Patent GB-1461230-A (GB)
Title: Reference value transmitter for a phase clipping control for starting a squirrel cage-asynchronous motor
IPC: H02P1/28
Filed: 1975-06-17
Abstract: 1461230 Control of AC motors SPRECHER & SCHUH AG 17 June 1975 [27 June 1974] 25796/75 Heading H2J For starting a squirrel cage asynchronous motor Mo having a phase clipping control 1, a reference value transmitter 4 is provided and comprises a start-up regulator (11) which regulates, during a terminal start-up phase, the increase of an output voltage U# from a startup voltage value U AL , which is constant during a primary start-up phase, to a rated voltage value U N  having a predetermined build-up speed, and a command transmitter 6 incorporating a motor current measuring device (7) and a threshold value indicator (8) which provides a threshold value indicative of a predetermined fraction of the motor current magnitude at the start of the primary start-up phase, the command transmitter delivering a signal to the regulator to trigger the terminal start-up phase when the motor current reaches the threshold value. As shown, the primary start-up phase may be preceded by a start-up starting phase during which the voltage is regulated either upwardly from a value U A  or downwardly from a value U N  to the voltage value U AL . As shown in Fig. 6, the command transmitter 6 receives a voltage U I  from a motor current measuring device in the form of a current-voltage converter, Fig. 7 (not shown). This voltage is applied to one input of a comparator 18 and to the input of a peak storage device 15 via a blocking circuit 14 which blocks the voltage except during the time when the transmitter provides the voltage U AL . The peak value stored, which represents the motor current at the start of the primary start-up phase, is applied to an impedance converter 16 whose output is applied to a voltage divider 17 on whose tap appears a voltage representing a predetermined fraction of the peak value stored and applied to the other input of the comparator 18. When the two inputs become equal, the comparator provides an output signal to an output stage 20 which applies the command signal to trigger the regulator in a voltage transmitter 5. The regulator comprises an integrator 25 which receives an input voltage realizing a slope γ in the start-up starting phase, a null input voltage during the primary startup phase, and an input voltage realizing a slope a in the terminal start-up phase, the switching being achieved by a switching mechanism 24 which receives an input from a differential amplifier 23 and responds to the command signal. The output of the integrator is applied via an AND gate 27 to a summation stage 28 whose output is fed to an inverting amplifier 31 to provide the output voltage U#, and to the input of amplifier 23 via a feedback device 29 also under control of the command signal. Initially, the summation device 28 provides an output - U A  or - U L  which is fed back to the differential amplifier 23 where it is compared with the voltage U AL , the amplifier 23 providing a voltage to the integrator 25 which provides a voltage having slope γ until the inputs to the differential amplifier 23 become equal. The summation stage 28 then provides an output - U AL  until the command signal is initiated which disconnects the feedback device 29 and causes the mechanism 24 to provide a voltage so that the integrator 25 provides a voltage having a slope α. The integrator is saturated at the rated voltage value U N . Details of the circuitry of the system shown in Fig. 6 are described with reference to Fig. 7 (not shown).