Patent GB-746213-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: H02P7/06
Filed: 1954-05-03
Abstract: 746,213. Automatic speed control systems. BROWN, W. J. May 3, 1954 [May 1, 1953], No. 12728/54. Class 38 (4). [Also in Group XXXVII] Relates to a control system as described in Specification 697,609 in which a series motor is supplied from a controllable converter and the output of the converter is regulated in accordance with the relationship between armature voltage and field voltage. The motor, having armature 6 and field winding 5, is supplied from converter 1, the control voltage for which is derived from terminal 7 and tapping 11 of a potential divider 10. A variable rheostat 303, forming part of the potential divider, is connected to a fixed inductive portion 301 consisting of inductance 304 and resistor 305 which may be wholly or in part the inherent resistance of the element 304. The value of the inductance 301, relative to that of the operative part of the rheostat 303, is chosen to provide a time constant for the potential divider which is the same order of,magnitude as that of the motor when the tapping 11 is in the position tending to cause the greatest instability due to such time constant. Thus, a sudden change of voltage between supply conductors 3 and 4 will cause the potentials at points 7 and 11 to change at approximately the same rate, and the signal voltage applied at terminals 8, 9 does not suffer from severe transient changes which could occur if the time constants of the motor and potential divider were widely different due, for instance, to the voltage divider being wholly resistive. In another arrangement, the upper end of the rheostat 303 is connected to the lower end of the inductance 301 so that the potential divider has a time constant substantially independent of the position of the tapping point 11, Fig. 2 (not shown). A non- linear resistor may be associated with the circuit for deriving the control voltage to compensate for the non-rectilinear variation between the motor field flux and the field current, caused by magnetic saturation. The non-linear resistor may have a high negative temperature coefficient and be connected in series with the inductive portion of the potential divider, the signal voltage being tapped from the connections between the inductive portion and the resistor and between the motor armature and series field winding respectively, Fig. 3 (not shown). A second non-linear resistor may be connected in parallel with the first at low motor speeds to modify the compensation to suit the motor field flux-current characteristic at such speeds. Compensation for the non-linearity of field flux in relation to field current is afforded, in a modification, by connecting a non-linear resistor between the inductive portion of the potential divider and the junction between the armature and field winding, Fig. 5 (not shown). Another arrangement for modifying the signal voltage fed to the converter to compensate for the time constant of the motor comprises a winding inductively coupled with the field winding and connected in the control circuit of the converter. This additional winding may be an auxiliary field winding wound on the poles of the motor and having the same number of turns as the main field winding, Figs. 6, 7 (not shown). Thus, during periods of rapid voltage change across the main field winding of the motor, a voltage equal and opposite to the transient component is induced in the control circuit.