GB618050A - Improvements in and relating to the testing of dynamo-electric machines - Google Patents

Abstract

618,050. Indicating and locating faults. GENERAL ELECTRIC CO., Ltd., and WARD, L. G. Sept. 12, 1945, Nos. 23526/45 and 30164/46. [Class 37] Apparatus for testing the rotor or stator windings of a dynamo-electric machine comprises an electromagnet, means for producing relative rotation between the electromagnet and the rotor or stator under test in such a manner that the individual windings thereon or windings in individual slots thereof are successively linked by magnetic flux from the electromagnet, and means for indicating changes in the instantaneous value of the E.M.F. in the winding of the electromagnet. As shown in Fig. 2, a stationary electromagnet 1 is disposed below the rotor 2 under test which is held between a chuck on a shaft and a back centre, the shaft being driven by an electric motor. The poles of the electromagnet 1 are so shaped that each conductor 9 is linked by the flux as the rotor 2 rotates. The windings 8 of the electromagnet 1 form one arm of a Wheatstone bridge, of which a second arm comprises a variable inductance in series with a variable resistance. The other two arms are formed by a resistance with a variable tapping. The bridge is supplied with alternating current and the output voltage, amplified if necessary, is fed to the vertical deflection plates of a cathode-ray oscilloscope. Alternatively, instead of using an A.C. bridge, the windings 8 may be connected across a D.C. supply and the ripple voltage developed across the windings 8 applied through condensers direct to the oscilloscope amplifier. A saw-tooth voltage is applied to the horizontal deflection plates and is arranged to have a period equal to that of one complete revolution of the rotor. This may be effected by means of an insulating disc mounted on the driving shaft and provided with a metal insert which short-circuits a pair of brushes once during each rotation of the shaft so as to discharge a condenser and thereby trigger the time-base generator. Alternatively, an amplifier serving to amplify the voltage variations in the electromagnet winding may be provided with a class C stage which furnishes pulses for triggering the time base. The trace produced on the screen consists of a number of vertical lines corresponding to the number of slots or individual conductors and any difference in impedance of a winding is indicated by a line of greater or lesser height than the remainder. The pair of brushes are mounted on a support which can be rotated about the axis of the shaft so that a vertical line of different length from its neighbours may be brought to one end of the trace on the screen. The rotation is then stopped and the shaft is turned until the brushes are short-circuited, the winding under the influence of the electromagnet 1 being that giving the fault indication. In a modification, Fig. 4, for testing stators, an electromagnet 22 is mounted so as to be rotatable by a motor 23 about an axis 24, and a spigot is provided on the test mounting 26, whereby the stator 25 to be tested is supported coaxially with the axis 24. The electromagnet 22 is U-shaped having two radial limbs spanning two stator teeth. The spacing of the poles may be adjustable. A weight 42 counterbalances the electromagnet 22. An energizing winding 27 is connected through slip-rings 28 to a source of alternating or direct current and to the vertical deflection plates of a cathoderay oscilloscope. A four-part commutator may be mounted on the shaft of the electromagnet to interchange the connections to the horizontal and vertical deflection plates as the poles of the electromagnet pass each pole of the stator under test. In the case of a four-pole stator the image is then in the form of a square, each side of which contains markings representing the condition of the windings of one pole of the stator. If any coil is faulty, this is indicated in the side of the square corresponding to the faulty coil only. A time base giving a circular scan may be used, the deflecting voltages being obtained from a two-phase alternator driven by the motor 23. The signal voltage may alternatively be used to modulate the electron beam either by applying it to an annular grid in the oscilloscope tube or to a disc electrode held against the screen.