Patent GB-665842-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: H02P7/34
Filed: 1948-08-19
Abstract: 665,842. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. Aug. 19, 1948. [Aug. 28, 1947] No. 21915/48. Class 38 (iii). [Also in Group XXXV] An electric motor control system comprises an adjustable voltage generator connected to the motor, a source of substantially constant voltage, means for adjusting the voltage of the generator including dynamo-electric means for producing a voltage proportional to the motor speed, a master switch having means for deriving a reference voltage, means for supplying exciting current to the generator comprising a dynamo-electric machine provided with a control field winding connected to be excited by the difference between the speed voltage and the reference voltage, means for supplying exciting current to the field of the motor comprising a second dynamo-electric machine having a first control field winding connected to be supplied from the constant voltage source, and an opposing control field winding connected to be excited by the difference between the reference voltage and a voltage derived from the armature voltage of said generator. The application to a machine for cutting veneer from a rotating log which is driven by a motor 2 is shown. The motor is supplied from a variable voltage generator 3 which is driven at constant speed and has field winding 3a energized from a metadyne exciter 6. A main control field winding 6c of the exciter is energized in accordance with the difference between a reference voltage derived from a potentiometer 7d and a voltage proportional to the speed of the motor which is obtained from a potentiometer 16 connected across tachometer generator 14. A field winding 2a of the motor is energized from a metadyne exciter 20 having a main control field winding 20a connected to bus-bars 10, 11. A field winding 20b opposes the winding 20a and is excited in accordance with the difference between a reference voltage obtained from resistor 12 and potentiometer 7d, and a voltage derived from the armature voltage of the generator 3 and applied across potentiometer 24. A rectifier 26 prevents excitation of the winding 20b until the voltage of the generator attains a value corresponding to its full speed and field excitation. Limitation of the armature current of the motor 2 is provided by field windings 6d and 20c on machines 6, 20, respectively. These windings are connected in series and are energized in accordance with the difference between a reference voltage and one proportional to the armature current of the motor. The reference voltage is provided by a potentiometer comprising resistors 28, 29, which are connected to a constant voltage source 30, 31. The voltage proportional to the motor armature current is that across commutating field windings 2b, 3b. Rectifiers 32, 33, enable the voltage-drop across windings 2b, 3b, to be compared with the voltage across one or the other of the active portions of resistors 28, 29, depending upon the direction of the armature current of the motor 2, and also serve to prevent energization of windings 6d, 20c, when the armature current is below a pre-determined value. If the currentlimit is exceeded, the winding 6d becomes energized to oppose the winding 6c thereby reducing the excitation of the generator field winding 3a. Simultaneously, the excitation of the winding 20c adds to that of winding 20a and the motor field is strengthened. The machine 6 is provided with an anti-hunting field winding 6e which is energized from transformer 35. Hunting is also prevented by applying voltages proportional to the rate of change of the armature voltage of the machine 20 to the field windings 6c and 20a by means of transformers 19 and 39 respectively. Initially the exciter 20 supplies maximum current to the motor field winding 2a and to the coil 45a of a field loss relay 45. A line contactor 46 is energized through contacts 47 to close an under-voltage relay 50. Forward operation of the motor 2 is provided by movement of a master switch 7 to energize contactor 8. A small current is supplied to field winding 6c from resistor 12 and the motor starts from rest. During the starting period the field winding 20b is not energized so that the motor field is fully excited. When the master switch is moved to adjust the potentiometer 7d for increase motor speed, the winding 20b is energized to weaken the field excitation. As a blade 20 moves towards the centre of the log the slider 16a of potentiometer 16 is moved in correspondence to decrease the portion of the voltage of the tachometer 14 which opposes the reference voltage. The generator voltage is thereby increased and the motor excitation weakened in a continuous manner so that the motor operates at substantially constant horse power to provide substantially constant velocity and tension of the veneer. When the master switch is moved towards the "off" position the generator voltage is lowered and the motor field excitation increased. The field windings 6d and 20c become effective, however, to prevent excessive pump-back current. With further movement of the master switch towards the zero position, the motor field strength reaches a maximum and the generator voltage falls gradually, the motor 2 being regeneratively braked to a standstill. Reverse operation is provided by moving the master switch to energize contactor 9.