Case Study
Characterization of a Traction Motor
This article describes how the Illinois Institute of Technology - technical partners of HBK - tested and analysed an electric for noise and vibration.
The test machine is a three-phase traction motor, unique in that it is a PM machine with a wound field excitation, so it has a variable field current for variable magnetization. The machine under test has eight poles. A point by point efficiency map with ramps at different loadings was run, for which the measured quantities were inverter voltage and current, torque and speed, vibration and noise.
Figure 1. Acoustic camera picture of a test motor.
Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)
The HBK gadget in Figure 1 is an acoustic camera. This is a camera with 36 microphones in a very particular array, and with it comes a video capture can produce a frequency plot. Using these plots, noise sources, their origins and their frequencies can be identified.
Electric Powertrain and NVH Testing
The test setup used was a device under test (DUT), measuring three-phase voltage, current and acceleration. These were synchronously and continuously recorded and brought into one common data acquisition system, the Genesis eDrive. Real-time display, plotting and measurement was carried out with HBK Perception software, enabling real-time feedback of voltage current frequency signals. Since all data is recorded and stored synchronously, the data can be exported to the BK Connect sound and vibration software for in depth post-process analysis.
This is a very smooth format for carrying out good analysis quickly with a small set of equipment –measurement equipment, different sensors, visualization software and a hardcore post-process software.
Figure 2. Electric powertrain and NVH testing.
Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)
Efficiency Mapping with Vibration
Efficiency Mapping
Efficiency mapping displays a series of static torque and speed points to measure efficiency. Many signals can be recorded: torque and speed, voltage and current, control and vibration. They can be used to monitor how control affects vibration, allowing avoidance of certain states and faster communication with other teams.
Figure 3. Recorded and calculated data points for a motor efficiency map.
Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)
Figure 2 shows a simple 12-point efficiency map with a fixed field current. Torque or speed values were increased, and efficiency values are taken. Triggered segments of measurement were taken, each one of these segments being half a second.
Triggers are thrown in, and measurements of voltage (blue) and current (red) are taken, too. This is also done for torque speeds and efficiencies. Vibrations are also measured – in this case, an RMS of vibration was noted, rather than raw real-time vibration values.
There are two accelerometers for which RMS vibration is being measured. The graph is a nice way to have all these signals displayed in one place, and what can be gained from it is that for a given speed torque combination, there is a specific level of vibration. Here, as torque increases, there is a strong presence of vibration at 2k RPM for a given field current. This might indicate to someone going through these efficiency maps that as engineering changes are made this could be a point to avoid.
It is very useful to get this instantaneous feedback and to be able to identify places where there could be resonances due to changes being made. It is a great way to monitor how controls and changes affect vibration.
Efficiency and Vibration Mapping
Figure 4. Efficiency and vibration heat maps for DUT.
Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)
Figure 3 shows a heat map plotted from all those efficiency points with speed (RPM) on the X-axis and then torque (Nm) on the Y. The contour shows the percentage efficiency. These maps give an idea of the optimal operation. Again, this example shows only 12 points. A very high-efficiency area of high torque and high speed can be seen, with an upward trend of efficiency with loading.
The same 12 points were then plotted for RMS vibration. This can show general trends in vibration and allow the identification of hot spots. Through this map, it can be seen that in the circled area there is a really high vibration area. By finding thresholds, this can indicate any issues that are going to be caused downstream.
The maps allow for analysis of system-level interactions, giving engineers the knowledge to increase communication and delivery time, by reducing back and forth between groups.
Ramps with Efficiency
Electrical Analysis of Motors and Drives During Sweeps
Figure 5. Ramp test showing state, efficiency, noise, vibration, and control variables during a ramp test. Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)
The next example is ramps tests. A ramp test is a common NVH test which involves setting a fixed torque and ramping speed. For example, say torque equals X, speed is then ramped from zero to a certain speed and back down to zero. This gives a nice frequency spectrum versus speed for noise and vibration signals. In Figure 4, voltage and current were measured and translated to control variables. In this plot, there is IQ in blue and ID in red, and these control variables are being monitored during the speed sweep for a fixed torque.
Efficiency can be measured with a dynamic measurement technique allowed by HBM equipment. Here, the field current value indicates the rotor state of magnetization. Speed, in green, is being ramped up, and torque, in pink, is fixed. This allows you to see how efficiency is changing dynamically.
Accelerometers and a microphone are examined in one place with a common measurement tool and a common visualization tool. Post-process analysis allows you to really understand how electrical state affects vibration.
Changes that affect efficiency can immediately be identified and easily communicated to be fixed. One graphic location, one common vocabulary set and one data set enables knowledge of the state of your vibration, the state of your torque, the state of your speed, and the state of your voltage current.
Spectral Analysis of the Inverter
Figure 6. Spectrum plots of acceleration, microphone and
current for full loading ramp tests from 0-3000RPM.
Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)
Spectral plots were carried out for further analysis. For the spectral plots in Figure 5, FFT frequency is on the X-axis and relative time on the Y. The bands show increased frequency presence with increased speed for voltage, current, acceleration and microphone.
This test took place in an acoustically noisy environment. When doing this acoustic analysis, you can see the inverter switching at 10 gigahertz. The voltage gives a fantastic signal for translating to acceleration or microphone because it does not have filtering - it is a pure signal that gives clear traces of switching noise.
The analysis provides information on the relationship between voltage, current, acceleration and noise, giving a full picture of all these things in one place.
When running this test, the assumption was that current and voltage had a direct correlation with acceleration. The brighter bands in voltage and current would result in stronger frequencies and would result in the same order bands in acceleration and microphone.
What was found for this particular machine was that the stronger current and voltage bands correlated with weaker acceleration bands, and then the weaker current bands correlated with stronger acceleration bands. This merits further analysis, as the reason behind it is not clear yet. It shows that assumptions cannot be made about correlations.
At some level, there is a transfer function between current and acceleration or between voltage and acceleration. Understanding these things is powerful, and having all this data in one place during these tests provides a way to identify where problems are coming from and possible solutions for them.
Efficiency from Ramps
Ramps efficiency can be used to plot efficiency dynamically. They require dynamic power measurement and high bandwidth torque. HBK measurement equipment involves some special tools that can measure dynamic efficiency very accurately as torque and speed increase.
Figure 7. Efficiency heat maps for DUT.
Image Credit: Hottinger Baldwin Messtechnik GmbH (HBM)
In this example, those measurements were taken from two different loadings of ramps tests - 13 Nm and 26 Nm. Efficiencies were plotted in the contour versus speed on the X-axis and torque on the Y.
The same upward trend is achieved - high efficiency in the upper right-hand corner. With the higher loading, the same trend happens, and it can be correlated nicely.
These efficiency maps can be used by NVH to identify how their changes to noise and vibration affect efficiency from one state to another. The cycle detect feature gives a really strong, close correlation on the point by point map. This requires a high dynamic power analyzer and a high bandwidth work sensor.
This information has been sourced, reviewed and adapted from materials provided by Hottinger Baldwin Messtechnik GmbH (HBM). HBM is an HBK company.
For more information on this source, please visit Hottinger Baldwin Messtechnik GmbH (HBM).