Introduction to Measuring Electric Power During Transients

Transient testing is very important in the engineering of vehicles and can provide results superior to steady-state power measurements. However, its measurement has many challenges. This article will discuss transient testing, its importance, and the challenges it involves, as well as outlining the HBK value for transient power testing and its systems that are ideal for obtaining accurate measurements.

Drive Cycle Tests – Understanding the Energy Use of a Vehicle

One of the current big topics in electric vehicles is range and how far they can go on a given battery charge. In order to determine how far a vehicle can go, the EPA and governing bodies around the world have determined drive cycles. These are determined considering how an average driver might act in different circumstances. See Figure 1 for an example of a variety of different drive cycles.

Variety of different drive cycles truncated to 10 minutes.

Figure 1. Variety of different drive cycles truncated to 10 minutes. Image Credit: HBK

Essentially, a drive cycle is a speed profile against time with given torques to provide acceleration rates. What is found is that the losses while accelerating or decelerating are different from measuring each of these points at a steady state. In order to find out how far a vehicle can go, power during a transient needs to be measured accurately.

These drive cycles are used to give fuel economy numbers when customers buy vehicles. Getting that certification is important to the vehicle manufacturers, partly for marketing purposes as this type of data can be used to benchmark competitor vehicles. These types of tests can be used to determine how far competitors can go.

The tests are also very useful in an R&D setting because different factors can be looked at and how they need to be changed determined. For example, say an examination of acceleration shows that a lot of energy is being lost there. Engineers and R&D sectors can then look at the power during that transient and improve it for a given load change or given transit.

In a lot of settings, people run the whole test to test it in its entirety, or at other times they will run small portions so that they can optimize.

The important thing is that there is a constantly changing frequency and a constantly changing load, that makes measurement difficult and makes power losses different than they would be if they were steady-state measurements. However it is essential to measure transient power to understand how far a manufacturer’s vehicles are going to go.

Dyno and Vehicle Level Testing

These tests can be carried out at the dynamometer level. In this case, an electric motor and inverter will be put onto a dynamometer, and a load profile will be run with the motor in a lab. Alternatively, the whole vehicle can be taken, which helps to clarify how the motor interacts with the gearbox, the transmission and the axle in order to characterize the whole vehicle drive cycle.

Often people will also need to go in-vehicle for these tests. This is necessary to determine what real people actually do in the vehicles, and how individuals or groups of people drive differently. For example, how a 17-year-old may drive compared to an elderly person. The end goal is to characterize the final product and get certification for the farthest range possible.

To do this the motor and inverter calibration needs to be tuned. Motor inverter pairing needs to be optimized to achieve maximum range. However, this needs to be done whilst maintaining a good driving experience. It is important that the car does not sound terrible or jerk drivers around whilst starting or stopping.

These types of tests can also be used to compare supplier parts, which is also important for competitor benchmarking and knowing what competitors are doing.

Lastly, the tests are important for characterizing vehicle energy use. Measuring energy use using these dynamic tests is essential for understanding the product, the experience, and knowing how far the vehicle will go.

Challenges of Transient Power Testing

There are two reasons that stand out as to why this type of measurement is difficult.

For one, there is no steady-state frequency. No one ever gets into a car and travels at 60 miles an hour the whole time. Drivers speed up, slow down, start and stop, constantly changing speed and therefore constantly changing electrical frequency.

The graph in Figure 2 shows a single sinusoidal phase of current from an electric vehicle. This is a vehicle slowing down, with frequency over time decreasing until it comes to zero.

Dynamic current signal of an electric vehicle stopping.

Figure 2. Dynamic current signal of an electric vehicle stopping. Image Credit: HBK

Traditional power analyzers that are intended for the grid (around 50/60 Hertz), require a fixed frequency. Therefore, if the frequency is changing, these would provide a good measurement as they have a minimum hold time. This is a real problem when using a drive cycle that is constantly dynamic: how can power be measured and how can accurate results be obtained?

Looking at it from an instrumentation side, traditional data recorders were not intended to handle the high frequency that comes from inverters. Inverters are turning on and off thousands of times per second. Hence, if a traditional data recorder is recording at 2kHz, an incredible amount of information will be missed. This is really a difficult method of measurement.

Another issue is that these machines are constantly changing states. Hybrid vehicles and pure electric vehicles have transients. If the accelerator is pushed in a full-throttle, machines are going to be clutching in and out. These have their own related transients.

There are also going to be multiple machines in cases like hybrid vehicles that can have multiple machines in the transmission. This involves a lot of things interacting. Furthermore, temperature is going to influence all of this.

The simplest example is ice. When the wheel hits the ice and it speeds up very quickly, how does that affect power? These are interesting topics that need to be understood in order to tune the machine.

The Importance of Dynamic Power Measurement

What is shown in the Figure 3 graph is a machine with a load step on. While spinning at a given speed, torque was stepped and there was a load response. There is a big inrush current.

Top - current suddenly applied to an electric motor (maroon), cycle detect (black), RMS current (red). Bottom - Power, reactive power and apparent power for a dynamic load change.

Figure 3. Top - current suddenly applied to an electric motor (maroon), cycle detect (black), RMS current (red). Bottom - Power, reactive power and apparent power for a dynamic load change. Image Credit: HBK

The black line is a cycle detect. When measuring electrical power, it needs to be measured at a minimum on a half-cycle basis. Every half-cycle a power calculation is done, which again is unique from that phase lock loop. This results in that initial inrush current and this very large reactive power spike.

Reactive power is in purple and apparent power is in yellow. Real power is in black, real power being torque and speed. There is a big reactive power spike. These are actually very increased losses for that period of inrush. There is an increased loss just from starting the machine - this could be a clutch or a transmission clutching in, or acceleration from a stop, or going from one speed to another.

To know how far a vehicle is going to go and more importantly improve how far it will go, these numbers need to be understood and power spikes minimized. This is why it is so important to understand what is happening in transition states.

HBK Value for Transient Power Test

eDrive Value

HBM makes a product called a dynamic power analyzer or eDrive system that can make dynamic power measurements. It timelines all signals - torque, speed, CAN bus, temperature, voltage current and power measurement - knowing that all these things contribute to an understanding of transience. For example, knowing the response time for when the inverter applies current and when it is seen at the wheels can help save energy.

The power measurements made are very accurate, even in the dynamics and even as a fundamental frequency is changing. The measurements allow future-proofing of testing capabilities. Power measurements can easily be added to the HBM system. If a customer has a three-phase system today, and a six-phase system the next day, cards can simply be added to add power measurements.

The HBM tests are auditable. Every power measurement is given with all the voltage and current data behind it so that it is clear why a test has obtained the results it did. HBM also makes the equations public and editable. The customer has the data and the equations so that they know exactly why they got a given power measurement. If something goes wrong, tests can be reviewed without having to rerun it.

HBM simplifies the measurement chain because they make world-class amplifiers, world-class torque sensors, and a suite of software to help not only measure efficiency but improve it by knowing why a system is doing what it is doing. HBM provides transparency, data and accuracy.

Dynamic Testing with Cycle Detect

What makes HBK credible and different in this dynamic power world is how they measure power. Many traditional power analyzers for the grid used a phase lock loop system. HBM uses something called a cycle detect that measures the signal coming in from a current, such as the current shown in Figure 4. A digital signal processor resides in the hardware that filters the signal. By identifying successive zero crossings, the time period for measurement can be calculated.

Current (red) and cycle detect (black) for a single phase of a three phase system. This highlights the cycle detect identifying 1/2 cycles for calculation

Figure 4. Current (red) and cycle detect (black) for a single phase of a three phase system. This highlights the cycle detect identifying 1/2 cycles for calculation. Image Credit: HBK

All RMS power averaging is carried out during this time period. Every half cycle there is a power measurement. Multiple cycles can be averaged together, or it can be done half cycle by half cycle which is the most dynamic method. This is the best method in order to optimize the system, as it allows measurement of things at a changing fundamental frequency.

Figure 5 shows an example of an electric scooter vehicle starting. Current is shown in red, PWM voltage in blue, power in black, reactive power in pink, and apparent power in yellow.

Scooter acceleration from 0 speed showing a ramp from 0 to full power. Top - Three-phase currents (red) and cycle detect (black). Middle - Three-phase voltages (blue). Note back emf and PWM operation. Bottom - Apparent power (orange), reactive power (purple) and real power (black).

Figure 5. Scooter acceleration from 0 speed showing a ramp from 0 to full power. Top - Three-phase currents (red) and cycle detect (black). Middle - Three-phase voltages (blue). Note back emf and PWM operation. Bottom - Apparent power (orange), reactive power (purple) and real power (black). Image Credit: HBK

The beginning portion is at zero speed. At this point current is applied and so fundamental frequency is ramping up in both amplitude and frequency. As the machine accelerates the cycle detect, the black square wave, is at one cycle at a time, taking each one of those measurements. The machine tracks the frequency all the way up, averaging the harmless voltages and currents and, more importantly, calculating power.

At the start an inrush of regenerative power can be seen due to the fact that the vehicle was a scooter and needed kicking. This causes an inrush of reactive power as energy goes back into the battery. Power is not a straight line as it reacts with stimuli from the environment such as hills and wind.

When the machine hits its speed limit or current limit, whichever it was limiting itself on, there is a small spike in power that can be seen in the current. This is an additional loss that did not need to happen – energy could have been saved and range extended.

This is just one small instance of tracking frequency and getting accurate measurements in the dynamics.

Energy Used During a Drive Cycle

Figure 6 shows a full test example of a short drive cycle where dynamic power is being measured. RMS voltage and RMS current are in the red and blue respectively, and the fundamental frequency is shown in black. Power is in black in the middle graph and the energy used is in red at the bottom.

Cycle averaged signals and energy usage for an example vehicle drive cycle.

Figure 6. Cycle averaged signals and energy usage for an example vehicle drive cycle. Image Credit: HBK

Periods of the test where there are starts and stops can be analyzed. Other periods where the scooter is becoming airborne and getting a power spike can be identified. By tracking that frequency and giving good power measurements the whole time, the energy used value can finally be obtained.

The engineering process can then begin and places at which losses were too high can be determined. This information can be taken to R&D. This is all enabled by having that cycle detect, a frequency tracking algorithm that is measuring power dynamically. This can be used to solve the problems of how to use less energy and how to go farther.

Conclusion

In conclusion, the HBM eDrive system is an effective way of measuring in-vehicle power flow. It can be used to measure vehicle energy on a dyno, a chassis dyno or in-vehicle. Cycle detect allows for accurate dynamic power measurement, and then raw data ensures that calculation of power and energy are correct.

For more information on their products, please visit the HBK webpage. The details of these topics will be touched on in the second part of this series.

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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