Versatile Measurement Method for Characterising the Permanent Magnets used in Electric Vehicles

Abstract

When rare earth permanent magnets such as NdFeB, or in the future their technically equivalent replacements, are used in the motors of electric vehicles the properties need to be known for the conditions of use in order to optimise the efficiency of the motor and increase the range of the electric vehicle. A Pulse Field Magnetometer measurement method based on measuring the properties of permanent magnets in air has been developed and provides considerable advantages over existing standard methods, such as the ability to measure the actual geometries used and at operating temperatures experienced within the motor. However, for this method to be accepted by the electric vehicle manufacturers, and in particular the motor supply chain, it is necessary to establish the agreement with the standard methods of measurement. An International Electrotechnical Committee, Technical Committee 68, comparison on the measurement of the magnetic properties of permanent magnets is presented and the achieved International agreement is established. Measurements were performed on 6 NdFeB magnets with intrinsic coercivities ranging from 1000 to 2600 kA/m by 8 institutes based in China, Japan, Italy, Belgium, Germany and the UK. During the last 2 decades, many variations of a Pulsed Field Magnetometer (PFM) have been developed, all of which are capable of determining the full BH curve in as little as 100 ms. By comparing measurements made using an internationally accepted electromagnet (closed-circuit) method and the pulsed field (open-circuit) methods, the influence of the dynamic effects of the latter could be investigated and established. For the quantities such as remanence, Br, magnetic flux density coercivity, HcB and energy product, BHmax the measurements agree within the combined uncertainties. However, for the intrinsic coercivity, HcJ, the dependence on the speed at which the magnetic field is reversed was found to be significant with the largest changes in value occurring as a DC measurement condition is approached. For the supply chain to adopt the PFM as a standard method the accuracy of the method needs to be established at an uncertainty level that allows the significant benefits of this technique to be fully realized. The metrology this involves is performed by National Measurement Institutes and in the UK this is one of the roles of NPL. The measurements being performed at NPL to calibrate a 50 mm bore PFM commissioned in August 2013 will be introduced and early results presented.