Patent GB-669850-A (GB)
Title: Improvements in electric motor control devices
IPC: H02P7/292
Filed: 1949-04-11
Abstract: 669,850. Control of D.C. motors. HEWITTIC SOC. A.R.L. April 11, 1949. [April 9, 1948; Feb. 17, 1949] Nos. 9783/49 and 9784/49. Class 38 (iii). [Also in Group XXXV] The speed of a D.C. motor is controlled by applying to the grid of the rectifier supplying the motor armature, a saw tooth voltage, the phasing of which is controlled by a voltage dependent on motor speed. The armature voltage of the separately excited motor 7, Fig. 1, is opposed by an auxiliary D.C. reference voltage, the resultant appearing across potentiometer 32. A portion of this voltage is added to a saw tooth voltage (generated by condenser 23 charging through rectifier 22 and discharging through resistor 24) to control the firing point on the saw tooth of an auxiliary gas filled valve 10 which in turn controls the main rectifier 1. A transformer 50 connected across a series resistor 8 produces a current limiting voltage across resistor 53, if the peak voltage across the transformer secondary exceeds a limiting voltage from a bias source 51. The voltage between the point L and the cathode of the rectifier 1 is limited by a load limiter comprising a biasing source 41 and a rectifier 42. In a modification. Fig. 2 (not shown) only a portion of the motor armature voltage tapped off a further potentiometer is used for control purposes. In another modification, Fig. 4, the field is supplied, through reversing switches 72A, 72B and contactors 70, 71, from a transformer winding 63 and auxiliary electrode 60; 61 in the main rectifier. A rectifier 34 supplies the reference voltage to the forward and reverse speed controlling potentiometers 32<SP>1</SP> and 32<SP>2</SP>, the output of the appropriate potentiometer being selected by a switch 72c and applied to the control circuit at point L as in Fig. 1. The full reference voltage may be applied to the control circuit in positions r and s of switch 72c in order to brake the motor. In another modification, Fig. 5 (not shown) an A.C. reference voltage is compared in a ring modulator with the output of an A.C. generator, the resultant being used as the control voltage at point L of,Fig. 1. The A.C. sources may be generated: by rotating interruptors and photo-cells. In another modification, Fig. 6 (not shown), the D.C. reference voltage of Fig. 1 is regularly interrupted by a timing relay. This enables a momentary peak to be obtained for motor starting. In another modification Fig. 8 (not shown) a field winding opposing the separately excited main field is connected across the motor armature. In another modification, Fig. 10, the field 9 is excited from an A.C. source via a rectifier 106 and an, impedance 107 which is, itself connected via a rectifier 108 across the motor armature. When the armature voltage exceeds the normal voltage . drop across resistor 107, rectifier 108 conducts and the resulting circulating current. reduces the field excitation. In another modification, Fig. 11 (not shown) the D.C. reference voltage is obtained by rectifying the output of a saturated current transformer,. the primary of which is in series with the A.C. supply to the. rectifier which supplies the motor field. Specification 657,031, [Group XXXV], is referred to.