A Quick Introduction to Transient Power Testing

Dynamic power testing is a complex process. Test equipment must cover a broad scope of critical tasks, including durability testing, mobile power testing, vehicle level testing, and transient power testing. Each of these test and development processes poses unique challenges. There is no steady-state frequency for any vehicle, from scooters to passenger cars, which is one such challenge. It would be incredibly rare for a driver to get in a car and travel at a fixed speed for their entire journey. Vehicle speed is constantly changing, increasing, or decreasing. Understanding the transient response of critical parameters is at the heart of dynamic power testing. It is also ever-more crucial as electric vehicles become more commonplace on our roads.

There are some issues with the way traditional instruments handle tracking frequency. Most traditional instruments were originally developed for the grid, so they use a DC power supply with a phase lock loop system and require a fixed electronic load. Using them in a vehicle where speed and electrical frequencies are constantly changing creates some real challenges in obtaining accurate, reliable measurements. This can impact an array of key performance metrics, including vehicle duty cycles.

There are constantly changing states in these electric machines, specifically for road testing. Think of vehicles accelerating from a stoplight and hybrids clutching in and out. Transients can occur, such as hitting things or wheels slipping on ice. There are a variety of issues that can come up.

Transient testing allows users to track machines as they slow down over time, which is difficult with traditional equipment. Conventional test benches may be incapable of measuring fast transient events, requiring careful consideration and control of load steps, slew rates, recovery time, rise time, etc.

Tracking a machine as it slows down over time is difficult with traditional equipment. HBK combats this issue by using a cycle detect system. Digital algorithms that track zero crossings are used to determine processes for making power calculations. This allows for dynamic testing, specifically transient power testing.

How to Carry Out Transient Power Testing

Electric Scooter Mobile Power Test

Electric scooter mobile power test.

Figure 1. Electric scooter mobile power test. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

This example of a transient power test was done on an electric scooter. The three-phase voltage cables were pulled out and mounted onto an HBK Genesis test system onboard and hooked up to a 12-volt battery. It was then connected to a computer for data acquisition and visualization. This set-up is shown in Figure 1. The scooter was then ridden around a track, a track with a hill in it, and data was taken live.

Visualize Real-Time Data in a Variety of Ways

Visualization of real time data from the scooter.

Figure 2. Visualization of real time data from the scooter. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

There are some interesting things that this transient power testing system, specifically the software, enables while acquiring data. The data can be visualized in a variety of ways. A random sampling of things showing some raw traces can be seen in Figure 2. The traces are high-speed BWM raw signals. This is what the signals look like at the high sample rate.

Some RMS averages are displayed, which gives the opportunity to look at a steady-state value or an average value of the signals to see the changes happening live. It is hard to see the change between one current peak and another, but the RMS is a lot more prominent, so watching the RMS in-vehicle can be a potent tool.

Digital readouts of that RMS current and RMS voltage power give drivers feedback on what points they have achieved. A driver or passenger can then throw a trigger on any value after hitting a specific voltage or current that they want, which is vital for reliable transient power testing.

As shown in Figure 2, in the real-time display that one might have in a vehicle or during a drive cycle, ID and IQ are calculated live. These are shown in a time trace and as an X, Y plot. There are a lot of ways to visualize the data, and any one of these points can be triggered. 

A live heat map of triggers

Figure 3. A live heat map of triggers. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

These triggers can be thrown to plot out a live efficiency heat map, as shown in Figure 3, and reverse engineer something to understand what the full efficiency is and then view all the signals to have a good understanding of what our vehicle is doing.

Dynamic Power Testing with Cycle Detect

Dynamic testing of a drive cycle with a cycle detect during acceleration and regeneration

Figure 4. Dynamic testing of a drive cycle with a cycle detect during acceleration and regeneration. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

Drive cycles and in-vehicle testing require dynamic power measurements. Dynamic power has much importance.

The first graph in Figure 4 shows the scooter accelerating from zero to a certain speed. Current is shown in red, voltage in blue, and then power, reactive power, and apparent power in black, pink, and yellow, respectively. The kickback needed to start the scooter can be seen. The scooter's inverter then activates, and the PWM kicks on. There is a little in-rush, and then as that frequency increases and the scooter speeds, the bands can be seen getting tighter. There is a little bit of overshoot and then a more steady state.

Throughout the recording, the cycle detect in black can be seen tracking that current all the way. The cycle detect allows measurement of signals as frequency changes. As the current value gets close to zero, the frequency increases. Then there are power values that are steadily increasing with vehicle power. There is a little in-rush of current where the scooter was kicked, as power was being put into the system. As that power is increasing, there is windage from the hills that are causing real-world oscillation in power as it is increasing. After this, there is an overshoot from the extra power. In these dynamic scenarios, more power is being used up than would normally be used.

This is a valuable tool for understanding how far a vehicle is going to go with an actual driver, an actual powertrain and actual road conditions. Dynamic testing lets users characterize real-world scenarios. It gives an accurate estimate for the range of the vehicle and allows optimization of the powertrain to perform better on EPA-driven or governing body-driven transient tests.

The second graph in Figure 4 shows deceleration and going into regeneration, which is a particularly fascinating state. The current can be seen reducing in frequency, and the PWM decreases in frequency. As speed decreases, power rolls off.

Regeneration affects driver experience and creates additional losses with poor management. Real power, in black, becomes negative as power is put back into the system. At a certain point, whilst slowing down, that power becomes positive again. Energy is being wasted operating regeneration. This consumption of energy during regeneration has to do with the losses in the machine due to heat being more than the amount of power being put back into the battery.

This is not only to do with experience but also to do with range and how power flow is managed. There is a lot of very valuable engineering information that can be gained from taking the raw data on a drive cycle and translating it back to how a vehicle can be improved for a given operating state.

Jump and Land

Test showing changes at different stages of a jump carried out with the scooter.

Figure 5. Test showing changes at different stages of a jump carried out with the scooter. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

Figure 5 shows where a jump was carried out with the scooter. It is operating in a steady state and then with the jump, the frequency speeds up rapidly because there is now no load. It is trying to hold a torque, but it is just open, with wheels spinning. Then the scooter lands, and there is a big inrush of current, which is a traumatic state for the system. Speed increases and there is a decrease in power, and then the inertia of the actual wheel enters regeneration. After that, there is a big jump in power again. This is a perfect example of transient power testing.

These are not necessarily things that we would account for in a drive cycle test. However, it is an important thing to examine when looking at vehicle operation – how do systems handle unexpected disturbances such as the wheel becoming airborne or the vehicle hitting ice and slipping?

Understanding these kinds of cases can add to vehicle experience and operation, helping to prevent many issues downstream and in the life of the vehicle. This is very valuable information for catching all the weird things that could happen and for identifying how to keep currents from going over a certain threshold to maintain the life of the system, prevent overheating and demagnetization, and avoid breaking the gearbox with quick torque responses.

Energy Used During a Drive Cycle

A drive cycle to determine energy usage.

Figure 6. A drive cycle to determine energy usage. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

Drive cycles can be performed on a dyno, a chassis dyno and vehicle runs. Figure 6 shows a lap around a circuit that emulates a drive cycle where there are accelerations and decelerations. Voltage (blue) and current (red) spike, fall and come down to zero. Vehicle speed or vehicle frequency (black) shows that the scooter is accelerating, decelerating, at zero, kicking, and so on. The vehicle path can be followed, giving an understanding of which things, like the jump, are happening where.

By dynamic power tracking and using a cycle detect to follow the fundamental, it can be seen when the system enters regeneration, when there are hard accelerations, and when power is being wasted. This can all be translated in real time to energy. This allows users to follow energy used over time, which is important because energy usage translates to range. The lesser the amount of energy that needs to be used for the same distance, the more range, and the more efficient the vehicle is.

By understanding how a certain power spike is influencing the energy used, the system overdrive cycle can be optimized by doing things like handling regeneration more effectively.

Dynamic Efficiency Testing on a Test Stand

Drive Cycle Testing

These drive cycles depicting real-world scenarios can be taken and replayed on a dynamometer. You can take the equipment from the vehicle, bring it to a test stand, and then replicate the torque-speed combination.

Dynamic efficiency testing of the scooter on a test stand.

Figure 7. Dynamic efficiency testing of the scooter on a test stand. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

Figure 7 shows a torque pattern (red) and a speed pattern (blue) for the World Harmonized Light Vehicles Test (WLTC). To get given speeds for this drive cycle, a certain torque needs to be applied. In this 30-minute test, as well as torque and speed motor loss were also measured, which showed that the losses from the machine came from different elements. Winding temperatures. Were also measured and lastly, some real time estimations were carried out to get magnet temperature.

These are all things that could be done in-vehicle on a chassis dyno, in-vehicle on the road, or on a dyno for R&D purposes. Transient power testing helps you to understand where your losses are highest so you can start to minimize them.

Dynamic Controls Work

3-Phase Scooter – Jump and Land

A lot of the time, especially when conducting transient power testing, it is important to understand how a controller handles the situation. Does the controller behave acceptably during transients? Getting an understanding of what the machine controller may be doing is useful for calibration and for reverse engineering or benchmarking. Understanding calibration is important for customer experience.

A space vector plane plotting of the jump scenario

Figure 8. A space vector plane plotting of the jump scenario. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)

The jump scenario was plotted in the space vector plane shown in Figure 8 to form an idea of how the controller was handling this given transient test.

The first graph shows that there is a high fundamental frequency of low torque and that then a lot of current and torque is applied when the vehicle lands. Flux in the magnetization can then be seen.

The brown area in the initial graph is then plotted in a stationary frame DQ0. The system goes from operating in the center, to the current applying itself and gaining traction with the machine, eventually giving rise to the circular pattern. This information has a lot of power in terms of tuning and understanding how to optimize the controller.

There is a lot of interesting information that can be obtained from transient power testing, and HBK provides some great tools to do these types of analysis. All these tests were run using HBM power analyzers. If you would like to learn more, please refer to the e-Drive power analyzer product page on the HBK site. Or contact a member of their team today.

This information has been sourced, reviewed and adapted from materials provided by Hottinger Baldwin Messtechnik GmbH (HBM).

For more information on this source, please visit Hottinger Baldwin Messtechnik GmbH (HBM).

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