| 摘要 |
1,267,345. Control of D.C. motors. WESTINGHOUSE BRAKE & SIGNAL CO. Ltd. 26 March, 1969 [11 April, 1968], No. 17436/68. Heading H2J. [Also in Division G3] The mark-space ratio of current supplied to a traction motor through a controlled rectifier is controlled by a pedal coupled to a potentiometer 21 associated with a capacitor 20 which prevents excessive change of motor current and is connected across the base drive circuit of a transistor 26. A ramp generator comprises transistors 29-32 and operates at constant frequency to produce a saw-tooth waveform through resistor 43 at the base of transistor 44 which, with a transistor 45, forms a comparator circuit. When the ramp voltage reaches the signal voltage on line 28 derived from the pedal control, transistors 44, 45, become conductive and non- conductive respectively. Transistor 51 turns on to generate an "off" signal for the controlled rectifiers in the motor circuit. A signal representing the motor current is derived through amplifier 52. When this current-signal is less positive than the signal on line 28, diode 53 conducts and the pedal control signal is clamped to the level determined by the amplifier 52. A thermistor 23 is connected in series with the potentiometer 21 and is in close thermal relationship with the main rectifier (3, Fig. 2, not shown) in the motor circuit. Accordingly, an excessive mean heat loss causes an automatic reduction of the mark-space ratio regardless of the current-limit control provided by the amplifier 52. The amplifier 52 may produce a signal indicating any tendency for the current-signal to exceed a reference. The output of the current sensing device may be integrated, the pedal control being over-ridden when the integrated value exceeds a reference level. In another arrangement the current-limit is modified in response to the temperature of the cooling fins or cooling medium of the main rectifier.
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