Patent GB-750995-A (GB)
Title: Improvements relating to apparatus for testing inductive windings
Inventor: REID CARL REVERE
IPC: G01R31/06
Filed: 1954-03-01
Abstract: 750,995. Fault indicating. HOOVER, Ltd. March 1, 1954, No. 5879/54. Class 37. Apparatus testing inductive windings for short circuits comprises means for repeatedly charging a condenser and for successively and suddenly discharging it through a test circuit containing or coupled to the winding under test, and means for detecting variation, from the datum value corresponding to a satisfactory winding, of the reverse voltage to which the condenser is charged due to the collapse of the magnetic field of the winding which is created by the condenser discharge. A wound motor armature 1 to be tested (Fig. 1) is rotatably supported on V-blocks 4 between the poles of an electromagnet 2 excited by winding L, and is rotated through a reduction gear by motor 7 energized from a 60 c.p.s. supply 8 over normally open switch 11 and normally closed switch 12 shunted by manually operated normally open starting switch 13. Cam 15 driven by motor 7 through further reduction gear opens switch 12 once per three revolutions of the armature, whose rotation is continuous from closure of switch 11 and momentary closure of starting switch 13 until the armature has made three revolutions after which the cam opens switch 12 and arrests the rotation. Winding L is pulse excited from capacitor C shunted by a high resistor 18, which is charged once per cycle from the supply through diode 22 from the secondary of a variable transformer 21, and is discharged to earth once per cycle subsequently to the charging diode 22 ceasing to conduct, by a thyratron 23 whose firing is controlled by impressing on its grid a negative direct voltage derived by rectifier 25 from half the secondary of transformer 24 and smoothed by capacitor 27 and resistors 26, 28, together with an alternating voltage from the junction of resistors 29, 30 across the other half secondary; the transformer being energized 180 degrees out of phase with transformer 21 and the resistors and capacitance being proportioned so that the thyratron fires # cycle after diode 22 becomes non-conductive; and suddenly discharges condenser C through winding L, which is magnetically linked to the test armature to induce a pulse voltage in the conductors linked by the test field (Fig. 2, not shown) and on completion of the discharge the collapse of the field induces an inverse pulse voltage tending to maintain the current flow in the same direction as the original pulse. This voltage pulse is stored on condenser C until the next charging pulse through diode 22, and any short circuits in the armature windings distort the field due to coil L and vary the value of the inverse pulse voltage from the standard value for a satisfactory armature; a single short circuit tending to increase the negative-going 'pulse while a multiple shortcircuit tends to decrease it (Fig. 3, not shown). The test armature is rotated at 60 r.p.m. for 3 revolutions before cam 15 de-energizes motor 7 so that it is subjected to 180 test pulses during rotation, ensuring that each conductor is subjected to plural current pulses while lying in maximum inductive relation to the field 2. The voltage across capacitor C is applied over series resistances 41, 42 whose junction is earthed over resistance 43 to normally saturated limiting amplifier 40 which eliminates the positive charging pulses across capacitor C, while preserving and amplifying the negative going inverse pulses which are resistance capacitance coupled to a reversed diode 49 fixing the zero level to which the now positive going pulses are referred, and to the grid of a triode 52 connected with milliameter 58 in a retroactive valve voltmeter circuit adjusted so that a normal armature winding produces half-scale deflection for each test pulse, and a faulty winding produces greater or less deflection as the armature is rotated by motor 7. The invention is applicable to test single inductive coils (Fig. 4, not shown).