1. Field of the Invention
The present invention relates to a cycloconverter for converting an AC power having a given frequency into an AC power having an arbitrary frequency and the method of controlling the same.
2. Description of the Related Art
A two-stage-cascade cyclic-current cycloconverter for driving an induction motor will be described below.
FIG. 6 shows an arrangement of a main circuit of this cycloconverter.
A main circuit 9 of the cycloconverter has U-, V-, and W-phase converter units in correspondence with U, V, and W phases of an induction motor 13, respectively. In the U-phase converter unit, positive converters 23 and 24 for flowing a positive component of an AC output current are two-stage-cascade-connected to negative converters 25 and 26 for flowing a negative component of the output AC current, and transformers 21 and 22 are connected to the converters 24 and 26 in the first stage and the converters 23 and 25 in the second stage, respectively. The U-phase converter unit further includes a reactor 27 for suppressing a cyclic current.
The V-phase converter unit is constituted by connecting transformers 31 and 32, positive converters 33 and 34, negative converters 35 and 36, and a reactor 37 in the same manner as in the U-phase converter unit. The W-phase converter unit is constituted by connecting transformers 41 and 42, positive converters 43 and 44, negative-converters 45 and 46, and a reactor 47 in the same manner as in the U-phase converter unit.
Outputs from the U-, V-, and W-phase converter units of the main circuit 9 are connected to the induction motor 13.
In the cycloconverter having the above arrangement, firing signals generated by an asymmetrical controller 20 are supplied to the positive and negative converters of each of the U-, V-, and W-phase converter units. For example, firing signals U1TB and U1TA for determining an output voltage U1S are supplied to the converters 24 and 26, respectively, in the first stage of the U-phase converter unit, and firing signals U2TA and U2TB for determining an output voltage U2S are supplied to the converters 23 and 25, respectively, in its second stage.
The two converters of each of the positive and negative groups output equal voltages at the same time upon application of the firing signals, and an average voltage V of the voltage outputs from the two converters is supplied to the induction motor 13. This average voltage V is given by the following equation: ##EQU1##
Although effective values of the output voltages from the positive and negative converters are equal to each other, a cyclic current flows from the positive converters 23 and 24 to the negative converters 25 and 26 due to a differential voltage caused by output voltage waveforms. The level of the cyclic current is suppressed by the reactor 27 for cyclic current suppression. The foregoing goes for the V- and W-phase converter units.
The asymmetrical controller 20 will be described below. FIG. 7 shows waveforms of an output voltage E.sub.u 1S from the positive and negative converters in the first stage, an output voltage E.sub.u 2S from the positive and negative converters in the second stage, and a total output voltage E.sub.u of the U-phase converter unit. FIG. 8 is a flow chart for generating an output voltage reference.
In order to obtain the output voltages E.sub.u 1S and E.sub.u 2S in the corresponding stages of the U-phase converter unit, the two converters in each stage are controlled as follows. That is, the output voltage of the two converters in one stage is fixed at a maximum voltage E.alpha. of the converter, and the output voltage of the two converters in the other stage is so controlled as to obtain the total output voltage E.sub.u of the U-phase converter unit.
Converter output voltage references in the respective stages are obtained by the following equations. That is, if the total output voltage reference of the converter unit is E.sub.u &gt;O:
1st-stage converter output voltage reference EQU El.sub.u =E.alpha. PA1 2nd-stage converter output voltage reference EQU E2.sub.u =Esin.theta.ov-E.alpha. PA1 for .vertline.Esin.theta.ov.vertline..ltoreq.2E.alpha., where E.alpha. is the maximum output voltage of the converter. If the total output voltage reference of the converter unit is E.sub.u .ltoreq.O: EQU E.sub.u 1S=Esin.theta.ov+E.alpha. EQU E.sub.u 2S=-E.alpha.
By controlling the output voltages from the converters as described above, the voltage to be applied to the motor acquires a sine wave as indicated by the waveform E.sub.u shown in FIG. 7.
When the asymmetrical control as described above is performed, the converters in the first stage constantly generate the maximum voltage with a high input power factor. As compared with symmetrical control in which the same sine-wave output voltage is given to both the first and second stages, therefore, the input power factor can be largely improved.
The symmetrical control system can be applied with no problem when a frequency of the output voltage reference is low, i.e., in a low-speed operation range. If, however, a frequency is as high as 20 to 30 Hz or more, for example, an output voltage waveform in each stage for outputting the above asymmetrical control cannot be correctly output. As a result, no sine wave can be obtained as an output voltage to a motor, and a distortion in sine wave becomes a disturbance such as a torque ripple. The above asymmetrical control, therefore, can be applied in only a low-frequency range.
In order to realize a cycloconverter capable of driving a motor at a high speed and having a high input power factor, control must be performed such that the asymmetrical control is used to improve an input power factor as a cycloconverter in a low-speed range (at about a base speed) in which the input power factor is particularly degraded and the symmetrical control is used in a high-speed range to correctly supply an output waveform during a high-frequency operation.