Patent GB-626125-A (GB)
Title: Improvements in or relating to means for effecting speed control of electric motors
IPC: H02P25/32
Filed: 1945-10-26
Abstract: 626,125. Automatic control systems for electric motors. IGRANIC ELECTRIC CO., Ltd. Oct. 26, 1945, No. 28346. Convention date, Oct. 26, 1944. [Class 38 (iv)] [Also in Group XXXVII] In an automatic system for maintaining a predetermined speed of an electric motor subject to varying load, the supply to a dynamo-electric machine coupled to the motor is controlled in dependence on speed deviation and the power supply to the motor is adjusted in steps each time the change in power supply to the dynamo-electric machine becomes equivalent to such a step. In Fig. 1 a three-phase woundrotor induction motor 1 and a separately excited D.C. motor 2 are coupled to a common load shaft. Operation is initiated by a start button 15 to energize a self-holding relay 13a which changes over its contacts from the position shown. The supply to the stator 1a of motor 1 is then from A.C. mains L1 L2, L3, and the D.C. supply to motor 2 is from a single phase of the A.C. supply through a full-wave rectifier including valves 6 and 7. A fixed A.C. bias for rectifiers 6 and 7 is obtained through a transformer 40 and a variable D.C. obtained from the combination of voltages across resistances 24, 22 and portions 19, 29 of potentiometers 19a, 29a, constitutes the control for the supply to motor 2. Mains 16, 17, 18 provide constant D.C. voltages as shown for the control circuit. The voltage across resistance 29 is dependent on the armature voltage of motor 2 and thus introduces a component of control for motor 1 dependent on the speed of the motors.. The voltage across resistance 19 constitutes a reference voltage which is manually adjustable for speed setting. The voltages across resistances 24, 22 are determined by the anode currents of valves 20, 23, the grids of which are respectively biassed by the output voltages of potentiometers 32a, 35a, the inputs to which are obtained from the armature circuit of motor 2 through current transformers 30, 31 and a full-wave rectifier including valves 33, 34. Soon after. starting valve 23 is biassed to cut-off and valve 20 passes current. As the armature current of motor 2 increases the increase in voltage from potentiometer 32a decreases the anode current of valve 20 to provide a component of control to compensate for armature volt-drop. At a predetermined maximum allowable value of armature current, say 125 per cent full load, valve 20 ceases to conduct and the voltage from potentiometer 35a controls the anode current of valve 23 to prevent any further increase of armature current. When the armature current exceeds a predetermined value, say 100 per cent full load, relays 9 and 11 operate to start a single-phase split-field motor 4 in such a direction as to decrease in steps a three-phase resistance 3 associated with the rotor 1b of motor 1. When the armature current is below a predetermined value, say 10 per cent full load, relays 10 and 12 operate to cause adjustment of resistance 3 in the reverse direction. In a modification, Fig. 3 (not shown), an additional valve and relays are arranged so that if, due to excessive speed, the torque of motor 2 becomes negative, its armature connections are reversed to brake motor 1. Under these conditions the armature current of motor 2 is still limited to the allowable maximum and if it is above a predetermined value the rotor resistance of motor 1 is automatically increased.