage charged in the lower DC link capacitor C
2
, V
2
, is supplied to the 3-phase motor
1
. In this state, no voltage is supplied from the upper DC link capacitor C
1
to the 3-phase motor
1
.
In order to allow the 3-phase motor
1
to rotate, it is necessary to generate voltages of three phases each exhibiting a phase difference of 120° from one another, Va, Vb and Vc, as shown in FIG.
3
.
In order to generate these voltages of three phases, one of the 3-phase voltage vectors from the B
4
inverter
2
respectively applied to the 3-phase motor
1
is coupled to the connection node between the upper and lower DC link capacitors C
1
and C
2
, and the remaining two voltage vectors are coupled to respective legs between the upper switches and the associated lower switches.
Also, a voltage vector of an inverted phase is also applied to the connection node between the upper and lower DC link capacitors C
1
and C
2
in order to generate voltages having the same effect as balanced 3-phase voltages. As a result, two voltage vectors respectively denoted by “Vu” and “Vw” in
FIG. 2
are generated. These voltage vectors serve as respective switching functions of the legs S
1
and S
2
in the B
4
inverter
2
.
The voltage vectors Vu and Vw serve to generate balanced 3-phase voltages along with a voltage of zero-phase. That is, it is possible to obtain 3-phase balanced voltages using four switches.
The two voltage vectors Vu and Vw generated by the B
4
inverter
2
have a phase difference of 60° therebetween, as shown in FIG.
3
. In the case in which the c-phase of the 3-phase motor is connected to the connection node between the upper and lower DC link capacitors C
1
and C
2
, as mentioned above, the phase of the voltage vector Vu is retarded from the a-phase voltage Va by 30°.
Therefore, where the B
4
inverter
2
is controlled using pulse width modulated (PWM) pulses, it is possible to control the 3-phase motor
1
using a switching logic of the B
4
inverter
2
expressed by the following Equation 1:
V
u
=
V
a_dc
=
[
1
2
+
1
2
·
ma
·
sin
(
θ
-
π
6
)
]
·
T
samp
V
w
=
V
b_dc
=
[
1
2
+
1
2
·
ma
·
sin
(
θ
-
π
2
)
]
·
T
samp
[Equation 1]
where, “&thgr;” represents the rotor position, “ma” represents the modulation rate, and “T
samp
” represents the switching sampling time.
The above Equation 1 is associated with the case in which the c-phase of the 3-phase motor is connected to the connection node between the upper and lower DC link capacitors. Referring to Equation 1, it can be found that the voltages Vw and Vu have a phase difference of 60° therebetween, and the voltage Vu is retarded in phase from the voltage Va by 30°. That is, there is an ON time ranging within the phase difference and phase retardation, for the sampling time.
Accordingly, the sampling time can be controlled using the ON time. Therefore, it is possible to control the 3-phase motor using four switching elements.
The interline voltage in the above mentioned conventional B
4
inverter corresponds to the voltage across the upper DC link capacitor or the voltage across the lower DC link capacitor in accordance with the switching status of the B
4
inverter. However, since the interline voltage results from the current supplied from only one capacitor, that is, the upper or lower DC link capacitor, the voltage ripple of each capacitor is rendered to be considerably high. Where the respective voltages across the capacitors are equal to each other, no phase variation occurs in the 3-phase voltage vectors. However, where the voltages respectively across the capacitors are different from each other, respective interline voltages corresponding to the switching states may have different levels and different phases, thereby resulting in a degradation in performance. For example, where the voltage V
1
across the upper DC link capacitor and the voltage V
2
across the lower DC link capacitor
4
are equal to each other, voltage vectors generated from the B
4
inverter are orthogonal from one another, as shown in the left side of FIG.
5
. However, where the voltages V
1
and V
2
are different from each other, voltage vectors distorted in level and phase are generated, as shown in the right side of FIG.
5
. Also, where the voltages V
1
and V
2
are equal to each other, a normal voltage, V*, is applied, as shown in the l
Cheong Dal Ho
Lee Dong Myung
Oh Jae Yoon
Yang Soon Bae
Birch & Stewart Kolasch & Birch, LLP
LG Electronics Inc.
Riley Shawn
LandOfFree
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