Introduction
The following measurement example demonstrates use of the Teledyne LeCroy Motor Drive Analyzer (MDA) to characterize a vehicular propulsion motor. The characterization in this case uses analog torque load cells and analog tachometers to measure drive output to motor-shaft mechanical efficiency. It also measures the motor drive’s output harmonics and total harmonic distortion (THD)
Electrical/Mechanical Acquisitions and Calculations
The left side of Figure 1 shows the drive output line-line voltage and line-current waveforms. We performed these acquisitions using the two-wattmeter method. Note that the green (C4) line current trace appears to be noisier than the blue (C3) line current trace; this is due to the use of different current sensors with one being noisier than the other is.
The top right of Figure 1 shows the analog torque load-cell signal. Trace C5 is the actual output signal, while the associated zoom trace (Z5) provides a far more detailed look. The zoom betrays high-frequency ripple in the torque. A zoom trace of C6, the analog tachometer output, in all likelihood would show a corresponding ripple.
Among the MDA’s powerful analysis capabilities is its ability to plot measurement results vs. time. In this fashion, the instrument makes it easy to spot trends in data. At lower right in Figure 1 are per-cycle waveforms of speed and efficiency, both of which derive from the mean per-cycle values of the analog load-cell and tachometer signals. The figure shows the per-cycle plot of speed over time. A similar plot of torque over time would have strong correlation with the speed plot.
Moreover, the per-cycle waveforms make it a simple matter to correlate power behaviors with other signals. For example, Figure 2 shows the per-cycle speed waveform scaled to ±1 RPM peak-to-peak. At this scaling, the waveform’s periodicity stands out. By applying cursors, one might measure the frequency if desired. At bottom right, we overlay the per-cycle efficiency waveform plot from Figure 1 (in pink) onto a per-cycle torque plot to highlight the strong correlation between torque and efficiency.
Likewise, it’s easy to correlate the per-cycle speed waveform with the analog tachometer signal (Figure 3). In this instance, the MDA’s Zoom + Gate feature comes in handy. Zoom + Gate adds measurement gating to a zoom trace so that the measurements reflect only the zoomed part of the full waveform. In Figure 3, C6 is the analog tachometer output and Z6 is a zoom trace of C6. After applying a low-pass filter to Z6 (F11) to suppress high-frequency noise, we clearly see that the tachometer signal has a nearly identical shape to that of the per-cycle speed waveform at bottom right. Further, we can see that the ripple in the current yields ripple in the speed.
Another useful aspect of analyzing vehicle propulsion motors is measurement and display of per-cycle total harmonic distortion (THD) waveforms (Figure 4). The MDA can filter all but the fundamental, the fundamental + N harmonics, or a range of harmonics (it can also filter for a single harmonic). Setting the harmonic filter to either fundamental + N or a range of harmonics returns an average value for the full acquisition or a zoom trace from within it. Clicking on that result opens a plot of THD vs. time.
Comparison to Other Analysis Methodologies
One could undertake analysis of a vehicular propulsion motor and its associated drive circuitry in other ways, namely using a traditional power analyzer or a four-channel, 8-bit oscilloscope. The power analyzer performs the static analysis but not the dynamic analysis, nor does it provide the MDA’s Zoom + Gate facility to isolate a section of interest within an acquisition for analysis. The power analyzer also cannot compile statistics, display per-cycle waveforms, or take long acquisitions. It is able, however, to characterize the motor and drive’s input/output efficiency with 0.25% accuracy, although such accuracy generally is not necessary in this application. The MDA offers speed and torque analysis and THD measurement, while the power analyzer has very limited capabilities in both respects.
Meanwhile, the oscilloscope is only able to acquire and display voltage signals and has limited calculation capabilities in this application. The MDA, on the other hand, delivers powerful analysis capabilities in its ability to plot measurement results vs. time. In this fashion, the instrument makes it easy to spot trends in data.
Conclusion
The Motor Drive Analyzer provides a far more insightful view into the performance of vehicular propulsion motors and their drive circuitry. The ability to plot trends over time for attributes such as torque and speed can be an invaluable tool in troubleshooting such systems. When coupled with the MDA’s long acquisition memory, that facility enables users to leap far beyond static mean measurements into the domain of full dynamic analysis
