Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Utility Patent
2000-04-05
2001-01-02
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S095000, C363S131000
Utility Patent
active
06169670
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a PWM inverter apparatus. More particularly, the invention is concerned with a PWM inverter apparatus which can ensure a high-speed transmission of command data for generation of a PWM pulse signal while ensuring an enhanced accuracy for the control of the inverter output voltage.
The inverter apparatus receiving an AC or DC power for conversion thereof to an AC power of a given frequency for performing a speed control of an AC motor connected to the output of the inverter is well known in the art and employed widely in practical applications.
In the inverter apparatuses developed recently, bipolar transistors, insulated gate bipolar transistors (hereinafter also referred to as the IGBT in short) or the like are used as the switching elements constituting an inverter main circuit for the inverse power conversion. Further, a pulse width modulation control (referred to as the PWM control in short) is generally adopted for making the waveform of the inverter output voltage to approximate a sine wave while decreasing higher harmonic components.
As the inverter apparatus known heretofore, there may be mentioned one disclosed in JP-A-7-143735 (hereinafter referred to as the publication (
1
)). In this known inverter apparatus, a photocoupler is inserted in a transmission line for supplying a driving signal or signals from a signal generator circuitry to the switching elements constituting major parts of the inverter main circuit for electrically isolating the signal generating circuitry operating at a low voltage (i.e., low-voltage-rated circuitry) from the inverter main circuit operating at a high voltage (i.e., high-voltage-rated circuit).
As is described in the publication (
1
), in the inverter apparatuses in general, the switching elements constituting major parts of the inverter main circuit are electrically connected in the form of a bridge circuit, wherein the switching elements of upper and lower arms of the bridge circuit are connected in series to an input DC power supply. For performing the on/off control of the switching elements, a non-lap period (hereinafter referred to as the dead time period or simply as the dead time) is provided during which the driving signals are cleared simultaneously in order to inhibit the switching elements of the upper/lower arms from being turned on or closed simultaneously. Parenthetically, the turning-on/off of the switching elements is accompanied with time delays. Further, the turn-on/off characteristics of the switching elements are not uniform but vary from one element to another. In other words, dispersion is found among the switching elements in respect to the on/off characteristics. In addition, the on/off characteristics of the switching element are likely to vary under the influence of ambient temperature changes and dispersion of the time taken for the transmission of the driving signals. For these reasons, the dead time mentioned above is ordinarily set with some margin or tolerance.
Furthermore, it is noted that a period in which the switching elements of the upper/lower arms assume simultaneously the off-state occurs during the dead time period. Consequently, the output voltage of the inverter is affected by the direction in which the output current flows during the dead time period, presenting thus an uncertain factor.
For the reasons mentioned above, the output voltage of the inverter apparatus contains non-negligible error components, giving rise to an acute problem.
For coping with the problem mentioned above, there has certainly been proposed such an arrangement of the inverter apparatus that the direction of the output current is detected in an effort to cancel the output voltage error which depends on the flow direction of the output current, as disclosed in JP-A-5-316737 (hereinafter referred to as the publication (
2
)). However, with the arrangement described in this publication (
2
), it is impossible to compensate for the output voltage error which is ascribable to the dispersion of the on/off characteristics among the switching elements, differences in the susceptibilities of the switching elements to the temperature changes and dispersion of the transmission time of the driving signals.
Furthermore, in JP-A-10-23756 (referred to as the publication (
3
)), such arrangement is described in which the output voltage of the inverter is actually detected for suppressing the actual error of the output voltage relative to the command voltage. More specifically, according to the teachings disclosed in this publication (
3
), the PWM command pulse signal is compared with the pulse signal derived from the output of the inverter main circuit to detect the error contained in the inverter output voltage in terms of temporal error or deviation for thereby correcting the timings of the rising or leading edge and the falling or trailing edge of the PWM pulse so that the error contained in the inverter output voltage can be suppressed. Owing to this arrangement, there can be obtained the inverter output voltage of the pulse width which conforms to that of the PWM command pulse. In other words, with the techniques taught in the publication (
3
), error components of the inverter output voltage can certainly be canceled by and large. However, in the inverter apparatus disclosed in the publication (
3
), a photocoupler is employed in the output voltage detecting circuit as a means for electrically isolating the low-voltage-rated control circuitry and the high-voltage-rated inverter main circuit from each other. This photocoupler however exhibits not a little dispersion in respect to the delay time involved in the pulse transmission. Moreover, the transmission characteristics of the photocoupler tend to change significantly as a function of the temperature. For these reasons, with the arrangement of the inverter apparatus disclosed in the publication (
3
), it is very difficult or impossible to suppress satisfactorily the error contained in the inverter output voltage, also giving rise to a problem.
Such being the circumstances, there has been proposed an inverter apparatus of the arrangement shown in
FIG. 1
of the accompanying drawings as an approach to solve the problems mentioned above. This prior art inverter apparatus is so arranged that the PWM command pulse signal and the inverter output voltage signal as detected are straightforwardly compared with each other without intermediacy of the electrical isolating means to thereby determine the deviation or difference between the command pulse signal and the output voltage signal for correcting the error contained in the inverter output voltage on the basis of the deviation as determined. In this inverter apparatus, the electrical isolating means for isolating electrically the control circuitry designed to operate at a low voltage from the inverter main circuit operating at a high voltage is interposed between an arithmetic unit for arithmetically determining the pulse command numeric data and the pulse width modulation unit.
This known inverter apparatus will be described in more detail by reference to
FIG. 1
of the accompanying drawings. The arithmetic unit
1
which can be realized by using a microcomputer is designed to generate as the output signal thereof a pulse command numeric data signal
7
indicative of pulse generation timings required for generating a PWM command pulse signal
9
and the like. The electric isolation device denoted by reference numeral
2
is provided for the purpose of electrically isolating the low-voltage-rated circuitry including the arithmetic unit
1
from the high-voltage-rated circuitry including the inverter main circuit
6
while allowing the signal to be transmitted from the former to the latter through the isolation device. To this end, the electric isolation device can be implemented by using a photocoupler. Further referring to
FIG. 1
, reference numeral
3
denotes a pulse width modulation unit for generating the PWM command pulse signal
9
, numeral
4
denotes
Hiragi Masahiro
Okubo Tomofumi
Tokashiki Mutsuo
Antonelli Terry Stout & Kraus LLP
Berhane Adolf Deneke
Hitachi , Ltd.
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