GB1458792A - System of test coils and method for determining properties of test pieces - Google Patents

Abstract

1458792 Electromagnetic test coil systems F FORSTER 22 Jan 1975 [10 April 1974] 2850/75 Heading G1N A test coil system for determining properties such as hardness, strength, heat treatment condition, depth of hardness of test objects includes excitation winding 16 connected in series either aiding or opposition with a primary winding 19 of a transformer 14 and receiver winding 17 in series opposition or aiding with the secondary 20 of the transformer. Transformer 14 is magnetically decoupled from windings 17, 19 and is mounted on a bracket 59, Fig. 4 of a potted assembly, the exciter winding being in the form of two series, connected windings 52, 53 to oppose the demagnetizing effect which occurs with short test pieces. The test coil 13 and transformer 14 are connected together as shown to pins 21, 22, 23, 26 of plug socket 15 for connection to test unit 11. Calibration balancing. With no test piece 18 present constant-current generator 31 is adjusted to produce the maximum current i and the slider of potentiometer 35 is moved to its lowest stop so that there is no voltage at input 42 of difference amplifier 36. Screwed plug 63 of the transformer 14 is then rotated to vary its inductance until a zero display is shown on meter 37 indicating that the voltage across windings 17, 20 U a and Ug are equal and opposite. Alternatively the turns of the transformer may be chosen to achieve this. Measuring. Constant current generator 31 is adjusted to give the current for the particular test and the first test piece 18 is inserted. The slider of potentiometer 35 is moved to a setting to produce zero deflection on meter 37 and the position of the slider is read off to give the desired reading. Alternatively a reference test piece may be inserted, the slider moved to zero the meter 37 output, and then replaced by a test piece the meter reading then giving the deviation from the reference properties. A phase sensitive detector may be used instead of rectifier 34. Instead of a through coil system the coil may be wound on a ferromagnetic core which is placed on the item under test.