| 摘要 |
1279277 Static frequency changers; control of A.C. motors SIEMENS AG 26 Aug 1969 [2 Sept 1968] 42536/69 Headings H2F and H2J [Also in Division G3] A frequency changer having a predetermined constant current output (e.g. to supply a variable speed A.C. motor 1), comprises a controllable rectifier bridge 2 (Fig. 1) and a controllable inverter bridge 6, the D.C. output of bridge 2 being smoothed by chokes 3, 4 and a capacitor C. In parallel with the inverter bridge 6 is a controlled commutating rectifier bridge 8. Commutating capacitors 7 form with the motor stator windings 1 oscillatory circuits through the elements of bridges 6 and 8 for switching OFF the rectifier elements of inverter bridge 6. Means are provided to maintain substantially constant the difference between the voltages set up at each end of choke 3, which means operates on the control angle of the rectifier elements in bridge 2. Thereby fluctuations in the current I 1 through the choke 3 and thus also in the current I 2 due to counter E.M.F. from the current converter 6, 8, are eliminated. Control arrangement. For pre-setting the direct current I 2 to the inverter, a signal representing the difference between a desired value current I 2 * and the actual value obtained by a device 10, is applied to a current controller 9 which produces an output voltage E. The voltage U 2 at the lower terminal of choke 3 is added to the control voltage E through, a summing amplifier 12 giving a resultant voltage # which through a control device 11 determines the firing angle α of the rectifiers in bridge 2. Thus the current I 1 will depend only upon the control voltage E and will be independent of any variation in U 2 since if E is unchanged any variation in U 2 will produce a corresponding variation in the input voltage U 1 to the choke 3. The current I 1 , and thus also the motor armature current, is maintained substantially constant independently of the mechanical load on the motor. For the purpose of linearizing the relationship between the arithmetical mean value of the output voltage U 1 of the rectifier bridge 2 and the control angle α, over a relatively wide range of output voltage, there is provided a function generator 13 (Fig. 3, not shown), comprising diodes and tapped resistors. In an alternative embodiment, Fig. 2 (not shown), the function generator is not used, and the resultant voltage is produced by adding E and U 2 and subtracting U 1 . The resultant voltage is applied to a proportional plus integral controller which causes the voltage U 1 to follow U 2 , i.e. to be substantially equal to it. |
