Patent GB-621782-A (GB)
Title: Improvements in and relating to electric motor control systems
IPC: H02P7/34
Filed: 1945-06-13
Abstract: 621,782. Automatic control systems for electric motors; exciting dynamo - electric machines. BRITISH THOMSON-HOUSTON CO., Ltd., and POOL, W. T. June 13, 1945, No. 15011. [Classes 35 and 38 (iv)] [Also in Group XXXVII] Speed control of an electric motor 1 is provided by combined regulation of its armature voltage and field strength, an exciter 7 for the motor field winding 6 having a main field winding 11 energized from a constant voltagesupply 5 at such a value that the exciter armature passes full current through the motor field winding, the exciter having also two auxiliary field windings 12, 13, controlled by relays 14, 15, so that only one is excited at a time according to the direction of rotation of the motor armature, and is arranged so as normally to oppose the main field winding, the auxiliary field windings being excited by the difference between the voltage generated by a tachometer generator 16 driven by the motor armature and a voltage tapped from a potentiometer 17 connected across the terminals of the motor armature. When the potentiometer is set at such a position that the voltage across a portion 19-20 equals the output of generator 16, the auxiliary exciter winding is not energized and full motor field obtains. Reduction of motor speed due to reduced armature voltage does not appreciably effect a change of excitation since the two compared voltages vary at substantially equal rates. When the potentiometer is set so that the voltage across a section 20-21 exceeds the voltage of the generator 16, an auxiliary field winding is energized to oppose the main exciter winding and reduce the exciter output and the motor field excitation. Until the motor speed has increased to its final value, the voltage of the generator 16 will be below the corresponding steady value, and the auxiliary exciter field winding will so reduce the voltage of the exciter 7 as to force down the motor field excitation. When the potentiometer slider is returned to the position 19, the auxiliary exciter field is reversed and assists the field winding 11. To obtain a degree of forcing action when strengthening differing from that when weakening the motor field excitation, rectifiers 22 and 23, shunted by resistors 24 and 25, may be connected in series with the field windings 12 and 13 so that, in one direction of current-flow, a resistor is included in the exciter auxiliary field circuit. The motor armature circuit may include an arrangement comprising a resistor 28-29 (see Fig. 3) which is connected through rectifiers to a potentiometer 26, supplied at constant voltage, and an additional field winding 30, such that, at a high predetermined motor armature current in either direction of flow, the winding 30 is excited to assist the exciter winding 11. A resistor 38 is added to this arrangement so that, when a difference of potential exists across the resistor 28-29, a current will flow through a part of the winding 30 to excite it additionally to the winding 11. This provides compensation for the reduction in motor excitation due to the fall in the tachometer generator voltage when the motor speed falls on load. The exciter 7 may be an amplidyne, or an exciter of normal design may be connected in cascade between an amplidyne exciter and the motor field winding. The motor armature may be applied with variable voltage from a generator or rectifier. Specification 547,613 is referred to.