Introduction

In the examples to follow, we’ll show how to use the Teledyne LeCroy MDA810 Motor Drive Analyzer to measure the voltage, current, and power delivered to a motor used in a precision cylindrical grinding machine, and assess the phase balance under complex dynamic loading conditions.

Voltage and Current Acquisitions

Figure 1 shows a long (50-second) capture of three line-to-line voltages and three line currents at the output of a motor drive supplying the grinding machine motor. The three line-to-line voltage and line current pairs are displayed top to bottom. Below the voltage and current waveforms, we show the Numerics table of mean calculated per-cycle values for the complete acquisition, and below that, the Drive Output Wiring Configuration and setup dialog. For this acquisition, the MDA sampled at 1 MS/s using 50 MS of acquisition memory.

Figure 1:

Acquisition of three line-to-line voltages and three line currents, Numerics table, and setup dialog

The peak-to-peak voltage on each grid is ±800 V while the peak-to-peak current is ±80 A (as shown). Note that the wiring is in a three-phase, three-wire configuration and we perform a line-to-line to line-to-neutral conversion on the data to provide Numerics table per-phase calculations. We are also using the VR-S line-to-line voltage as the “Sync”, or cyclic, period for all power calculations. Also note that the Harmonic Filter is set to Fundamental, invoking the DFT-based per-cycle filter. Doing so ensures that the Numerics table values are based on the fundamental frequency component only of the drive output voltage and current signals, regardless of how much the frequency changes during the acquisition.

Calculating Per-cycle and Per-phase Power Quantities

Figure 2 shows a larger view of the Numerics table. It might seem odd that the three-phase reactive power (Q), power factor, and phase angle values are not calculated from an addition or average of the per-phase values. These values are calculated on a per-cycle basis, and all the cycles are averaged for each phase. In the presence of complex reactive power flows, the three-phase summation of Q as a mean value in the Numerics table (and the subsequent derivation of power factor and phase angle) may look misleading, but a closer examination of these per-cycle reactive power flows makes the behavior more clear.

Figure 2:

Larger view of the Numerics table shown in Figure 1

Displaying Per-cycle and Per-phase Power Behaviors

Figure 3 shows the same acquisition with the calculated per-cycle RMS line-neutral voltage and line current values plotted versus time as waveforms.

Figure 3:

Calculated line-neutral RMS per-cycle and per-phase voltages and current

The three per-phase RMS line-neutral voltage waveforms and three line current waveforms appear in the top and bottom right grids, respectively. Both are overlaid and identically scaled. We can see that the per-phase RMS voltages are nearly identical whereas the per-phase RMS currents show large differences during two of the heavy loading conditions.

In Figure 4, the waveform at top right shows the calculated per-cycle, per-phase φ values plotted versus time; while the bottom right grid shows the calculated per-cycle, per-phase reactive power (Q) values plotted versus time. The vertical center for each waveform is 0° or 0 VAR, respectively. These waveforms make it clear that there are very complex reactive power flows as the motor takes on loading.

Figure 4:

Calculated per-phase phase angle and reactive power waveforms

We then use the Zoom+Gate feature of the MDA to zoom to a single cycle of the drive output when the reactive power is negative (Figure 5, left image) and compare that to a single cycle of the drive output when the reactive power is positive (Figure 5, right image). In each case, the Numerics table calculations are restricted (gated) to this single period, and it is easy to understand derivations for all of the per-phase and three-phase power quantities in these cases. In each case, we can also observe a very different phase relationship between the line-to-line voltage and line-current waveform pairs.

Figure 5:

A single cycle when reactive power is negative (left) and a single cycle when reactive power is positive (right)

Conclusion

The Teledyne LeCroy Motor Drive Analyzer contains a powerful set of algorithms that permit complete understanding of the dynamic operation of a motor drive. It provides very long capture time, a rigorous set of per-cycle power analytics, a DFT-based per-cycle Harmonic Filter that can accurately isolate the fundamental frequency only for use in all calculations, and per-cycle Waveforms to visually show the dynamic voltage, current and power behaviors over time. Using Zoom+Gate to isolate measurements and view waveforms for a single power cycle makes it easy to correlate complex drive system behaviors to drive output signals.