GB1066057A - Improvements relating to inductive clearance measurements - Google Patents

Abstract

1,066,057. Measuring physical dimensions electrically. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. May 28, 1964 [June 14, 1963], No, 23802/63. Heading G1N. A clearance monitoring system utilizes the change in apparent impedance of an air cored probe coil with distance from an electrically conductive surface as a measure of the distance. A system for measuring clearances between a disc or shaft of a steam turbine rotor 3 (Fig.1) and its casing comprises measuring and reference inductive probes 1, 15 respectively. Both probes are stationary and have air cored coils fed from an oscillator 19 at a frequency between 10 K c/s and 10 M c/s (e.g. at 1 Mc. ) and are connected to a differential amplifier 31 the output of which is indicated on a meter 31A, An extension bracket 9A of metal having similar magnetic characteristics when hot as the turbine rotor is located in the field of a coil 15A and both probes are in the turbine casing and in the steam flow path. Deviation of the impedance of coil 1A of probe 1 from that of probe coil 15A produces an output from the amplifier which may be rectified so that the meter 31A gives a steady deflection indicating the positive and negative changes in clearance over one revolution of the rotor. Preferably the probe coil 1A is arranged in a non resonant circuit and the oscillator frequency is sufficiently below or away from the natural resonant frequency of the coil and its self capacity together with the capacity and inductance of the connecting cable to avoid any deleterious effects of change in the said parameters on the measured values. Instead of the oscillator 19 a regenerative amplifier may be used Fig. 2 (not shown) so that the system of coil and cable capacity oscillates at a frequency dependent on the inductance of the probe coil. A voltage output from the amplifier varies as the impedance of the probe coil and cable and thus as the clearance being monitored.