Patent GB-696268-A (GB)
Title: Improvements in or relating to constant speed electric motor drives
IPC: H02P7/06
Filed: 1951-11-27
Abstract: 696,268. Exciting electric motors. DICTAPHONE CORPORATION. Nov. 27, 1951 [Nov. 29, 1950], No. 27774/51. Class 35. [Also in Group XXXVII] A shunt-wound motor 2 is adapted to drive the mandrel of a dictating machine comprising an amplifier 42 and is supplied through a rectifier 22 and a filter circuit so that it will operate at substantially the same constant speed from either D.C. or A.C. sources without the need for adjustment. The current supplied to the motor field winding 8 is filtered by resistor 24 and electrolytic capacitor 26, whilst the current for the armature 28 is filtered by resistors 24, 32, and capacitors 26, 34. In order to reduce circulating currents in the motor circuit the ratio of the values of the capacitors 26 and 34 is made equal to the ratio between the field winding and armature currents. A resistor 44, which has a negative temperature coefficient, is connected in the field circuit and has the same temperature as the field windings so that the total resistance of the field circuit remains substantially constant despite changes of heating. The resistor 44 may have its characteristics modified by a shunt-connected fixed resistor 46, and it may be adapted to over-compensate for change of speed due to resistance variation of the field windings in order wholly or partly to compensate for changes due to other elements in the system. The filter resistances 24 has a positive temperature coefficient so as to compensate for changes in the temperatures of the rectifier 22 and the condensers 26, 34. The variation of load due to the change, with temperature, of the viscosity of the lubricant in the machine bearings is allowed for by a negative temperature coefficient resistor 48 which is connected in shunt with the motor armature. The resistor 48 is arranged to respond to changes of temperature of one of the motor bearings and may be connected respectively in series and parallel with fixed resistors 52 and 64. When starting the motor, the resistor 54 may carry a greater current than the resistor 48 so that the temperature of the compensating resistor will be fixed primarily by the temperature of the bearings, the arrangement being such that the value of the resistor 48 will vary at a rate necessary to provide the necessary compensation. Other bearings in the apparatus, such as that which supports the mandrel 4, may have temperature compensating circuits, similar to that including the resistor 48, connected in parallel with the armature 28. A start-stop switch 56, which is actuated by a push-button 58 on hand microphone 62, controls a dynamic braking resistor 72. The excitation of the motor field is maintained when the motor is at rest, so providing rapid starting operation. A second rectifier, Fig. 2 (not shown), may be connected between resistors 24 and 32 to prevent discharge current from the capacitor 34 passing to the motor field circuit. The presence of the rectifier 22 enables the correct polarity to be readily ascertained when the supply is direct-current in character. Specifications 653,378, [Group XXXVII], and 696,267 are referred to.