Patent GB-662761-A (GB)
Title: Improvements in or relating to protective devices for alternators or synchronous alternating current motors
IPC: H02P9/10
Filed: 1949-01-04
Abstract: 662,761. Testing dynamo-electric machines. GENERAL ELECTRIC CO., Ltd., and FRIEDLANDER, E. S. Jan. 4, 1950 [Jan. 4, 1949]. No. 211/49. Class 37. [Also in Group XXXV] A protective device for a synchronous machine comprises a mechanical rectifier having two terminals arranged to be bridged periodically in dependence upon rotation of the rotor, these terminals being connected in a circuit supplied from the A.C. supply to or from the machine, and means responsive to the output from this circuit to give an indication or effect a control when the angle between the magnetic axis of the field system of the machine and the axis of the effective magnetic field thereof exceeds or departs from a predetermined value. On the shaft 10, Fig. 1, of a three-phase twopole alternator 11 is fitted a rectifier 12 having a conducting segment 13. Two brushes 14, 15 are mounted in a holder 16 which is adjustable about the shaft and carries a pointer 19 indicating on a scale 20. The segment 13 subtends an angle such that the brushes 14, 15 are bridged during half of each cycle. Other forms of mechanical half-wave rectifier may alternatively be employed, for example a vibrating contact controlled by a cam disc driven by the rotor shaft. The brushes 14, 15 are connected in a circuit across the alternator terminals, either directly or through a transformer 17, including an ammeter 18. With the alternator running on no load the brushes are adjusted until zero reading is obtained on the meter 18, the position of the pointer 19 then corresponding to the magnetic axis of the field system. With the alternator running on load the brushes are again adjusted to obtain zero reading, the displacement of the pointer 19 from its original position, then indicating the angle by which the magnetic field has been displaced. Alternatively the meter 18 may be used to indicate maximum current, or it may be calibrated in phase displacement. In a modification the meter 18 is replaced by one winding of a ratiometer, the other winding of which is connected through a full-wave metal rectifier across the secondary winding of the transformer 17. The ratio of the direct current in the circuit including the mechanical rectifier to the arithmetic mean voltage obtained from the second rectifier is a measure of the phase angle and may be directly indicated. In a further arrangement, Fig. 4, the output from the circuit including the mechanical rectifier is applied to one winding 40 of a polarized relay 41 while the output from a rectifier 30 connected across the secondary winding of the transformer 17 is connected through a variable resistance 42 to the other winding 43 of the relay. Condensers 44, 45 are connected across the relay windings 40, 43 for smoothing. The relay 41 has a pivoted armature 46 carrying a contact arm 47 which engages fixed contacts 48, 49 when deflected owing to a change in the ratio of the currents in the coils 40, 43. The operating coil 50 of the voltage regulator of the alternator 11 is connected in series with a resistance 51 across the secondary winding of the transformer 17, and the arm 47 and contact 48 of the relay 41 are arranged to connect a shunt resistance 52 across the operating coil 50 when the phase angle exceeds a predetermined value and thus the field excitation of the alternator is increased until the angle is reduced below the predetermined value. The polarized relay may include a torque-producing device as described in Specification 607,793, and movement of the armature may be arranged merely to give an indication or operate an alarm instead of controlling the excitation.