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

Ryan Maughan explains the existing ways to measure motor torque, and considers a new approach Twists in the tale A s the market for electrified vehicle powertrains develops, so the quest to deliver improved efficiency and performance in electric drive systems continues. Maximising vehicle range for a given battery capacity is a key target of every powertrain development programme, while maintaining and improving the powertrain system’s safety integrity is paramount. Most automotive motor inverters use a range of sensors to monitor and control the traction motor. A typical sensor scheme comprises a rotor position sensor, which for propulsion applications is usually a position resolver that uses magnets mounted on the motor’s shaft, and Hall effect sensors to create a sine and cosine voltage waveform with the purpose of reporting the rotor position in relation to the stator and rotational speed. An alternative to magnetic sensors uses inductive sensing elements to create the same waveforms. This absolute position signal tells the motor controller where the motor’s rotor is relative to the stator phases and its rotational speed. In addition to speed and position, another critical measurement is temperature, and typically a number of temperature sensors are embedded into the motor’s stator to ensure that the stator’s temperature does not exceed design parameters. Rotor temperature is also critical, but because it is difficult to measure temperature on the rotating part it is not directly measured; instead its relationship with stator temperature measurement is established through simulation and experimental measurements. Motor phase currents are also measured, using several current sensors inside the motor controller. They are usually Hall effect devices that can measure current from the magnetic fields created by the electric current as it flows through the conductors. This method is preferred for high- efficiency motor drives over the use of shunt resistors, which while providing a relatively simple and low-cost solution to obtaining accurate current measurements, incur significant power losses over the shunt itself. Motor torque is typically controlled by an inner current control loop. In its most simplistic guise, this comprises a feedback loop, where the measured motor phase current is compared to current demand, and an error/ correction value is calculated. This value is used to modify the PWM generator that in turn modulates the DC link voltage to the motor in a sinusoidal manner, effectively inducing current into the machine’s stator windings. This induced current is proportional to the magnetic flux generated in the stator’s windings, which in turn is proportional to the torque generated. The controller will source as much current as is needed to hit the requested torque input to the drive (assuming the controller is tuned correctly). Sensorless control, as opposed to the classical sensored control schemes described above, differ as no position sensor is used to determine A SAW device can be shaft-mounted inside the motor to measure shaft torque and temperature in close proximity to the motor rotor 44 Winter 2022 | E-Mobility Engineering