Patent GB-660106-A (GB)
Title: Improvements in or relating to alternating current commutator machines
IPC: H02K27/04
Filed: 1947-09-03
Abstract: 660,106. Dynamo-electric machines. VICKERS, Inc. Sept. 3, 1947 [Sept. 10, 1946], No. 24270/47. Classes 35 and 38 (ii). In an A.C. shunt excited commutator motor or generator the stator carries an armature reaction compensating winding for maintaining the flux in the excitation axis, whilst the brushes, which contact more than one commutator segment are split and have means for preventing short circuit currents through the brushes between the segments under commutation. In one form, Fig. 1, the stator 31 is excited by a winding 38 composed of two coils 47, 48 and flux distortion due to armature reaction is minimized by means of shortcircuited bars 37 lying in slots 41 in the polefaces, as shown in Fig. 6. The armature 32 has two similar lap windings 55, 56 wound in the same slots but connected to alternate commutator segments which are twice as numerous as the slots, each winding having an odd number of coils. The commutation principle involves the use of six brushes spanning four segments. The brushes a-f and a<SP>1</SP>-f<SP>1</SP> of compound brush structures 33, 34 engaging the commutator are, in this form, connected to common connections 68, 69 through windings 66a-66f and 66a<SP>1</SP>-66f<SP>1</SP> on separate legs of the core 67 on a reactor 66, resistors 69<SP>1</SP> being also connected across the brush leads. The common lines are connected through the primary winding 70 of a resonator transformer 36 whose core is provided with an air-gap 77 to reduce saturation and whose secondary winding 73 is connected through a condenser 74. Curves are given showing how the value of the condenser, which determines the reactance of the transformer 36, offsets the reactance of the motor armature and so maintains the current in phase with the applied voltage. Various values of condenser and shape of core produce different motor characteristics, e.g. the motor may be made to carry considerable overload, or to stall without substantial increase in armature current on slight overload. The stator and rotor may be fed from lines L1 and L2 through variable autotransformers 99, 94 respectively, the latter being centre-tapped at CT to permit the motor to be reversed. In a modification, the field and armature are fed by separate steps on the same auto-transformer, reversal being obtained by a change-over switch in the armature circuit. The field excitation settings provide constant torque irrespective of the speed. The phase of the field current is corrected by means of a condenser 98, but in a modification, Fig. 2, the same effect is produced by running the machine from a three-phase supply, the reactor 66 being replaced by a reactor 107 which is a simple core ring 108 carrying the windings, a transformer 103 being employed, if desired. Vector diagrams are given illustrating the action of single and polyphase machines acting as motors or generators. Various modifications of the reactor circuit are shown in connection with different commutation positions. In one form, the two sides of the reactor may be partly bridged by a magnetic shunting arm having an air-gap, and the winding 73 connected to the condenser 74 may be mounted on this shunt, in which case a further reactor is necessary but the transformer 36 is dispensed with. The windings may be arranged in various ways on the reactor core and the shunt may extend from end to end of the core. Additional shunts may be situated outside the core to bridge the coil carrying limbs, and in one form three separate reactors with magnetic shunts are used. A modified motor is described in which the field winding is distributed around the stator and is energized on the field axis but short circuited on the quadrature axis to produce the same effect as the bars 37. In single-phase machines the winding is short-circuited through a low impedance winding on the resonator transformer 36. In three-phase machines the connection is direct, or alternatively two distributed field windings are provided, one energized on the field axis and the other connected in series with the armature and resonator transformer on the quadrature axis so as to provide a series compensating winding. In a further three-phase modification, the excitation current is applied to the armature by means of brushes in quadrature with the main brushes, the commutation circuits above described being used for both sets of brushes. Tap-changing transformer.-A tap-changing system similar to the commutation system used  in this invention may be applied to the tapchanging auto-transformers used in the various modifications. Fig. 22 shows such a transformer which has a pair of identical windings connected to alternate tapping points in accordance with the principle. The supply from L1, L2 passes through coils 155, 156 on a reactor 153 to winding 151 and through coils 157, 158 to winding 152. Fixed tappings FT and CT are also connected to the windings through coils 165, 166, 169, 170, on the reactor and in addition a sliding assembly comprising six brushes a-f spanning four commutator segments, as described above, is connected through coils on a reactor 171 to a tapping terminal AT.