Patent GB-692659-A (GB)
Title: Improvements relating to electrical testing apparatus
IPC: G01R31/06
Filed: 1951-03-14
Abstract: 692,659. Detecting faults. ELECTRICAL & AUTO PRODUCTS (1931), Ltd., and ROTHWELL, G. A. March 14, 1952 [March 14, 1951], No. 6116/51. Class 37. Means for testing armature, field coil, and stator windings comprises a search head containing a driver coil and a detector coil adapted to be moved over the surface of the article being tested, an oscillator coupled to the driver coil, and an amplifier having the detector coil in its input circuit and a meter or indicator in its output circuit, a signal being transmitted from the driver coil to the detector coil when the search head is placed near a closed winding. The search head 1, Fig. 1, includes an ironcored driver coil 2 and an iron-cored detector coil 3. An oscillator valve 4 is coupled to an output valve 5 which has in its anode circuit a transformer 6 feeding the driver coil 2. A variable resistance 7 in the grid circuit of the valve 4 is set to give a frequency of approximately 1 kc. The valve 4 is a twin triode connected as a two-stage resistance capacity coupled amplifier with the whole of the output fed back to the input, forming a multi-vibrator circuit which is resistance-capacity coupled to the output stage. The detector coil 3 is connected to the anode of a shunt-fed diode 9 coupled to a triode amplifier 10. A meter 11 is connected in the anode circuit of the amplifier 10 in a bridge circuit, which is balanced in the absence of an input signal by a resistance 12. When a signal is applied to the diode 9 the grid of the triode 10 is driven negative, the bridge becomes unbalanced, and the meter pointer is deflected. The sensitivity is controlled by a variable resistance 13. A relay 14 is provided in the anode circuit of the triode 10, with normally closed contacts across the meter terminals, so that the meter is short-circuited until the triode becomes conductive. Fig. 2 shows a search head for testing field coils. There are two driven coils 2 in series and two detector coils 3 in series. The detector coils 3 are mounted on the outer limbs 15 of a laminated iron core, the driver coils 2 being mounted on a cross-piece on the centre limb 16. A field coil to be tested is placed over one of the projecting ends of the limbs 15 and causes coupling between the coils 2, 3 if there is a shortcircuit in the winding. A signal is thereby transmitted and the meter is deflected. The field coil can be tested for continuity by joining its ends together, when a deflection should be obtained on placing. it over the core 15. Absence of deflection indicates a break in the winding. Fig. 3 shows an armature search head comprising a handle 17 and two parallel casings 18, 19, each carried by a slotted limb 20 which is secured to the handle 17 by a bolt 21 passing through the slots so that the distance apart of the casings 18, 19 can be adjusted. In the casing 18 there is an insulated support block 22 carrying the driver coil 2, the core of which has laminated polepieces 23. The detector coil 3 is similarly mounted in the casing 19. The search head is moved over the armature core face and as it passes over a series of closed windings a continuous reading is obtained. A modification is described for use inside a stator.