Introduction
The following measurement examples demonstrate use of the Teledyne LeCroy Motor Drive Analyzer (MDA) to characterize a 480-V, three-phase, 1.5-hp AC induction motor drive. Examples will include measurement of drive input and output performance and efficiency when measured for a short time period under no-load and under loaded conditions, as well as dynamic analysis of the drive from a no-load startup to a steady-state load. The latter example shows how the MDA leverages its long acquisition time to gain a full understanding of the motor drive’s performance characteristics.
Input-Output Performance Analysis with No Load
To begin with, Figure 1 shows acquisition waveforms of the drive’s input and output signals.
The four waveforms on the upper left side of the screen capture are the AC inputs to the motor drive (acquired using the two-wattmeter method). The setup summary at bottom left shows that the input is configured for three-phase, three-wire, 2V2A. Channels 1 and 2 (yellow and magenta, respectively) are the input voltages, while channels 5 and 6 (white and lavender, respectively) are the input currents.
Shown at top right are the acquired waveforms present at the motor drive’s output. We have defined the waveforms in the setup summary at bottom left using the two-wattmeter method. Thus, the MDA's eight analog input channels afford viewing of inputs and outputs of a three-phase system.
In this example, the acquisition is relatively short as shown by the Timebase descriptor box at bottom right. The acquisition spans 500 ms and comprises 2.5 Mpoints. From a cursory glance, it appears to be a steady-state load. Below the waveform display grids is a numeric table of mean values (see inset above). The input voltage is 480 V and output voltage is 120 V, which is substantially less because the drive is running at a low speed with no load. From the currents, we can calculate the real, apparent, and reactive power; power factor; phase angle; and efficiency. The latter is very low, which one would expect with no load on the drive.
Plotting of Efficiency vs. Time
Among the MDA’s powerful analysis capabilities is its ability to plot measurement results vs. time, time-correlated to the original acquisition data. In this fashion, the instrument makes it easy to spot dynamic changes in data and correlate them to other acquired signals. In Figure 2, the AC input signals appear at top left. Atop the right-side display grids, note the selection of the Efficiency tab to display the calculated sync signals derived from Channel 1. These sync signals indicate the period of time over which the various power values are measured, and these periods are different between the AC input and drive output. Shown in Figure 2 are two color-coded grids that show the power period for the AC input sync signal (center right) and drive output sync signal (bottom right). Viewing the sync signals serves to verify the power periods for input and output.
Clicking on the red-shaded box in the Mean Value Numerics table (Figure 2, bottom) opens a per-cycle efficiency vs. time waveform (Figure 2, top right). Efficiency is on the vertical axis while time is on the horizontal axis. The time on this plot correlates to the time in the Channel 1 voltage trace.
The plot appears as a stair step-shaped waveform because it reflects the two differing power periods in the two sync signals. Per-cycle efficiency vs. time derives from the ratio between the two measurements. The plot evidences the differences in the periods as can be seen in the differing widths of the steps in the waveform. With cursors, we measure the variation in efficiency between peaks and valleys. In the example of Figure 2, the yellow descriptor box at lower right tells us that efficiency peaks at about 36.3% and diminishes to 32.3%. Thus, a signal that appears to be more or less steady state in nature does in fact display some variation in efficiency.
Leveraging the Power of Complete Statistics vs. Simple Mean Values
Unlike traditional power analyzers that perform only static mean measurements, the MDA also provides full statistical breakouts of measurements and efficiency values (Figure 3). From the statistics, we note that the MDA made 50 efficiency calculations on the displayed per-cycle vs. time waveform, based on 28 AC input periods and 22 drive output periods. Note that the mean efficiency value in the Numerics table matches the mean efficiency value in the full statistics table.
Input-Output Performance Analysis with Constant Load
As we have observed, an AC induction motor drive’s efficiency under no-load conditions will be rather poor. However, the same drive under load behaves somewhat differently.
The drive now consumes substantially more current (Figure 4). Phase angle, power factor, and efficiency all improve, with the latter rising from about 36% to 73%. With the short timebase in use, this still qualifies as a static test with constant load, speed, and torque.
Input-Output Dynamic Performance Analysis from No-Load Startup to Steady-State Load
The above test scenarios-no load and steady-state load-are both static in nature. However, the MDA shines in dynamic performance analysis, thanks to its long acquisition memory.
Shown in Figure 5 is a 10-second acquisition set up as 1 s/div with 10 horizontal divisions. The two left-hand columns of display grids are the full 10-second acquisition waveforms of the AC input at far left, and 20X zoom traces of the areas highlighted in the full acquisition traces at left center. The two columns at right are the drive output waveforms, again with full acquisitions at right center and zoom traces at far right.
The output traces, and especially the zoom traces, show us the drive’s startup with no voltage on the output. The first 1-s division displays a very slow startup, coupled with some of what might be termed “inverter misfire.” The zoom traces help spot such waveform abnormalities.
We may highlight any part of the full acquisition for display in a zoom trace: beginning, end, or middle. Not only that, but the zoom ratio can be changed from, say, 5X instead of 20X. Doing so might help observe how the frequency increases as the motor begins to rotate. We can also keep an eye on the drive’s current draw during the period of rising frequency.
Alternatively, the zoom might show us the latter part of the acquisition when the drive is reaching full load capability and full speed (Figure 6). Here, we might narrow the zoom even further to 50X, or to whatever level needed to show pertinent waveform details.
A capability present in the MDA called Zoom + Gate takes this selectivity even further. Zoom + Gate combines a given zoom ratio and position with a “gating” of Numeric table calculations. Access this feature from within the touchscreen display’s MAUI user interface or from a front-panel button. With Zoom + Gate enabled, the Mean Value Numeric Table will include calculations from only the zoomed part of the full acquisition (Figure 7).
Zoom + Gate lets you isolate a desired section of the full acquisition to gain a fuller understanding of what is happening in that segment without the calculations being muddied by areas of non-interest.
As with static analysis, clicking on any item in the Mean Value Numerics Table generates a per-cycle vs. time waveform plot. Figure 8 shows the result of clicking on the mean efficiency value. The plot at top right depicts efficiency rising after startup, falling off, and then steadily increasing as the motor comes up to speed. The dip in efficiency is likely due to the effects of charging of the bus capacitor. Once charging completes, efficiency ramps up in near-linear lockstep with the load.
You may examine many other parameters in this fashion. At bottom right, Figure 8 also shows per-cycle vs. time plots of RMS voltage, current, and power for the drive input and output. For these and other parameters, the MDA810’s dynamic analysis capability reveals details of the drive’s performance unavailable with traditional power analyzers.
It is worth noting that the MDA’s built-in PC supports a UHD extended desktop, allowing you to see all this information on a larger monitor if desired. The MDA’s Q-Scape mode provides 4X the viewing area for enhanced understanding of details while viewing multiple waveforms at once.
Conclusion
The Motor Drive Analyzer allows you to more closely investigate the performance attributes of an AC induction motor drive than is possible with a traditional power analyzer or four-channel, 8-bit oscilloscope. All of the dynamic analysis capabilities described above are simply impossible to do with a power analyzer or oscilloscope. The traditional power analyzer provides “gold standard” accuracy for static analysis but is unable to perform the dynamic analysis of an MDA810, nor can it allow overlay of a sync signal to confirm accuracy of power calculations.
