Introduction
In the examples to follow, we’ll show how to use the Teledyne LeCroy MDA810 Motor Drive Analyzer to measure dynamic AC input power, harmonics and distortion for a 240V three-phase variable frequency motor drive.
AC Signal Acquisition
Figure 1 shows a full 20-second capture of the three-phase 240VAC input voltage and current signals to the motor drive. For this acquisition, the MDA sampled the input at 5 MS/s using 100 MS of acquisition memory.
Figure 2 shows the same acquisition but zoomed in on only a couple of cycles of the input voltage and current signals. The AC input voltage signals are probed line-neutral. Note that the current signals show a non-linear behavior typical of a cascaded H-bridge variable frequency motor drive.
Setting up the Motor Drive Analyzer for Measurements
The MDA permits assignment of the three-phase voltage and current signals in the AC Input tabbed dialog so as to calculate three-phase voltage, current and power values. In Figure 3, we have assigned the six input channels to Va, Vb, Vc, Ia, Ib, and Ic per the wiring diagram and have selected the Harmonic Filter Fundamental + N (with N set to 50). Note that the calculations are being made during the period defined by VR-N (Channel 1, or C1), as shown by the selection of that signal as the Sync.
In the Numerics tabbed dialog, we have selected the three-phase sources and the various measurements for display as shown in Figure 4.
Displaying Per-cycle Quantities Over Time
Figure 5 shows the three-phase electrical quantities displayed in the Numerics table, as selected in Figure 4. The values displayed in this table are mean values for 1000 unique periods. By highlighting a cell in the Numerics table, the instrument displays a per-cycle Waveform of that value versus time. This fosters understanding of the dynamic behavior of Voltage and Current THD (blue and green traces, upper right), the RMS Voltage (pink trace, lower right) and power factor (orange trace, lower right).
We can see that the power factor drops precipitously during the load release while the input voltage remains relatively stable. Additionally, voltage THD is relatively low and stable, but current THD is high (as expected) and changes substantially with loading.
Using Zoom+Gate to Assess the Locations of Worst Harmonic Distortion
We can understand the contribution of the various harmonic orders to the THD using the Harmonics Calculations varying frequency method as shown in the Harmonics Calc tabbed dialog shown in Figure 6.
Having made the appropriate selections, we can display the Harmonics order table and Harmonics spectral displays for the various voltage and current signals (Figure 7). Note that according to the mean values from 1000 cyclic calculations in the full acquisition, the dominant harmonic components in the current signals are the 5th, 7th, 11th and 13th harmonics. We also see this in the spectral waveforms.
The varying frequency method uses a cycle-by-cycle DFT calculation of Harmonics by order and by individual cycle. Therefore, we can apply a Zoom+Gate to zoom the acquired voltage and current signals and gate the Harmonics order table calculations and spectral calculations to specific portions of the input signals. Figure 8 shows such a Zoom+Gate applied to the beginning of the acquisition, where current THD was the highest.
In Figure 8 we can see that the B and C phases add a significant 3rd and 9th harmonic component whereas the A phase does not.
Conclusion
Variable frequency motor drives are highly non-linear loads and create significant distortion on the input to the drive that changes with applied load. These dynamic behaviors are easily measured with the Teledyne LeCroy MDA. Thus, the motor drive can be quickly qualified and optimized to maximize performance and minimize distortion under a variety of different operating conditions.
