GB751615A - Improvements in control arrangements for electric motors - Google Patents

Abstract

751,615. Exciting dynamo-electric machines. BRITISH THOMSON-HOUSTON CO., Ltd., and METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. March 24, 1955 [Jan. 22, 1954], No. 1989/54. Class 35. [Also in Group XXXVII] Direct-current motors, which may drive the top and bottom rollers of a hot rolling mill, have their armatures M1, M2 supplied in parallel from a common Ward-Leonard generator G the output of which is adapted for rapid reversal. The field winding sections F1A, F1B and F2A, F2B of the first and second motors and connected in a bridge circuit for energization by exciter E having a sub-exciter AS of the crossflux type. The latter machine has a main control field winding ASF and an opposing feedback winding ASB. An exciter A is connected in the bridge circuit across terminals a, b, which are equi-potential with regard to the current derived from the exciter E, a field winding AF of the exciter A being energized in accordance with the difference between the armature currents of the two motors. If, due to load unbalance, the armature current of the motor M1 increases, for example, then the field winding AF is energized and the output voltage of the machine A becomes such as to produce current in the field windings F1A, F1B, which is in the same direction as the main excitation current. The corresponding current in the field windings F2A, F2B opposes the main current, so that, whilst the field strength of the first motor is increased, the excitation of the second motor is reduced. By this means the required load balance is restored. The excitation of the machine A may be controlled by a magnetic amplifier having its input circuit connected for energization in accordance with the difference between the motor armature currents. An additional field winding AFC of the machine A is provided with an adjustable pre-set voltage from potentiometer P 1 whereby a predetermined speed difference between the two motors may be obtained on no-load. The potentiometer P1 is connected across resistors R1, R2 in the bridge circuit. In another arrangement, the motor field windings each comprise a single section connected in a bridge circuit with two exciters, Fig. 2 (not shown). The two exciters are adapted for simultaneous control and may be placed in adjacent or diagonally opposite arms of the bridge. As before, the machine A is connected across opposite points of the bridge which are at equal potentials unless the motors have their loads unbalanced. When it is desired to maintain a predetermined relation between the loads other than equality, the machine A is arranged to respond to departure of the motor currents from the corresponding ratio with respect to each other. Thus, the winding AF has its ends connected to two potentiometers connected respectively across commutating windings C1, C2 or equivalent resistive elements in series with the motor armatures, Fig. 3 (not shown). The potentiometers may be ganged to be moved in opposite directions.