Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2002-05-08
2003-11-04
Miller, P. (Department: 2858)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S812000, C318S808000, C318S727000
Reexamination Certificate
active
06642690
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for measuring phase current for an inverter control apparatus and an inverter control apparatus to which the measuring method is applied, and more particularly, to a method for measuring phase current for an inverter control apparatus using a single current sensor and an apparatus therefor, which is capable of changing a reference voltage for modulating a pulse width in a cycle of a triangle wave and compensating for the reference voltage changed in the same cycle, to thus measure current every cycle of the triangle wave without measuring current with error.
2. Description of the Background Art
In general, when a three-phase alternating current motor (a three-phase induction motor) is controlled using an inverter, a current sensor is used for current control or to protect the motor and an electrical load from over-current. The three current sensors are installed to correspond to the respective phases between the inverter and the three-phase alternating current motor and measure current that flows through the three-phase alternating current motor. Also, in the case that three-phase phase current is in balance, current of two-phases is measured using two sensors and the remaining current is calculated. Accordingly, three-phase output current is obtained.
Recently, a method of measuring phase current of each switching state of the inverter by installing a single current sensor between a DC(Direct Current) output circuit and the inverter and estimating the three-phase output current of the inverter according to the phase current of each measured switching state is provided. Accordingly, it is possible to reduce expenses in the manufacturing of an apparatus for measuring three-phases alternating current.
FIG. 1
is a block diagram showing a constitution of an inverter control apparatus including an apparatus for measuring current of each phase of three-phase alternating current using a single current sensor according to a prior art.
As shown in
FIG. 1
, the inverter control apparatus includes a converter
11
for converting alternating current from a three-phase AC(Alternating Current) power source
10
into direct current, an inverter
14
including a pair of switching devices Q
1
and Q
4
, a pair of switching devices Q
3
and Q
6
, and a pair of switching devices Q
5
and Q
2
in the respective phases, the inverter
14
for converting direct current from the converter into alternating current and providing the alternating current to a three-phase induction motor
16
, a current sensor
12
for measuring current that flows through a line between the converter
11
and the inverter
14
, an analog-to digital (A/D) converter
13
for converting an analog direct current value measured by the current sensor
12
into a digital current value, an inverter controller
15
for generating a pulse width modulation (PWM) signal using a reference voltage of each phase and a triangle wave on the basis of the digital measured value from the A/D converter
13
, to thus control switching of the switching devices Q
1
and Q
4
, the switching devices Q
3
and Q
6
, and the switching devices Q
5
and Q
2
in the respective phases.
The operation of the inverter control apparatus using the current sensor according to the prior art of the above constitution will now be described.
The converter
11
receives the three-phase alternating current from the three-phase current power source
10
and outputs the direct current after rectifying and smoothing to the inverter
14
. The inverter
14
converts the direct current into alternating current and outputs the alternating current to the three-phase induction motor
16
. The value of the direct current that flows between the converter
11
and the inverter
14
(flows through a so-called a direct current link) is measured using the current sensor
12
. Thus the measured analog direct current value is converted to digital data using the A/D converter
13
and is output to the inverter controller
15
. The inverter controller
15
compares the digital current value from the A/D converter
13
with a command current value, calculates a new current command value obtained by compensating for a difference value between the two values, generates a PWM signal corresponding to the new current command value, and outputs the PWM signal to the inverter
14
.
The PWM signal is a rectangular wave for comparing the direct current reference voltages of the respective phases U, V, and W with the voltage signal of a triangle wave and for turning on or off the switch device of a corresponding phase. That is, the inverter controller
15
compares the triangle wave voltage signal with the reference voltage signals Vu, Vv, and Vw of the respective phases as shown in FIG.
2
. When the reference voltage signals Vu, Vv, and Vw of the respective phases are larger than the triangle voltage signal, the inverter controller
15
outputs a high level of square wave signal (refer to the waveforms of Q
1
, Q
3
, and Q
5
of
FIG. 2
) for turning on the positive switching devices Q
1
, Q
3
, and Q
5
of a corresponding phase and a low level of square signal (waveforms that are obtained by inverting the waveforms of Q
1
, Q
3
, and Q
5
of FIG.
2
and are not shown) for turning off the negative switching devices Q
4
, Q
6
, and Q
2
of a corresponding phase. When the reference voltage signals Vu, Vv, and Vw of the respective phases are smaller than the triangle wave voltage signal, the inverter controller
15
outputs a low level of square wave signal (refer to the waveforms of Q
1
, Q
3
, and Q
5
of
FIG. 2
) for turning off the positive switching devices Q
1
, Q
3
, and Q
5
of the corresponding phase and a high level of square wave signal (waveforms that are obtained by inverting the waveforms of Q
1
, Q
3
, and Q
5
of FIG.
2
and are not shown) for turning on the negative switching devices Q
4
, Q
6
, and Q
2
of the corresponding phase. In the positive and negative switching devices, when the direction, in which current flows out through the inverter
14
to the three-phases induction motor
16
, is considered a positive direction and the direction, in which current flows in from the three-phase induction motor
16
through the inverter
14
, is considered a negative direction, in the case where the positive switching devices Q
1
, Q
3
, and Q
5
are turned on, current flows out to the three-phase induction motor
16
. In the case where the negative switching devices Q
4
, Q
6
, and Q
2
are turned on, current flows in from the three-phase induction motor
16
through the inverter
14
.
Therefore, the switching devices Q
1
through Q
6
of the inverter
14
are turned on when the pulse width modulated square wave signal is at a high level and are turned off when the pulse width modulated square wave signal is at a low level. Accordingly, the switching devices are at a certain state among the 24 states of FIG.
3
. When the positive and negative switching devices of an arbitrary phase are simultaneously turned on, the circuit of the phase is electrically shorted. Accordingly, the inverter
14
and the motor
16
are burnt out. Therefore, it is essential for the inverter controller
15
to control the positive and negative switching devices of the respective phases to not to be simultaneously turned on. Therefore, it is assumed that the positive and negative switching devices are not simultaneously turned on. The current Idc measured by the current sensor
12
flows through the three-phase induction motor
16
and is one among the 8 values in the right most column of FIG.
3
.
The above will now be described in more detail with reference to
FIGS. 1 and 3
.
Referring to
FIG. 3
, in the states of the switching devices Q
1
through Q
6
in the first and the last rows, the positive switching devices Q
1
, Q
3
, and Q
5
are turned on and the negative switching devices Q
4
, Q
6
, and Q
2
are turned off or the positive switching devices Q
1
, Q
3
, and Q
5
are turned off and the negative
Birch & Stewart Kolasch & Birch, LLP
LG Industrial Systems Co. Ltd.
Miller P.
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