OIPEEC Conference / 3 rd International Ropedays - Stuttgart - March 2009 Aldo Canova (1), Fabio Degasperi (2), Francesco Ficili (4), Michele Forzan (3), Bruno Vusini (1) (1) Dipartimento di Ingegneria Elettrica - Politecnico di Torino (Italy) (2) Laboratorio Tecnologico Impianti a Fune (Latif) – Ravina di Trento, Trento (Italy) (3) Dipartimento di Ingegneria Elettrica - Università di Padova (Italy) (4) AMC Instruments – Spin off del Politecnico di Torino (Italy) Experimental and numerical characterisation of ferromagnetic ropes and non-destructive testing devices Summary The paper mainly deals with the magnetic characterisation of ferromagnetic ropes. The knowledge of the magnetic characteristic is useful in the design of non- destructive testing equipments with particular reference to the design of magnetic circuit in order to reach the required rope magnetic behaviour point. The paper is divided in two main section. In the first part a description of the experimental procedure and of the provided setup devoted to the measurement of the non linear magnetic characteristic of different rope manufactures and different typology is presented. In the second part of the paper the magnetic model of the ropes is adopted for the simulation of a non-destructive device under working conditions. The simulation are provided with a non linear three dimensional numerical code based on Finite Integration Technique. Finally the numerical results are compared with some experimental tests under working conditions. 1 Introduction In addition to satisfying the requirements for the eligibility of magneto-inductive devices for control of rope for people transport equipment, defined by relevant national and European directives, one of the major performances required from a magneto inductive device is to provide an LF or LMA [1] signal (LF: Localized Fault and LMA Loss Metallic Area) with high signal-noise ratio. There are several factors that can influence the performance but it was found that an increase in this provision requires a high level of magnetic saturation of the rope. It is obvious that the increase of magnetic saturation can be reached by increasing the size of the magnetic parts of the detector, but we must take into account also the user need of a device as light as possible. Is therefore necessary to make an optimal design of magnetic structure that minimizes the weight maximizing the magnetic saturation within the rope. In building a magnetic model that satisfactory approximates the magnetic detector, in many cases must be taken into account the three-dimensional geometry and magnetic characteristics ( B-H curve) of different materials. Usually the magnetisation performances of a device is deduced from the measurements of the internal magnetic induction along its main axis in the absence of the rope (no-load test). 1 Innovative ropes and rope applications In this case it simply must have the characteristics (first magnetization curve) of the soft magnetic materials, usually steel, with which the detector yokes are made and the properties of the permanent magnets (residual induction and coercive field) that generate the magnetic flux. If one want to evaluate the actual degree of saturation of the rope, measurement is usually not possible and one should proceed with the simulation of the instrument in the presence of rope (load test). The problem for this simulation is related to the difficulty of having the magnetic characteristics of the ropes. Although in this case are known the material characteristics of the raw material, the machining alter the magnetic characteristics and therefore is necessary to have the measurement of the B-H curve on a sample of rope made. This work will be presented with an experimental activities on a series of ropes that made it possible to have the real magnetic characteristics. With a full three-dimensional model, nonlinear equipped with all first magnetization curves of different materials, including rope, one can design any magneto-inductive detectors for any section or rope type. 2 Magnetic rope characterisation The magnetic characteristic has been measured for different types of ropes used for people transport systems, in the range 20 – 53 (mm) diameter. The measurement of the B-H characteristic of a magnetic material has been obtained as interpolation of the vertexes of several symmetric hysteresis cycles. The measurement has been done establishing a magnetic field H, with a controlled magnitude and direction, in the region where the magnetic material is placed. The value of the induction in the sample can be deduced by measuring the variation of the flux linked with a probe coil due to a known variation of the magnetizing field intensity. A toroidal configuration of the inductor, like the one shown in Fig.1, is the easiest way to set up a magnetic field where both the magnitude and the direction are controlled. Unfortunately, it was not possible to set up this configuration for the real ropes, because the resulting length of the torus was too much long. The measurement of the induction inside a toroidal configuration has been done for a single wire rod, made of the same material used for the real ropes. The wire rod, 8.1 (mm) diameter, has been bended and welded at the edges, the total length of the ring, l, was 60 (cm). The toroidal inductor has been built by winding 2050 (turns) around a rubber tube. With this configuration, the magnetic field inside the torus H 0 is = ⋅ H0 n I (1) where n is the turn density, N/l where N is the number of turns. The B-H characteristic of the sample rod is presented in Fig.3, while in Fig.4 the magnetic relative permeability µr is presented as a function of the magnetic field (magnetic the relative permeability µr permeability is defined as the ration between -6 the material permeability µ=B/H and absolute permeability of the vacuum µ0=4 π10 (H/m )). The measurement of the B-H characteristic of the real ropes has been done with the magnetic circuit shown in Fig.2. The magnetizing inductor is made by 4 separate coils, built by winding 1000 (turns) of copper wire around a plastic tube, 16 (cm) length, and 6 (cm) diameter. The magnetizing inductor made by the 4 coils is a ‘long’ 2 OIPEEC Conference / 3 rd International Ropedays - Stuttgart - March 2009 inductor: the total length of the system is 64 (cm), so the ratio between the length and the diameter is around 10. The long inductor assumption means that only the tangential component of the magnetic field should exist and the field magnitude in the centre of the system can be computed by (1), with n now equal to 6250 (turns/meter), when the inductor is empty. To measure the magnetic induction, a probe coil has been turned around the rope and connected to an electromagnetic flowmeter (Yokogawa, Electronic Fluxmeter, Type 3254), then the rope has been placed inside the magnetizing inductors and bended to create a closed magnetic circuit. The maximum current in the coils is limited to 16 A, so the maximum field H0MAX is 100 (kA/m). When the rope is present in the magnetizing system, the effective field H is different from H0 because the field magnitude is not constant along the rope axis and consequently also the intrinsic induction J in the rope is not constant. Fig. 1. Schematic of the experimental set up for Fig.2. Schematic of the experimental set up for the BH characterization of the simple wire rod. the BH characterization of the ropes. 2 350 1.8 300 1.6 1.4 250 1.2 200 R 1 µ B B [T] 150 0.8 0.6 100 0.4 50 0.2 0 0 0.0E+0 5.0E+3 1.0E+4 1.5E+4 2.0E+4 2.5E+4 3.0E+4 3.5E+4 0 5000 10000 15000 20000 25000 30000 35000 40000 H0 [A/m] H0 [A/m] Fig.4. Relative permeability of the sample wire Fig.3. B-H characteristic of the sample wire rod. rod. The field H can be decomposed into two components, H = H sol + H irr where Hsol = H 0. From: rot H = G (2) div J div H = − (3) µ 0 where G is the current density and J is intrinsic induction, the components of the H field can be expressed as: 3 Innovative ropes and rope applications div J rot H = 0 div H = div H = − (4) irr irr µ 0 = = = div Hsol 0 rot Hsol rot H G (5) Hirr , the demagnetizing component of the field, cannot be computed but in very simple geometric configuration and consequently only H0 is taken into account as magnetic field for the measurement of the B-H characteristic. The effect of the demagnetizing component depends on the cross section of the rope and it is higher for the biggest ropes. It is worth to notice that this component does not arise in the toroidal configuration. To take into account the effect of the Hirr , the measured value of the induction B or the magnetic bias J have been modified into B’ and J’ according to the methodology described in the following. The magnetic circuit has been studied in analogy with the electrical network theory; using the following definitions: Φtotal , the measured value of the flux, i.e. the total flux linked with the probe coil, Φmet the flux in the metal cross-section of the rope, Φair the part of the linked flux in the air, Scoil cross-section of the probe coil, Smet cross-section of the metal part of the rope, α filling factor of the coil ( Smet /Scoil ); dl dℜ = reluctance of an infinitesimal part of the metal part of the met µ ⋅ µ ⋅ 0 r Smet magnetic circuit, dl dℜ = reluctance of an infinitesimal part in air of the magnetic circuit.