Patent GB-834887-A (GB)
Title: Improvements in and relating to apparatus for testing magnetic material
IPC: G01R33/12
Filed: 1956-04-30
Abstract: 834,887. Electric tests; analogue electric calculating. GENERAL. ELECTRIC CO. April 30, 1956 [April 29, 19551, No. 13200/56. Class 37. The hysteresis loss of a lamination is determined by means of three coils, two surrounding the lamination and one near to it. One of the coils is an A.C.-fed magnetizing coil, another a " B " coil producing a voltage dependent on core flux and the third an " H " coil producing a voltage representative of a first derivative of the magnetomotive force in the vicinity of the lamination. General operation and theory (Figs. 1 and 2).-The core lamination 10 to be tested is inserted in a magnetizing coil 11 and the " B " coil 12 while the " H " coil 13 is in two parts positioned adjacent the axis 10. The output voltage from supply 14, regulator 15, step-down transformer 16 and band-pass filter 17 which removes harmonics from the source voltage is fed through potentiometer 18 and power amplifier 19 to control the magnetizing current in coil 11. The output voltage of coil 12 feeds a computer 20 which takes account of the size of the specimen under test and the signals developed at taps on resistors 24, 25 are fed respectively to a " B " amplifier 27 to energize one coil of a wattmeter and to a rectifier and stabilizer amplifier 26 where it is compared with a D.C. reference voltage to control a servo amplifier to maintain the core flux constant. The " H " coil feeds, through an integrating circuit 41 and amplifier 42, the other coil of the wattmeter which indicates directly power loss/unit volume. The Specification states that the power loss/unit volume is given by where f is the frequency, n the turns/cm. of the magnetizing coil, N the number of turns in a uniform secondary winding such as the " B " coil, A the cross-sectional area of the core and i is the magnetizing winding current. In the previously described arrangement e is determined directly from the " B " coil but i is determined from the fact that a voltage is induced in the " A " coil which is proportional to the rate of change of the magnetizing current. The output of the " H " coil is integrated at 41 to give i in the expression above and the integration of the product of these quantities is preformed by wattmeter 40. Practical arrangement 'Figs. 3a, 3b).-The A.C. voltage regulator 15 is connected to mains supply 50 and, as described in connection with Fig. 1, feeds a substantially sinusoidal voltage to potentiometer 18. The voltage tapped from the ,mobile arm of the potentiometer, driven by servomotor 33, is used to energize magnetizing coil 11. The " B " coil is connected through a test-calibrate switch 52 to a flux density computer to be described, and produces a voltage indicative of core flux density which is tapped from switch arm 56a, fed to an averaging rectifier and amplifier 26, compared with a reference D.C. developed across a resistor 84 by a conventional reference circuit 75 and the difference signal feeds a servo amplifier 32 which drives servomotor 33, adjusting potentiometer arm 34 varying the core flux until the input to the servo amplifier is zero. At a further arm, 56b, of switch 56 is developed a signal which is constant for any given setting of potentiometer 25 which determines the setting of the magnetizing flux and this signal is fed through an A.C. amplifier 27 having two resistance-capacity coupled stages followed by a direct-coupled cathode follower 130 and fed back by lead 136 to the cathode circuit of the first amplifier 101. The amplifier thus has negligible phase shift. The output of the amplifier feeds a coil 135 of a wattmeter. The output of the " H " coil is fed through switch 52 and relay contacts to an integrator circuit 41 and an amplifier 42 identical in design to the amplifier 27 and feeds the other wattmeter coil. The integrator 41 comprises a cathode follower 151 feeding a phaseshifting network comprising resistors 160, 162, 163, 165 and 171 and condensers 161, 164. The effect of this network is to produce a 90-degree phase shift in the input signal and this phaseshifted signal represents the integral of the applied signal. Relay 202 is arranged to connect the output of the " H " coil to the integrator only when the output of the servo amplifier is below a certain level, and comprises a gas-filled tetrode 207 which fires and operates the relay to close the circuit to the integrator only when the output of the servo amplifier is zero. Flux density computer.-The function of this computer is to compensate for the variation of the cross-sectional area of the specimens under test in computing the loss/unit volume. When a setting is made on a " width " rheostat 21, thickness rheostat 22a and " compensator " rheostat 22b, the servo controlled voltage supply for coil 11 establishes a predetermined flux density in the lamination. The arm of the computer consisting of resistor 23 and compensator 22b maintain a constant resistance value between ground and the bottom of the " width " rheostat 21 so that the width and thickness settings are independent. Ganged switches 56a, 56b and 56e are used to alter the range of thickness of laminations by substituting different values of thickness. Switch 60b acts to substitute automatic thickness computation for manual by replacing rheostats 22a, 22b by 222, 223 which are adjusted by a servomotor 220 driven by the output of a bridge 211. The thickness of a lamination is given by of the lamination, W h  is the weight of any material removed from the lamination, d is the density, L the length and b the width of the lamination. One arm of the bridge consists of rheostat 212 adjusted to be proportional to the length L. An adjacent arm consists of rheostat 213 whose resistance is inversely proportional to the width b of the lamination, the other adjacent arm consisting of rheostats representing W and W h  respectively. The opposite arm contains a fixed resistance 216 proportional to the expected minimum thickness of a lamination nominally 12 mils. thick. Rheostat 217 is driven by the servomotor which rebalances the bridge and at the same time adjust thickness rheostat 222 in the flux density computer. The width adjustment may be coupled to the width adjustment 21 of the flux density computer.. Switch 60 changes the thickness setting from manual to automatic. Calibration.-When switch 52 is in its lower position capacitor 231 is connected to the width adjustment rheostat 21 and the output of power amplifier 19 while the input of integrator 12 is connected to the junction of the top or bottom pair of resistors 232, 233, 234. This circuit simulates a lamination of known characteristics such as power loss/pound, width thickness and power factor.