Patent GB-1409725-A (GB)
Title: Induction motor control
IPC: H02M7/49
Filed: 1972-10-13
Abstract: 1409725 Control of AC motors WESTING- HOUSE AIR BRAKE CO 13 Oct 1972 [13 Oct 1971] 47445/72 Heading H2J The windings W1, W2, W3 and W4, W5, W6 of a polyphase induction motor 11 are supplied from inverters 41, 42 via the main and auxiliary windings 26, 31, 36; 28, 33, 38 and 27, 32, 37; 29, 34, 39 of a reactor 12, the inverters being supplied from a D.C. source 43 and controlled by a programmed logic circuit 15 having sections 45 and 51 which respectively control the inverters 41, 42 to produce constant torque operation of the motor up to a predetermined base speed and constant horse-power operation above this speed, such control being dependent on signals derived from a tachometer 64 sensing the motor speed and the logic circuit 15 also being programmed to alter the control settings of the inverters after failure of the source 43, whereby restoration of the supply whilst the induction motor is rotating causes it to be reenergized at a voltage lower than that at the instant of de-energization and at a frequency substantially equal to that representative of the motor speed at the instant of re-energization. The logic sections 45, 51 receive the signal from tachometer 64 via a voltage to frequency converter 68 and a ring and sealing counter circuit 69, the signal also being fed to low and high pass filters 58, 59 which respectively supply the sections 45, 51, each of which is enabled by the presence of both signals so that the sections 45, 51 are respectively operative below and above the base speed. In a typical starting operation to produce a voltage/frequency characteristic having a linear constant torque portion (variable voltage and frequency) and a linear constant horsepower portion (constant voltage, variable frequency), Fig. 2 (not shown), the section 45 controls the inverters by pulse width modulation and the section 51 phase shifts the inverter outputs away from a nominal 15 degrees with which the inverters 41, 42 respectively lag and lead the motor reference angle, this nominal phase difference between the inverter outputs ensuring equal power factors for the stator windings of the motor with the 30 degrees mechanical displacement thereof. The section 51 produces a six step inverter output waveform a typical example of which, together with the waveform produced by the section 45, is discussed, Figs. 3 and 4 (not shown). Conventional logic circuits and thyristor inverters are employed. A typical voltage/frequency re-energizing characteristic is discussed, Fig. 2 (not shown), and operations are described in which the motor is re-energized after de-energization during the constant torque or constant horsepower portion, the waveform shape and control during these operations also being discussed, Figs. 3 and 5 (not shown). The motor may drive a vehicle in a mass and/or rapid transit transportation system for which typical failures of its supply 43 are discussed, the logic section 45 or 51 being programmed to rapidly increase the voltage to the motor after it has been reenergized at the lower value. The interaction of motor 11, reactor 12 and inverters 41, 42 to eliminate harmonics is also discussed.