Method and circuit for automatic speed control of asynchronous motors supplied by static frequency converters

摘要

1, 083, 568. Automatic control systems. MINISTERUL INDUSTRIEI CONSTRUCTIILOR DE MASINI ROUMANIA. March 21, 1966 [March 25, 1965 (2)], No.12338/66. Heading G3R. [Also in Division H2] An asynchronous motor 2 is supplied via a rectifier/inverter 8, the frequency of which is controlled in response to the difference between the motor current i and a reference value i* the value i* being derived from the difference between actual speed (from tachogenerator 3) and an adjustable reference 1. The r.m.s. voltage of the inverter is controlled to produce constant flux, by comparison of flux transducer 10 and fixed reference 9, at speeds below nominal, while at and above nominal speed the voltage is kept constant by comparison with a fixed maximum valve reference 13; a device 11 produces a control signal from whichever error (flux or voltage) is the lesser. Reversing is effected by change of phase sequence of the inverter and change of polarity of the current reference i*; it is effected by switch devices 15, 16 which operate when the actual speed drops below a threshold value following a reverse command set on speed reference 1. The inverter comprises two thyristor circuits per phase, each of which switch on and off several times per cycle; they are triggered on by high frequency pulses applied to gating circuits, the sequence rate (and hence output frequency) being determined by a ring counter whose rate is controlled by the current error signal. Fig. 2 (not shown). The thyristors are extinguished by LC circuits and commutating thyristors, triggered at a time determined by the voltage or flux error signals, so as to adjust the pulse lengths applied to the motor. Three inverter circuits are described; in Fig. 5 (not shown) each thyristor (T1 ... T6) has its own, single, commutating thyristor (T7.... T10), in Fig. 6 (not shown) commutating thyristors (T19.... T21) are common to all phases and in Fig. 7 (not shown) each thyristor is commutated by its own thyristor bridge network.