Patent GB-697609-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P25/32
Filed: 1950-04-24
Abstract: 697,609. Rectifying; automatic speed control systems. BROWN, W. J. April 23, 1951 [April 24, 1950; Aug. 1, 1950; Sept. 4, 1950], Nos. 10015/50, 19205/50 and 21758/50. Classes 38 (ii) and 38 (iv). [Also in Group XXXVII] In a control system for a series motor which is supplied from a controllable converter, the output of the converter is controlled by a quantity dependent upon the relation between the voltage across the motor armature and that across the series field winding. In Fig. 1, a controllable converter 1 has input terminals 2 and delivers power to a motor having a series field winding 5 and armature 6. The control terminals 8, 9 of the converter are connected respectively to the junction-point of the field and armature windings and to a tapping-point of a potential divider 10 which is in parallel with the motor. The converter is adjusted so that, when there is no signal voltage applied to terminals 8, 9, a small priming voltage is supplied to the motor, and, as the motor speed increases following adjustment of tap 11, the potential at point 7 approaches that at point 11 I until the converter output is just sufficient to keep the motor at a desired speed determined by the adjustment of the voltage divider. If the motor load increases the output is increased in correspondence, the ratio of the voltages across the armature and field winding, however, remains substantially equal to the resistance ratio of the voltage divider. When operating under unsaturated conditions of the motor, this arrangement tends to reduce speed-variations. The converter may comprise a single-phase gasfilled rectifier, Fig. 2 (not shown), the voltage divider consisting of a potentiometer connected between two fixed resistors which determine the minimum and maximum speeds of the motor. A phase-shifting network, similar to that described in Specification 656,816, for controlling the rectifier comprises a saturable reactor having the signal voltage from the motor applied to one of its regulating windings. A rectifier is connected in the control circuit of the transductor to ensure that the output of the converter is only increased when point 11 becomes positive relative to point 7, so preventing loss of control when the potentiometer-slider is adjusted to reduce the speed of the motor. An anti-hunting arrangement, similar to that described in Specification 686,799, comprises an additional winding on the transductor which is energized in accordance with the rate-of-change of motor field voltage. A modified form of potentiometer, Fig. 3 (not shown), has an insulated stud at its low-speed end. When the potentiometer-arm is moved on to this stud the converter output is reduced to its priming value and the motor armature is simultaneously short-circuited by spring-contacts actuated by the arm. To obtain a rectilinear relation between the motor speed and the position of the potentiometer-tap, the connection from the converter output to the high-speed terminal of the potentiometer is replaced by one through a fixed resistor to the potentiometer arm, Fig. 4 (not shown). In another arrangement for braking the motor the signal voltage supply conductors and the motor armature are simultaneously short-circuited. In an arrangement for regenerative braking and reversal of the motor (Fig. 5) the converter has two outputs and the motor two field windings, a currentlimiting circuit also being provided. A mercury arc rectifier 120 has a main anode transformer 121 with two sets of star-connected secondary windings and a delta-connected primary winding. The motor has armature 6 and opposing field windings 5, 5A. Voltage dividers 10, 10A, are respectively connected across the first and second outputs of the rectifier, the taps, 11, 11A, being joined to terminals 9, 9A of control devices 15, 15A. The common terminal 7 is connected through a two-way switch 155 to terminals S or 8A. The control device 15 comprises a phaseshifter 158 and a phase-splitting and peaking network 159 for delivering a three-phase output from the single phase output of the phaseshifter. The network 159 comprises three meshes 164, 165, 166, each providing a peaked output voltage, together with means for displacing the phase angle of the output from the three meshes by 120 degrees relatively to one another. Each of the grids 129-131, which are normally biased negatively, is supplied with a peaked voltage having a similar phase relationship to its corresponding anode 123-125, and this relationship is varied by the phaseshifter 158. The control device 15A is similar to the device 15 and operates to vary the firing angles of grids 132-134. The position of switch 155 may thus determine the direction of rotation of the motor, or it may be changed over whilst the motor is running to institute regenerative braking. A current-limiting device, similar to that described in Specification 656,778, becomes effective, if the voltage across the operative field winding rises to a value equal to the reference voltage between points 212, 213, so that rectifier 214 or 215 conducts to prevent further rise. The effect upon the input of the converter due to saturation of the motor field is compensated for by taking the signal voltage from the tap on the voltage divider and the junction of two resistances connected across the field winding, Fig. 7 (not shown). One of these resistances may be non-linear in character and variable to compensate for the non-rectilinear relation between the flux and the voltage across the field winding. In addition, a currentlimiting circuit becomes effective when the voltage across the whole of the field winding exceeds a reference value, Fig. 8 (not shown). Since the current-limiting voltage is taken from the whole of the winding it exercises a predominating control over the signal voltage and sharply restricts the rise of current under overload conditions. The controllable converter may be of the dynamo-electric type and comprise a polyphase motor driving a D.C. generator and its exciter which is controlled by the signal voltage from the motor. A tuning resistance may enhance the amplifying properties of the exciter, Fig. 9 (not shown). The controllable converter may take the form of a dynamo-electric amplifier of the amplidyne or other cross-flux type.