Why are new techniques needed for testing EV motor efficiency?

New testing techniques are needed for electric vehicle (EV) motors because they’re used in different ways than motors in industrial or consumer systems. EV motor efficiency is a critical parameter to ensure maximum driving range, and it’s complex to quantify.

New testing techniques have been developed that measure:

The thermal efficiency of high-power density motors
The regenerative braking efficiency of these motors
Their contribution to noise vibration and harshness (NVH) and ride comfort
Their electromagnetic compatibility (EMC) to ensure safety
Their reliability to support a long operating life

In addition, EV motors cannot be tested in isolation. They’re an integral part of the overall EV drivetrain, and their performance must be measured with the dc input power from the battery, the traction drive inverter, and the load (Figure 1).

For example, motor efficiency can be affected by changes in battery performance as the battery pack discharges and ages.

Figure 1. The dc input (battery), inverter, motor, and load must all be tested when measuring EV motor efficiency. (Image: Yokogawa)

EV motor testing requires a data acquisition (DAQ) system that can handle high voltages like 900 Vdc and higher, placing new demands on the test system. Instead of conventional voltage probes, for example, embedded sensors are better suited for testing EV motors and drivetrains.

The DAQ systems used for EV motor testing need high bandwidths. Conventional DAQ systems have maximum sample rates of about 100 kS/s. While 100 kS/s is well-suited for low-frequency power measurements like 60 Hz ac, much higher sample rates are needed to test EV motors and traction inverters that can operate at 20 kHz and higher frequencies.

In addition to operating at higher frequencies, the frequency is not fixed and varies widely during operation. This makes it challenging to measure power values using a conventional phase-locked-loop (PLL) based measurement system. PLLs can have settling times of several seconds. That’s unsuitable for measuring power values where the frequency constantly varies, like the output of an EV traction-drive inverter under dynamic loading conditions.

The correlation of measurements across various systems can also be an important factor when measuring EV motor efficiency. For example, EV motor efficiency must be measured across changing environmental or operating conditions, and the conditions must be correlated with the corresponding performance data to get a complete and accurate picture.

Examples of EV motor operating conditions that must be tested include:

Constant load testing for performance while cruising
Step load testing to determine the motor’s load capacity under varying conditions
Dynamic load testing, where the load is changed continuously to simulate accelerations and decelerations
Peak load testing to determine the motor’s ability to handle brief bursts of power, sometimes slightly more than its rated capacity
Endurance testing to simulate continuous high-power delivery needed for long-distance cruising.
Regenerative load testing for evaluating the performance of the regenerative braking mode
Thermal load testing stresses the motor’s thermal management system to ensure long-term reliability

HIL and EV drivetrain testing

Full-size EV drivetrain testing platforms must be better suited for use in the design phase. They’re large and challenging to reconfigure. Smaller-scale modular hardware-in-the-loop (HIL) platforms can address the needs of EV drivetrain developers.

HIL can be used for a range of testing needs, such as implementing the Urban Dynamometer Driving Schedule UDDS drive cycle and the Highway Fuel Economy Test (HWFET) from the US Environmental Protection Agency.

Signal-level, power-level, and mechanical testing can all be implemented using HIL (Figure 2):

Signal-level testing (a) of the device under test (DUT) requires injecting sensor signals from a simulated environment into the DUT and having the HIL platform read the data returned from the DUT.
Power-level testing (b) adds a layer of power electronics and an electronic load on top of the signal-level environment. The DUT process controller receives the same sensor signals from a simulated environment. Still, in this case, it controls a power electronics block (the traction inverter) complete with a feedback loop and an electronic load that can change the loading conditions for various testing requirements.
Mechanical-level testing (c) builds on the power-level HIL testing and replaces the electronic load with electric machines to simulate traction motors. The machines must be scaled to support meaningful simulation of the final full-sized system.

Figure 2. HIL can be used for signal, power, and mechanical-level testing of EV platforms. (Image: MDPI vehicles)

References

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Figure 1, Yokogawa, top of page
Figure 2, MDPI vehicles, Page 2, Figure 1

HIL and EV drivetrain testing

References

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