| 摘要 |
<p>1338644 Control of A.C. motors; inverting ERNEST SCRAGG & SONS Ltd 20 Jan 1972 [27 Jan 1971] 3251/71 Headings H2J and H2F A squirrel-cage motor 6 is controlled to maintain constant any selected speed within a wide range under a variable motor load and good torque regulation is obtained by maintaining the slip as a constant fraction of motor speed. Feedback signals of voltage and current supplied to the motor 6 are employed by the control system such that voltage feedback predominates in regulating motor torque at high motor speeds and current feedback predominates in regulating motor torque at low motor speeds. A speed demand signal 1 controls the frequency of an oscillator 2 which drives a three-stage ring counter 3. The six pulsed outputs, staggered by 60 degrees, pass into a waveform generator 4 which gates pairs of outputs together to synthesize six 120 degrees on and 60 degrees off pulse train waveforms to drive a three-phase inverter 5 powering the motor 6. Three mixing gates 7 modulate the waveforms and the input to gates 7 is derived from a control amplifier 8 in series with a pulse width modulated, high frequency oscillator 9, the amplifier 8 controlling the mark space ratio of oscillator 9. The amplifier 8 has three inputs, of which two are motor voltage and motor current, passed through dividers 10, 11, to render them as low voltage signals for the control circuit, and the third input is a signal derived from the speed demand signal 1 and modified in a device 12 according to the low I D =k(Kx), where I D = demand current, x= frequency (motor speed), and K is a constant. This ensures that the gain of the current feedback loop decreases with increasing frequency. The full inverter and control systems are set out in Figs. 2, 3A, 3B, 3C and 3D (not shown).</p> |
