Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2002-10-17
2004-10-05
Berhane, Adolf (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
active
06801441
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multi-phase voltage-controlled Pulse Width Modulation (PWM) frequency converter, comprising a control unit, a rectifier bridge designed to be connected to a multi-phase supply line, a direct-voltage intermediate circuit and a controlled inverter bridge for supplying a multi-phase alternating voltage into a multi-phase load.
2. Description of Background Art
Three-phase voltage-controlled PWM frequency converters have a rectifier bridge for rectifying the three-phase alternating voltage of a supply line to produce a d.c. voltage for a direct-voltage intermediate circuit, and an inverter bridge for the inversion of the intermediate circuit direct voltage into a variable-frequency three-phase alternating voltage while power is flowing in the direction from the supply line to a load, such as a cage induction motor. A cage induction motor is generally used in many applications, e.g. pumps or fans. The inverter bridge is a full-wave bridge with pulse-width-modulation controlled semiconductor switches and with diodes connected in inverse-parallel with these. The rectifier bridge may be an uncontrolled full-wave bridge, in which case only diodes are used in it, or a controlled one, in which case it is provided with controlled semiconductor switches and with diodes connected in inverse-parallel with them. In the case of a controlled rectifier bridge, power may also flow in the direction from the load to the supply line, e.g. in situations where a motor is being braked. A known possibility for implementing a controlled rectifier bridge is a three-phase circuit as presented in U.S. Pat. No. 4,447,868, which allows power flow either from the a.c. circuit into the d.c. circuit or vice versa. According to the above-mentioned patent, conduction by the transistors of the rectifier bridge is so controlled that the transistor in the upper arm of the phase with the highest supply voltage instantaneous value and the transistor in the lower arm of the phase with the lowest supply voltage instantaneous value are conducting.
Prior-art solutions aim at maintaining a constant voltage in the direct-voltage intermediate circuit by using a high-capacitance d.c. capacitor as an intermediate energy storage. Prior-art solutions also generally use a three-phase a.c. inductor unit or a single-phase d.c. inductor unit in conjunction with the rectifier bridge in order to limit supply line current peaks.
The ratings of the capacitor unit are generally determined by the capacitors' ability to withstand the electric current and voltage loading applied to them and the required service life in extreme conditions. To determine the electric loading, the components generated by the rectifier and inverter circuits are generally first calculated separately and then summed quadratically. This is the procedure observed when the capacitor unit has a considerable capacitance, in which case the circuits can be regarded as separate circuits and their instantaneous values have no effect on each other. From these starting points it follows that the capacitance of the capacitor unit becomes fairly large because the preferable capacitor type, the electrolytic capacitor, has a relatively low current tolerance. On the other hand, a large capacitance value is advantageous in respect of various regulation functions (e.g. stability of motor voltage, operation in braking situations, operation in the event of a mains failure).
Due to the large capacitor unit, the direct voltage is nearly constant. As seen from the direction of the supply line, this has the consequence that, in order to limit the mains current peak values, a considerable amount of inductance is needed at some point along the current path. At present, this inductance is most commonly placed before the rectifier bridge, so it will simultaneously protect the rectifier bridge against supply line overvoltage spikes. The rating of the current limiting inductor is usually e.g. such that, with nominal current, the voltage prevailing across the inductor equals about 3-5% of the supply voltage.
Prior-art filter components are bulky and expensive. Therefore, they are a very great factor affecting the size and cost of a frequency converter.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the present invention is to eliminate the drawbacks of prior-art solutions and to achieve a control arrangement that will make both the capacitor acting as an energy storage in the direct-voltage intermediate circuit and the inductor used to limit supply line current peaks superfluous.
The control arrangement makes it possible to connect the rectifier bridge to the inverter bridge directly without a direct-voltage capacitor unit acting as an intermediate energy storage, so that the direct current produced by the inverter bridge will flow directly into the supply line without current limitation by an inductor unit.
A multi-phase PWM frequency converter according to an embodiment of the invention uses a rectifier bridge which has fully controllable semiconductor switches and, in parallel with these, inverse-parallel connected diodes, and in which a control unit controls the conduction of the fully controllable semiconductor switches of the rectifier bridge so that the fully controllable semiconductor switch in the upper arm of the phase with the most positive supply voltage instantaneous value and the fully controllable semiconductor switch in the lower arm of the phase with the most negative supply voltage instantaneous value are continuously conducting. Thus, regardless of its direction, the intermediate circuit current can flow freely into the supply line. This embodiment of the invention is characterized in that the frequency converter requires no large-capacitance capacitor unit acting as an energy storage to smooth the intermediate circuit voltage, no large-inductance inductor unit to limit the peak values of supply line phase currents and no measurement of the supply line phase currents or of the direct current as in prior-art solutions.
A voltage-controlled multi-phase PWM frequency converter according to a second embodiment of the invention having diodes in its rectifier bridge is characterized in that the control unit produces the output voltage pulse pattern via the controllable semiconductor switches of the inverter bridge by a pre-determined method in such manner that, regardless of frequency and load, the output power factor remains above a preset minimum value, with the result that only positive current values appear in the intermediate circuit current. Therefore, the frequency converter need not be provided with a large-capacitance capacitor unit acting as an energy storage to smooth the intermediate circuit voltage nor with a high-inductance inductor unit to limit the peak values of the supply line phase currents.
The details of the features characteristic of the frequency converter of the invention are presented in the attached claims.
Although the PWM frequency converter of the invention requires no capacitor for smoothing the intermediate circuit d.c. voltage and no inductor for limiting the peak values of the mains current, a capacitor with a low capacitance value may still be used in order to limit the voltage spikes produced in switching situations by the energy latent in the stray inductances of the direct-voltage circuit. Similarly, a filter unit consisting of inductors with a low inductance value and capacitors with a low capacitance value may be used on the supply line side to filter off high-frequency harmonics from the supply current. However, these components have no essential importance in respect of the present invention.
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patent: 6
Berhane Adolf
Ricotec Oy
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