Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2000-03-02
2001-01-30
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S047000
Reexamination Certificate
active
06181582
ABSTRACT:
Applicant claims, under 35 U.S.C. §119, the benefit of priority of the filing date of Mar. 5, 1999 of a German patent application, copy attached, Ser. No. 199 09 706.2, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for generating a representation of the supply voltage in connection with service devices that are capable of feedback to a supply line, in particular for frequency converters of electric motors. The invention further relates to a circuit arrangement for executing the method.
2. Discussion of Related Art
Frequency converters for electric motors of machine tools are known from the prior art, which feed the energy, which is being generated in the course of braking the electric motors, back into the supply system. To feed the energy back into the power supply, the electric motors are operated in a generator mode. The voltage output by the electric motors is initially rectified and thereafter converted to a voltage that has the same frequency and phase position as the supply voltage. In a three-phase rotary current system, at least two such voltages must be selected for feedback, the third voltage can then be calculated from the two known ones. The voltages determined in this manner are called representations of the supply voltages.
In order to keep the outlay for wires as low as possible, for determining the representation of the supply voltage it is advantageous to pick up the required control signals from the supply system downstream of the commutating chokes, to which a signal, which is proportional to the supply voltage and is pulse-width-modulated, has been applied. Since the supply voltage has been modulated by the pulse width modulation at this point, a low pass filter is required for demodulating the supply voltage. This filter should rotate the phase as little as possible, but should demodulate the pulse width modulation as well as possible. A low pass filter of a critical frequency of approximately 2 kHz is used for this.
A disadvantage of such a filter is that harmonics of the supply frequency are not suppressed. The result of this is that in case of a connection to a supply system with a relatively high internal resistance (for example because of the use of a power supply transformer for matching the supply voltage), an oscillation tendency can easily occur. This applies in particular in feedback, since a positive feedback relative to the harmonics can occur.
These disadvantages are avoided when using so-called −90 degree filters. Two of three supply voltages of a rotary current supply system are fed to such a filter, the difference between the two is formed in an upstream-connected subtracter, and this difference is rotated by −90 degrees of angle by the filter. By means of this a representation of the third phase is generated. A −90 degree filter is a low pass filter of a very low critical frequency (for example 2 Hz), which generates a phase rotation of almost −90 degrees at 50 Hz. The exact rotation of over −90 degrees is achieved by a second filter of a critical frequency of approximately 1.25 kHz and connected in series. If no rotation of exactly −90 degrees is required, the second filter can also be omitted. This circuit has the advantage that all harmonics are satisfactorily damped out of the supply frequency because of the low critical frequency, so that therefore no sinusoidal voltage with a very low distortion degree is available.
However, the disadvantage of this circuit arrangement, also known from the prior art, lies in that the determination of the representation of the supply voltage is erroneous if a supply voltage fails, since the other two supply voltages are determined from the failed supply voltage. The result of this is that, if one of the three supply voltages fails, the phase current and the intermediate circuit voltage very quickly assume extreme values because of the erroneous triggering.
OBJECT AND SUMMARY OF THE INVENTION
It is therefore a n object of the present invention to recite a method for reproducing the supply voltage, wherein an oscillating tendency because of the positive feedback of the harmonics is avoided, and also no impermissibly high currents are generated in case of the failure of a supply voltage phase. This method moreover is to be realized by the simplest and most cost-effective circuit arrangement possible.
The above object is attained by a method for generating a representation of supply voltages that includes generating control signals from supply voltages, supplying the control signals to a first path and a second path. The control signals of the first path are processed by forming a difference signal from two of the control signals, low pass filtering the difference signal and rotating the low pass filtered difference signal by −90 degrees of angle. Simultaneously, the control signals in the second path are processed by low pass filtering. The method further includes selecting either the rotated low pass filtered difference signal of the first path or the low pass filtered control signals of the second path as function of the difference between the rotated low pass filtered difference signal of the first path and the low pass filtered control signals of the second path.
The above object is attained by a circuit arrangement for generating a representation of the supply voltages that includes a first path that receives control signals, the first path having a subtracter for forming a difference signal from two of the control signals and a low pass filter to receive the difference signal. A second path receives the control signals, the second path includes a second low pass filters that passes frequencies higher than the first low pass filter does. Output signals of the first and second paths are fed to a combination network which, as a function of the difference between the output signals of the first and second paths, passes on the output signals of one of the first and second paths.
The method in accordance with the present invention has the advantage that an automatic switch is performed between the two known methods for reproducing the supply voltage. For example, if no impermissible values for phase voltages and phase currents appear, the reproduction of the supply voltages is determined in a known manner by a −90 degree filter. However, impermissible phase currents would be set as soon as a power supply voltage fails. As soon as the difference between the output voltages of the −90 degree filter and the filter with the higher critical frequency in the first and second path becomes greater than the threshold voltages of the diodes of the combination network, the low pass filters of the higher critical frequency in the second path automatically take over the control of the phase voltages. This is achieved in that the −90 degree filters are coupled in a high-ohmic manner to the summing point of the output signals of the first and second paths, but the filters of the first path via a diode series connection.
Thus, the reproduction of the supply voltages in the range of permissible values for the intermediate circuit voltage or for the phase currents is determined by the −90 degree filters. But as soon as the difference between the output signals of the first and second path exceeds a defined threshold value, which is set by the number and threshold voltages of the diodes, the supply system reproduction is determined by the low pass filters of a higher critical frequency. Because of the employment of the low pass filters of a higher critical frequency for determining the supply system reproduction, it is then possible to prevent impermissibly higher values of the intermediate circuit voltage and of the phase currents.
Details of the invention will be further explained in what follows by means of the embodiment represented in the drawings.
REFERENCES:
patent: 4507619 (1985-03-01), Dijkstra et al.
pate
Brinks Hofer Gilson & Lione
Dr. Johannes Heidenhain GmbH
Vu Bao Q.
Wong Peter S.
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