Efficiency Mapping for Electric Vehicle Powertrains: Digital is 10x Faster than Analog

A key objective of Electric Vehicle R&D is to maximize the range for a given battery capacity and vehicle weight. That means designing a powertrain that operates in the maximum efficiency zone as much as possible. The problem is that efficiency mapping is a costly and time-consuming process, often occupying expensive equipment for weeks on end. That’s where HBK eDrive Power Analyzers come in: they use an innovative digital algorithm to map motor efficiency in a fraction of the time required by analog methods.

HBK, electric vehicles, efficiency mapping

Efficiency maps are used to optimize the design of the powertrain in an EV. Image Credit: HBK

To test and optimize their powertrains, electric vehicle manufacturers turn to HBK for test and measurement equipment. HBK’s systems evaluate all aspects of an EV’s powertrain, including the motor, controller, gearbox, and inverter. The eDrive Power Analyzer solution reduces the time it takes to create an efficiency map by a factor of 10.

The efficiency of a powertrain varies primarily with speed and torque. HBK equipment measures battery power, inverter power, and motor power to determine each component’s efficiency at different torque-speed combinations.

Using a dynamometer, the user programs torque and speed parameters for every point of the vehicle’s operating range measures the input and output powers of various components, and uses those figures to calculate the efficiency of each section. Once the data points have been mapped, the analysis software reveals details about how the entire propulsion system operates, and the powertrain can be designed to optimize efficiency.

Real-time mechanical and electrical measurements

To create an efficiency map, engineers need to measure the DC bus voltage and current, inverter voltage and current, inverter input power and inverter output power. By measuring the amount of electrical power going into the motor and the amount of mechanical power produced by the motor, engineers can determine the motor’s efficiency. If the EV has a gearbox, engineers can measure its mechanical power output and calculate its efficiency as well.

All these tests are very temperature-dependent; when machines get hot, they lose energy and become less efficient. Keeping the machine wihin a tightly defined temperature range is vitally important for engineers. 

The HBK eDrive product measures all mechanical and electrical values simultaneously. Torque, speed, temperature, voltage, current, and other parameters are measured and stored in real-time, along with data from the powertrain’s control system.

Post-processing analysis will compare the actual physical quantities measured during the test with the values the controller is ‘thinking’ or estimating. This comparison allows engineers to optimize the design of the control system.

Efficiency mapping with cycle detect

Efficiency mapping sounds simple in principle: you only have to map a few hundred operating points on the torque-speed spectrum. But the efficiency also varies with gear ratio, temperature, and even the battery’s state of charge. Suddenly, all these real-world variables multiply into maybe 50,000 unique scenarios for a single powertrain, and that will tie up an expensive test rig for weeks.

Traditional power analysis uses a technology called a phase-locked loop to track a frequency. It’s an analog approach that is very slow and is usually tuned at 60 Hz. In reality, EV motors run at multiple speeds, so a power analyzer needs to sweep across a range of frequencies in order to test a motor at all those speeds.

The problem is that with each frequency change, the analyzer has to wait for the system to stabilize before taking another reading. This adds about 10 seconds to each measurement. Multiply that by 10-second delay by 50,000 setpoints adds up to several weeks of dynamometer time.

This is why HBK developed a digital algorithm that, reads the fundamental frequency of the sine wave supplied to the motor in real-time and delivers a measurement on every half-cycle of the wave, which is a fraction of a rotation of the machine. For example, for a 20-pole machine, a measurement is taken every 10th of a rotation.

HBK, electric vehicles, efficiency mapping

The HBK eDrive solution enables R&D teams to optimize powertrains more quickly and bring new products to market earlier. Image Credit: HBK

The HBK cycle detects algorithm allows a power map with 300 unique conditions to be completed in a few minutes. A map with 50,000 scenarios will take around 10 hours, whereas the old method might take 10 working days or more. This is a significant gain in productivity for Test bench engineers. In practice, it means that R&D teams can complete the optimization of EV powertrains more quickly and bring new products to market earlier.

This information has been sourced, reviewed, and adapted from materials provided by Hottinger Brüel & Kjaer GmbH.

For more information on this source, please visit Hottinger Brüel & Kjaer GmbH.

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