Patent GB-1372092-A (GB)
Title: Speed control systems for electric motors
IPC: H02P25/26
Filed: 1971-10-07
Abstract: 1372092 Automatic speed control LA TELEMACANIQUE ELECTRIQUE 7 Oct 1971 [7 Oct 1970] 46814/71 Heading G3R [Also in Division H2]  A speed control system for an induction motor having a polyphase stator 1 and a wound rotor 2 combines the gradual insertion of resistors 9, 10, 11 in the phases of the rotor with periodic interruptions of the current in the phases respectively by semi-conductor controlled rectifiers 20, 22, 24, each of whose ignition generators 25, 26, 27 is supplied by circuit elements 28, 32, 33, 35, 40 with three signals, a first of which is a periodic signal of rectangular waveform having a duration which is a predetermined fraction of the rotor voltage period, the second of which is the error between the actual and desired values of motor speed and the third of which is dependent on the period of the rotor voltage and determines the ignition threshold of the rectifier, each ignition generator containing a first unit receiving the first and second signals for generating a sawtooth voltage, the tooth duration of which is determined by the first signal, whilst the slope of the tooth is proportional to the amplitude of the second signal and the sawtooth voltage supplying an ignition signal when a second unit which it supplies establishes a coincidence with the third signal, whereby the error voltage which corresponds to the difference between the actual and desired motor speed has the effect of causing the slope of the sawtooth voltage to vary, the ignition of the controlled rectifier occurring for a given error, at a constant fraction of the period of the rotor voltage. Each generator, Fig.2 (not shown), comprises a constant current generator (46) controlled by the second signal and charging a capacitor (47) during time periods determined by a transistor (48) controlled by the first signal, this arrangement constituting the unit generating the sawtooth voltage with the length and slope of each tooth respectively determined by the first and second signals. The sawtooth voltage is applied, together with the third signal, to a differential operational amplifier (50) whose output is determined by the intersection of the third signal and the voltage, Fig.3 (not shown), and is applied to an oscillator, amplifier, transformer circuit (54, 55, 56) to produce the firing signal for the controlled rectifier. The element 28, Fig.4 (not shown), includes a delta-delta transformer (62) whose secondary winding feeds a diode bridge (63, --- 68) closed through resistors (70, 71, 72) across each of which is developed a voltage for application to a respective wave shaper (74, 75, 76), whose outputs form the first signal. Another secondary winding of the transformer (62) is star connected to supply a star network of diodes (83, 84, 85) and resistors (87, 88, 89), the latter producing voltages which are applied to respective waveshapers (91, 92, 93) and capacitors (95, 96, 97), whose resultant signal is combined and rectified to produce an output whose frequency is proportional to that of the rotor voltage. This output is converted to an analogue voltage in the element 32, which feeds the element 35 for deriving a signal indicative of the actual motor speed. The latter signal is applied to element 40 for comparison with a speed reference signal 140 and the consequent production of the second signal. Element 32 also feeds the element 33 which employs non-linear inversion to produce the third signal. Typical waveforms produced by the element 28 are described, Fig.5 (not shown). The output of element 35 is also applied to a unit 36 fed with upper and lower limiting values 37, 38, the arrangement being such that if the actual speed exceeds these limits the unit 36 operates, through a blocking oscillator and a controlled rectifier in an element 39, to close a contactor 41 which short circuits the terminals 6 ', 7', 8' and hence the rectifiers 20, 22, 24, the speed control then being determined by the setting of the variable resistors 9, 10, 11. The electronic switches 16, 17, 18 may be Triacs or pairs of thyristors, instead of the thyristor-diode pairs shown.