GB1108471A - Electric motor speed control apparatus - Google Patents

Abstract

1,108,471. Control of D.C. motors. ENGLISH ELECTRIC CO. Ltd. 12 Oct., 1964 [13 July, 1963], No. 27855/63. Heading H2J. [Also in Division G3] The speed of a D.C. electric crane motor is controlled at first by variation of the firing angle of thyristors 17 in its armature circuit in response to a comparison at 27 between the back e.m.f. and a voltage from control potentiometer 20; when the maximum speed position 44 is reached the field is weakened as the armature current (measured at 40) decreases. When the motor speed is greater than the desired speed the resultant negative error signal operates change-over switches 18, 30 to provide regenerative braking. An electromagnetic brake is held off by the field current through a relay 15. The armature current is limited by a circuit 34 which compares the actual armature current over line 35 with a field strength signal over line 50 and reduces the speed error signal if necessary. Field weakening is effected by a unit 16 which delays the firing time of further thyristors 13 is response to the output voltage of the comparison unit 39. This is illustrated in Fig. 2 (not shown), and comprises four similar bistable transistor circuits with graded threshold levels; initially these are all switched "off" by contacts 42, 43, which establishes full field, but when controller 20 reaches position 44, 42 and 43 change over and, after a delay, a number of the bi-stable circuits are switched " on ", depending upon the difference between the armature current and the reference 41, thus varying the output voltage and the field is weakened by a corresponding amount. Any subsequent increase in armature current does not affect the field, since such bi-stable circuits as are switched " on " remain so until contacts 42, 43 are changed back by movement of controller 20; a reduction in armature current could produce further weakening of the field but this may be prevented by an inhibiting circuit comprising a Zener diode (Fig. 3, not shown). With the speed controller in the off position a switch is closed to establish a weak field at standstill.