E-Mobility Engineering 016 l Aurora Powertrains eSled dossier l In Conversation: Thomas de Lange l Automated manufacturing focus l Torque sensing insight l Battery Show Europe 2022 report l Sodium batteries insight l User interfaces focus

for accurate torque measurement, this is not the case as there are many factors that make this method of torque estimation less than ideal. The main variables that affect the torque constant of an electrical machine can be split into two distinct categories. The first are design factors such as saturation, magnetic loading, and temperature that affects parameters such as the magnetic flux density (Br) in the air gap. Because the Kt and voltage constant (Ke) are linked to the Br in the air gap between the rotor and the stator, Br will change with an increase in magnet temperature depending on the magnet properties, rotor material permeability and machine loading factors, so it is a continually varying property. This challenge remains with the addition of induced magnetic fields from reluctance effects in IPM geometries or other machine technologies such as switched reluctance and induction motors. The second category is manufacturing variability, where each machine type has its own unique set of variances, such as air gap, magnet Br variance and so on. To some degree, the design variance of the machine and the effect on the Kt can be simulated or measured, and the values of Kt can be stored in a parameterised look-up table to increase the accuracy of the estimated torque. However, the sheer number of permutations needed to give a torque accuracy comparable to a physical torque value obtained via a rotary torque transducer is incredibly processor-intensive, and is unfeasible for all but the most basic design variables. Also, it should be noted that although there are many techniques to parameterise these variances for a specific machine drive combination, and to do so for general-purpose industrial inverters – where there is a need to deliver performance with a huge choice of motor sizes and technologies from multiple vendors – is practically impossible. In traction applications in particular, from a system safety integrity point of view the primary safety goal of the system will be to ensure that there are no unrequested torque disturbances or deliveries that could lead to unintended vehicle accelerations, or in the case of an EV, any regenerative braking capability decelerations. Realising this safety goal based solely on torque estimation derived from current measurement using the Hall effect is not possible, leaving failure modes open. The physical measurement of motor field currents using Hall effect sensors is prone to issues such as background noise from electromagnetic fields inside the inverter and transient signal effects on the current-carrying cables, which must be filtered out before reading the measured values, and that leads to a reduction in fidelity of the measured values. There is a fine balance between filtering the signals to remove noise and having a signal of sufficient integrity to be able to accurately detect faults in the system. This issue has become more acute as both drive and machine efficiency have been developed, making it harder to differentiate a potential short-circuit condition from the normal operation of the system. There is also a major issue with signal latency or time delay in positional feedback of the rotor; instead it can be calculated by analysing the measured back-EMF on the motor phases. Sensorless control is deployed primarily to reduce cost at the expense of accuracy, but the reduced control fidelity – particularly at low rotational speeds – and resulting safety integrity mean they are not feasible for traction motor applications. Speed/position encoders or resolvers vary in cost depending on specification, accuracy resolution and environmental performance. Depending on the requirement, the cost of position encoders or resolvers can be particularly significant in the overall bill of materials of the machine. Because it is a key part of the functional safety control system in traction applications, accuracy and reliability are critical, and more costly resolver technology is required than for less safety-critical applications. In order to estimate the torque in an electrical machine, the torque constant (Kt) must first be obtained by a combination of model-based simulation and physical measurements, the latter being time-consuming. We can say that the torque of an electrical machine is proportional to magnetic flux, which in turn is proportional to current. If the current flowing into the motor from the inverter drive can be accurately measured, the torque of the machine can be calculated. However, in reality, Axial and radial RF couplers can be used to communicate with a SAW device mounted on the shaft with a low-power RF signal Winter 2022 | E-Mobility Engineering 45 Insight | Torque sensing