Patent GB-1382297-A (GB)
Title: Method of and apparatus for controlling a synchronous machine
IPC: H02P21/10
Filed: 1972-06-29
Abstract: 1382297 Automatic control of motors  SIEMENS AG 29 June 1972 [29 June 1971] 30599/72 Heading G3R [Also in Division H2] The stator current of a synchronous motor is controlled in dependence on (a) two desired current values adjustable independently of one another, one of which determines the stator current m.m.f. vector component at right angles to the field vector and the other of which determines the component parallel to the Held vector and (b) a pair of signals which represent respective vector components defining the momentary position of the field vector. The embodiment shown in Fig.8 uses a frequency changer 2 (cycloconverter) having pairs of anti-parallel connected thyristors in respective supply leads to the stator windings of the synchronous motor 3, the firing angle of the thyristors being determined by control sets St and phase current controllers 20 whose inputs receive respective error signals produced by comparators 19 supplied with the actual values of stator current i R , i S , i T  and desired value signals from a transformation circuit 18. The latter has inputs i a *, i b * which are produced by a vector rotator VD1 having four inputs, two of which are the desired values I W *, I B * respectively representing the stator current m.m.f. vector components at right angles and parallel to the field vector, and the remaining two of which are the vector components sin #, cos # representing the momentary position of the field vector. The rotator VD1 thus rotates the field-related values I W *, I B * through an angle # to produce the stator-related desired values i a *, i b *, Fig.1 (not shown). The signals sin #, cos # are produced by a flux computer 5 which receives as input signals (a) the outputs from a transformation circuit 12 receiving the actual stator currents i R , i S , i T  and producing two stator-related, orthogonal, stator current components (i a , i b ), Fig.1 (not shown); (b) the outputs from a transformation circuit 16 receiving, via a digital three-phase current generator 15, the pulse output of an electromagnetic pulse generator 15a, 17 responsive to the rotor movement and producing signals sin α, cos α which define the momentary angular position α of the rotor in the stator-related coordinate system (a, b), Fig.1 (not shown), the generator 15 for this purpose producing three sinusoidal output voltages whose period is inversely proportional to the frequency of the pulse output of the generator 15a, 17 and comprising a counter, step-by-step switch, a distribution gate and a D/A converter; and (c) a signal at terminal 9 proportional to the excitation current i.e. supplied to the rotor winding by a converter 1. The signals I W *, I B * may be pre-set to provide a desired torque and power factor, Fig.2 (not shown), but in Fig.8 the signal I W * is made speed dependent by passing the signal from generator 15a, 17 through a frequency/voltage converter 57 to produce an actual speed signal n for comparison with a reference signal n*, the resultant error signal being passed to a speed controller 56 whose output is the dividend input of a quotient former 59 having a divisor input from the computer 5 and an output equal to I W *, the input from computer 5 representing the flux magnitude of the field vector. The converter 1 is controlled by an excitation current controller 60 whose input is the difference between the actual excitation current i e  and a reference value i e * produced by a flux controller 4, which in turn receives as input a signal representative of the difference between the signal from computer 5 indicative of the actual flux of the field vector and a reference signal #*of desired flux. To prevent over-voltage during load surges produced by change-over from one motor load to another, the input signal to the controller 60 is also applied to the vector rotator VD1 and constitutes the desired value signal I B *, the arrangement being such that the motor operates at unity power factor during steady state conditions but during load changes the stator current regulating loop assists the excitation current controller 60 to stabilize the rotor flux. The composition and operation of the circuits 5, 12, 16, 18, 60 and VD1 are described, Figs. 3 to 7 and 9 (not shown), the flux computer 5 including a field simulation circuit, two vector rotators and a vector analyzer incorporating threshold, limiting and integral section.