Patent GB-711128-A (GB)
Title: An apparatus for regulating the speed of electric motors by means of ward-leonard control
IPC: H02P7/34
Filed: 1952-11-03
Abstract: 711,128. Control of D.C. motors. THRIGE, T. B. Nov. 3, 1952 [Dec. 14, 1951], No. 27619/52. Class 38(3) A D.C. motor 2, which may drive a vehicle or a hoist, is supplied from a Ward-Leonard generator 1, and both the generator field winding 3 and the motor field winding 4 are supplied from separate A.C. sources through rectifiers 5, 6, the field circuits also comprising the main windings 7, 8, of transductors which are controlled in response to the motor armature current. The transductors are provided with control windings 11 and 12 which are energized from a separate D.C. source 25 through individual reversing switches 26 and 27. In addition to the armature current-responsive windings 9, 10, the transductors have reversible self-magnetizing windings 28, 29 connected across the rectifiers 5 and 6 respectively. Impedances 23 and 18 prevent the field windings from being regulated below pre-determined values. For operation of the motor at maximum output, contact 32 is closed to short-circuit resistor 31 in series with the transductor winding 29, and the switches 26, 30, are adjusted so that the windings 28 and 11 of the transductor controlling the generator field winding 3 oppose the current-responsive winding 9 during lifting or forward driving operation of the motor. At the same time, the transductor controlling the motor field excitation is arranged so that windings 29 and 10 oppose the winding 12. The field current of the motor thus remains at a steady low value until the armature current reaches a given value, and then the field current increases sharply to a steady high value. On the other hand, the generator field current has a high value until a given value of armature current is reached, whereupon the generator excitation falls to zero if the contact 24 is open. Thus, with increasing torque, the motor speed remains constant for a period, then falls gradually to a low steady value, and finally decreases to zero, Figs. 2 and 3 (not shown). The armature current never exceeds a predetermined value. For regulating the lowering speeds of hoists, a suitable speed-torque characteristic is obtained by arranging that the transductor windings 28 and 11 operate in the same direction as the current-responsive winding 9 during regenerative operation of the motor, whereas the self-magnetization as well as the magnetization in the control winding 10 produced by the armature current during lifting will oppose the magnetization of the control winding 12. In this arrangement, the generator field winding 3 is reversed by means of switch 19 and the contact 32 is opened. Large loads are thereby lowered at less speed than small loads, Figs. 4, 5 (not shown). For low speed operation of the motor, a suitable speed-torque characteristic is provided by arranging that the selfmagnetization of the transductor controlling the generator field winding 3 and that of the winding 11 oppose the magnetization of the control winding 9 produced by the armature current during lifting or forward driving operation, whereas the windings 10, 12, and 29 all act in the same direction, Figs. 6 and 7 (not shown). Another arrangement suitable for low speed operation has the winding 28 opposed to the windings 9 and 11 for forward driving operation, whereas the windings 10, 12, and 29 again all operate in the same direction, Figs. 8 and 9 (not shown).