| 摘要 |
1509451 Frequency changing SIEMENS AG 17 March 1975 [29 March 1974 22 Jan 1975 3 March 1975] 11091/75 Heading H2F In a combination of frequency changers, e.g. for feeding an A.C. motor 6, an odd number m of frequency changers 1, 2, 3 or inverters are connected in a symmetrical m-phase star arrangement. At least one of said frequency changers (3) is provided with a control loop arranged so that the output voltage follows a trapezoidal reference waveform so that, when used with a balanced load 6, a corresponding voltage appears across the output of the other frequency changers. No direct connection is made between the load and the star point Mu so that at least two of the frequency changers 1, 2 are provided with current control loops so that the line currents are sinusoidal. As described, direct or intermediate D.C. frequency changers 1, 2, 3 are connected via transformer 4 to threephase mains 5. The outputs are connected in star with reference to a point Mu for feeding a synchronous or asynchronous motor 6 the star point Mm of which is not directly connected to the star point Mu. Frequency changer 3 is provided with a voltage control loop which monitors the voltage between load terminal T and the star point Mu and compares it with a trapezoidal reference waveform U3 of adjustable voltage; the error from comparator 19 is fed to a voltage controller 18 so as to determine the conduction angle of the frequency changer 3. Frequency changers 1 and 2 have current control loops respective to the difference between currents I1 and I2 and a sinusoidal reference current I1* and I2*. The mth (third, sixth, ninth &c.) harmonics in the currents cancel because of the balanced load 6. The use of a trapezoidal rather than sinusoidal voltage waveform enables greater fundamental output components to be achieved with given thyristors and is stated to improve the input voltage/ current phase relationship for the fundamental. Modifications.-In Fig. 7 (not shown) the trapezoidal voltage waveform for controlling the output voltage of converter 3 is derived by monitoring the voltage across one of the load terminals and the load neutral point and applying to a multiplier/limiter arrangement so as to clip off a portion of each half cycle lying between 37À5 and 142À5 degrees. The amplitude of this arrangement is regulated by rectifying the voltage between one load terminal and the load star point and comparing it with a desired reference (DC). |
