Patent GB-655981-A (GB)
Title: Control systems for dynamo electric machines
Inventor: ASBURY ALLAN
IPC: B23D36/00
Filed: 1948-02-27
Abstract: 655,981. Automatic speed control systems. ENGLISH ELECTRIC CO., Ltd. Feb. 27, 1948, No. 6094. [Class 38 (iv)] [Also in Group XXXVII] A control system for an electric motor driving a flying shear comprises two main tachogenerators E3, E4, driven in accordance with strip speed and shear speed, an amplifying exciter, E9, E10, responsive to the difference of output voltage of the tacho-generators for varying the speed of the shear motor in a corrective sense in accordance with variations of strip speed, and rheostats independently adjustable for varying the length of cut strip and providing main and vernier adjustment for varying the relationship between the tachogenerators E3, E4 outputs. As shown, tachogenerators E1, E3 and exciters E5-E10 are driven in accordance with the speed of the last mill stand, whilst tacho-generators E2, E4 are driven by motors M1, M2, which are excited from constant voltage sources and supplied by a variable voltage generator G1 excited from an amplifying exciter E9-E10. The control field winding of the exciter E9-E10 is connected in a circuit including the exciter E8 and the tachogenerators E3, E4. The exciter E8 has two field windings, E8a, connected in circuit with a resistance 21 in the generator load circuit, a resistance 20 and rectifiers 22, 23, and E8b supplied from an electronic amplifier 24. The resistance 20 is also connected in series with the exciter E7 across the separately-excited tachogenerator E6, the field winding of which is supplied from the further separately-excited tacho-generator E5, which in turn is excited from a constant voltage source in accordance S<SP>2</SP> with the factor -, where S is the speed of the L<SP>2</SP> strip and L the desired length of cut strip. The predetermined relationship between shear speed and strip speed is obtained by differentially applying the voltages of the tacho-generators E3 and E4, the latter having two field windings, to the control field winding of the amplifying exciters E9, E10, so that the generator G1 is automatically excited in accordance with the difference voltage in a sense to maintain the speed ratio constant. To vary the cut length, the relative speed of the motors driving the last mill stand and the flying shear is adjusted by ganged resistors 25, 26, 29, whilst a vernier adjustment is obtained by resistances 27, 30. Adjustment for variations in the mill roll diameter is obtained by resistances 28, 31. The tacho-generators E3, E4 form a coarse control and the rheostats 26, 27 are adjusted so that the speed of the motors M1, M2 is slightly in excess of that required. The subsidiary tachogenerators E1, E2 serve to reduce this speed to that required by the differential application of their output to the electronic amplifier 24 which is such that an output opposing that of E3 is applied to the field winding E8b of the exciter E8 only when the speed of E2 exceeds that of E1, thus causing the speed of the motors M1, M2 to be lowered to the required value. If it is desired to start the flying shear from rest, assuming the last mill stand to be running, the output from the tacho-generator E4 will be zero so that a maximum voltage will be applied'to the control field winding of the exciters E9, E10, causing the generator to start and a current to flow in the load circuit. During acceleration, if the voltage across the resistance 21 exceeds that across the lower half of the resistance 20, which is in accordance with the factor S<SP>2</SP>/L<SP>2</SP>, current flows in the field winding E8a in a sense to oppose the effect of the tacho-generator E3, due to the action of the rectifiers 22, 23. If, however, the voltage across the lower half of 20 exceeds that across 21 no current flows in the field winding E8a. The excitation of E9, E10, during acceleration, is therefore limited to a value at which the voltage across 21 is equal to that across the lower half of 20. The electronic amplifier 24 also serves to provide a more rapid control of acceleration, by applying to the terminals 24b, when the acceleration current exceeds the predetermined value, a voltage, derived from the resistance 21 and the lower half of a resistance 33 in a similar manner to the acceleration control circuit of the field winding E8a, such that the field winding E8b is excited in a sense to assist the field winding E8a. For deceleration of the flying shear the tacho-generator E3 is removed from the circuit so that only E4 is operative. The operation is the same as for acceleration except that the voltage, applied to E8a and E8b are in the reverse sense to those applied during acceleration, i.e. in opposition to the output of E4, thus causing deceleration. When the flying shear has attained the desired speed the excitation of the control winding of E9, E10 may be made independent of the factor S<SP>2</SP>/L<SP>2</SP> by means of the relay 32 adapted to operate when the speed of the motors M1, M2 is within 20 per cent of the required speed. Operation of relay 32 causes a switch 32a to be closed, energizing the field circuit of E7, and 32b to be opened, de-energizing the field circuit of E6, so that the voltage across the resistances 20, 33 and therefore the excitation limit value of E9, E10 is made independent of the factor S<SP>2</SP>/L<SP>2</SP>. In an alternative arrangement of cut length adjustment, the tacho-generator E4 may have only a single field winding connected across two potentiometertype rheostats supplied in series from a constant voltage source, or the excitation of the tachogenerator E3 may be varied. Specification 655,982 is referred to.