Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2001-04-06
2002-12-17
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S098000
Reexamination Certificate
active
06496397
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to control devices of inverters for variable-speed control of alternating current (AC) electric motors.
Prior known methods for controlling an inverter of the type that is used for variable-speed drive of induction motors typically include a V/f-constant control technique for controlling an output voltage V
1
of the inverter in a way proportional to a primary frequency f
1
thereof. With this approach, while it is possible to permit the excitation current of an induction motor to stay substantially constant, application of a load can lead to an increase in voltage potential drop due to the electric motor's primary resistance r
1
, resulting in a decrease in induced electromotive force of such motor. This would disadvantageously result in a decrease in magnetic flux of the motor, causing the motor's output torque to decrease accordingly. Another problem is that the “real” velocity tends to decrease with respect to a velocity command, resulting in an increase in velocity variability ratios. One approach proposed to avoid these problems is to employ automatic torque boost control techniques for achievement of torque increase in middle and low velocity regions, in particular.
A typical known approach is to detect an electric motor current by use of separate current detectors corresponding in number to two phases, as provided on the inverter output side; then convert in magnetic flux phases this current from stator coordinates into rotating magnetic field coordinates (uvw/dq conversion) to obtain a torque current Iq; then determine through computation a primary resistance voltage drop component r
1
·Iq in accordance with the current obtained; and, thereafter, calculate a voltage command Vm through addition of it to a V/f-constant voltage (induced electromotive force). In this case, the voltage command is Vm=K·f
1
*+r
1
·Iq. Here, K is the proportional gain. Such control for permitting an output voltage value to increase with an increase in current in this way is called “automatic torque boost control.” Further, a slip frequency fs in proportion to the torque current can be added to a reference frequency f
0
* to thereby compute an inverter output frequency command f
1
*. In this case, f
1
*=f
0
*+Ks·Iq. This control is known as slip compensation among those skilled in the art. These schemes suffer from a cost increase due to the fact that two separate current detectors for independent detection of two phases are required on the inverter output side. On the other hand, another scheme is also available for computing the torque current equivalent components from the average value Idc of inverter DC input currents and then performing the torque boost in accordance therewith. This control method constitutes a scheme for detecting Idc to compute an effective power current Iqx approximating the torque current Iq for compensation of the inverter output voltage in accordance with the significance of this current, in view of the fact that Equation 1 is established due to the equality of power on the DC side of the inverter to that on the AC side thereof.
&AutoLeftMatch;
Vdc
·
Idc
=
3
V1
·
I1
·
cos
ϕ
=
3
V1
·
Iqx
(
1
)
Here in the Equation 1, Vdc is the DC voltage of an inverter, Idc is the DC current average value, V
1
represents the significance of an inverter output voltage (phase voltage), I
1
represents the significance of an electric motor current, and cos&phgr; is the power factor. However, in AC electric motors, such as induction motors, the output voltage V
1
is to be controlled in a way substantially proportional to the inverter frequency in order to control the value of V/f so that it remains constant. Due to this, V
1
decreases in low speed regions, resulting in a noticeable decrease in Idc, as apparent from Equation 1 above. Accordingly, the effective power equivalent current Iqx decreases in detection accuracy, which disadvantageously poses a problem in that the automatic torque boost control decreases in accuracy.
Alternatively, current limit control is designed to detect three phase components of an inverter output current; and, when a current value goes beyond a pre-specified current limit level even with respect to one phase thereof, the inverter output frequency is reduced to thereby lower the slip frequency of an induction electric motor for causing the motor current to stay below the limit level, thus preventing occurrence of unwanted over-current trip phenomena. In this case, at least two motor current sensors are required.
Due to this, several schemes for computation and detection of the inverter's output current from a DC input current(s) of the inverter to satisfy electric motor sensorless design requirements have been proposed to date, such as for example those as disclosed in Japanese Patent Laid-open Nos. 8-19263 and 6-153526, and IEE Proceedings Vol. 136, No. 4, Jul. 1989 Pages 196-204. Also, some important teachings as to the relation among the inverter's DC input current and output current plus gate states have been recited in Journal “D” of the Institute of Electrical Engineers of Japan, “Smoothing Capacitor's Capacitance Reduction and Rapid Stop/Restartup Control Method for Voltage-Type PWM Converters,” (Apr. 1, 1992) at page 33.
In these electric motor current sensorless techniques, as recited in the above-identified Japanese documents, the one described in Japanese Patent Laid-open No. 8-19263 is designed to sample-hold a DC current in all the gate states once at a time whenever a gate state changes, resulting in an output of a sample-hold circuit varying whenever the gate state changes. Further, a difference (DC current change component) between two sample-hold circuit output values is computed in units of gate states, which requires the use of “special” A/D converters and microcomputers of the type which may offer high-speed operabilities. As for the technique of Japanese Patent Laid-open No. 6-153526, this Japanese document is completely silent about any practical configuration including the arrangement of sample-hold circuitry and how to make sample-hold signals required. Additionally, the Japanese Patent Laid-open No. 6-153526 teaches inverter output current detection methodology; however, it fails to teach current limit control and automatic torque boost control methods.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems and its object is to provide a control device for use with an inverter operable to detect from the inverter's DC input current the amplitude of an electric motor current along with a torque current and/or excitation current thereof, and then using them to perform current limit control, automatic torque boost control, or velocity sensorless vector control and the like.
An inverter device in accordance with the present invention comprises a three-phase inverter which converts DC electric power into AC power as a supplement to an AC electric motor, and a control device operatively associated therewith. The control device includes a phase current detection section for outputting a phase current waveform based on a DC input current, and an arithmetic or operational processing section based on the phase current waveform for performing operational processing tasks for control of the inverter device.
According to the present invention, since a three-phase AC phase current is detected on the basis of the DC input current, it will no longer be necessary to provide a current detector for each phase. Owing to this, the inverter device may be reduced in size or lowered in cost. Further, since the inverter is controlled based on the phase current waveform, it is possible to adequately control the inverter without having to speed up the operational processor unit.
A control device of another inverter device also incorporating the principles of the instant invention is arranged to include sample-hold signal cre
Fujii Hiroshi
Hiraga Masahiro
Ishida Seiji
Kato Junji
Nakatsu Kinya
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Patel Rajnikant B.
LandOfFree
Inverter device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Inverter device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Inverter device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2937340