Patent GB-1486817-A (GB)
Title: Circuit arrangements for controlling the mark-space ratio of a solid state switch in a dc circuit
IPC: B60L11/18
Filed: 1974-12-16
Abstract: 1486817 Control of D.C. motors GENERAL ELECTRIC CO 16 Dec 1974 [26 Dec 1973] 54181/74 Heading H2J A circuit arrangement for controlling the mark-space ratio of a solid state switch connected to control the supply of power from a D.C. source to a D.C. load, comprises a power control circuit for generating a demand signal representative of a desired mark-space ratio, gating control means for controlling the markspace ratio of the switch as a function of the demand signal, a current limit signal stage coupled to the power control circuit to produce a first signal which varies as a function of the mark-space ratio, means for sensing load current and supplying a second signal representative thereof, a current limit detector connected to receive the first and second signals and to generate a limit signal as a function thereof, and means to supply the limit signal to the power control circuit to set a maximum mark-space ratio that the demand signal generated by the power control circuit is permitted to demand in view of that limit signal. Fig. 1 shows the application of the invention to control a D.C. series motor 13 supplied from a battery 10 through a thyristor 20, which is turned on by a gating control 22 and turned off by a known commutation circuit (not shown) represented by the second gate connection to the thyristor. This commutation circuit employs charge and discharge of a capacitor whose voltage is a function of the value of current supplied to the motor 13. The motor speed is determined by the setting of a potentiometer 24 supplying a controlled acceleration circuit 26 (when the motor drives a vehicle), whose control signal is applied to control 22 to determine the markspace ratio of the thyristor 20 and also applied to a current limit signal stage 36 which includes a temperature sensitive resistor, preferably a PTC resistor 44, responsive to the thyristor temperature. A limit detector stage 50 receives the output of stage 36 and that of an amplifier stage 55 responsive to the motor current sensed by resistor 14, the stage 50 having an output connected to circuit 26 to vary its control signal and hence the mark-space ratio of thyristor 20 in accordance with (1) the motor current, (2) the thyristor temperature and (3) the voltage across the commutation capacitor. The latter voltage is limited to a maximum value by the setting of a potentiometer 47 also connected to stage 50. Operation for conditions (1), (2) and (3) above is as follows: Condition (1).-Changes in the control signal by variation of potentiometer 24 and hence of the voltage across capacitor 32, produce corresponding changes in mark-space ratio providing the output voltage from stage 50 (combination of sensed current and markspace ratio) exceeds the voltage of capacitor 32. If not, diode 33 allows an appropriate discharge of capacitor 32 through resistor 34 to limit the control signal. Thus by appropriate selection of circuit values, a linear relationship between current and mark-space ratio may be obtained as shown in Fig. 2. By using resistors 38, 40 of substantially different values in the circuits 37, 38 and 39, 40, responding respectively to positive and negative control signals giving mark-space ratios above and below 50%, the linear relationship has a different value above and below the 50% ratio to obtain almost full utilization of the maximum current-carrying capacity of the thyristor over its range of operation at a given temperature. Condition (2).-Changes in thyristor temperature cause the resistor 44 to control stages 36, 50 to further decrease the voltage across capacitor 32 and so produce the characteristics T1, T2 and Tn of Fig. 2 corresponding to progressively increasing temperatures Condition (3).-If the current reaches an upper limit (determined by the setting of potentiometer 47), stage 50 functions to limit the mark-space ratio to a fixed predetermined value. Capacitor 32 has charge and discharge resistors 28, 29 and 30 chosen to obtain different maximum rates of acceleration and deceleration upon adjustment of the potentiometer 24.