Introduction
The following measurement examples demonstrate use of the Teledyne LeCroy Motor Drive Analyzer to test a small hand-held tool using a sine-modulated permanent-magnet synchronous motor operating at high speed. The tool operation involves reversal of the motor direction once per second. The measurements examine the correlation of drive control signals to tool behaviors and dynamic power behaviors, including the the power consumption during the transition inrotational direction and the difference in power consumption while rotating in each direction. The goal was to understand and mitigate undesirable power losses during these periods, which, if too high, could cause user discomfort and/or reliability issues.
Viewing Control and Speed Command/Feedback Signals
The example shown in Figure 1 shows five 12-bit, 1 GHz analog acquisition channels used to view two control signals (C1, yellow and C2, magenta), the encoder position of the rotor (C3, light blue), the actual velocity of the motor (C4, green), and the commanded velocity (C5, gray-green). An external controller board designed by the test engineer processes the C3 and C4 signals for input to the MDA810. Because this is a sensorless motor, the installed encoder is for test purposes only and will not be part of the final product.
The control signals C1 and C2 signal the motor to change rotational direction. The rising edge of C1 initiates the motor rotational reversal, while the falling edge of C2 represents the time when the motor reversal should be completed. Capturing data over a longer time period (5 seconds, in this example) enables many cycles of the transition to be viewed. Zoom traces (on the right side of the grid display) display details of one of these transitions to clearly show the timing of the control signals and the motor’s response. The speed change is also monitored at this transition point as shown in Z4 and Z5. From these signals, it can be seen that the motor rotational reversal is well-behaved and operating as expected.
Motor Drive Output Power Analysis
In this example, a two-wattmeter method is used to analyze the motor drive output and calculate three-phase power values. See Figure 2 for the probing configuration for the two-wattmeter method.
The two-wattmeter method permits three-phase system power to be measured with only four signals, leaving more signals available to acquire other drive control or power behaviors. Three wattmeter methods are also supported in the MDA810.
Two high-voltage differential probes (C1, the yellow trace, and C2, the magenta trace) and two current probes (C5, the green trace, and C6, the purple trace) connect to the motor drive output as described in the wiring configuration diagram in Figure 2. Figure 3 shows a single acquisition of the line-to-line voltage and line current waveforms. The line-to-line voltage waveforms and line current waveforms appear to be 120° out of phase, which is expected in a three-phase system. The waveforms on the right of Figure 3 are zooms of the waveforms on the left. The un-zoomed waveform appears to be noisy, but the zoom traces reveal that the appearance of the noise is really the switching characteristics of the devices in the drive output. This could not been seen using a traditional 8-bit oscilloscope, but the MDA810’s 12-bit acquisition system has the required resolution to make this observation possible.
Then, a longer acquisition is made, as shown in Figure 4, of the above to view the complete motor rotational direction change and calculate power values before, during and after the rotation change. Of interest is the amount of power consumed during the transition from one direction to another. The ideal situation is to not have a sharp increase in power at this transition point. This acquisition contains two transitions of the motor direction.
In order to determine the cyclic period over which all voltage, current, and power calculations should be made, a signal is chosen as the “reference period”. In the MDA810, this is referred to as the “sync” signal. The sync determines the measurement interval for computation of the per-cycle voltage, current, power, efficiency, mechanical, and other values. It is usually necessary to filter the sync signal to remove high-frequency content and obtain better periodicity, and it is simple to do this in the MDA810.
Figure 5 shows an example using the C1 line-to-line voltage as the sync signal with application of a 500-Hz low-pass filter. The sync signal in this figure was acquired at a timebase of 20ms/div to demonstrate a clear example of a sync signal. It does not directly correlate to the 200 ms/div acquisition in Figure 4. A colored overlay provides visual identification of the measurement periods. The sync signal is viewed to verify correct identification of the periods, which ensures correct power calculations. With the MDA810’s colored overlay view, it is easy to verify proper recognition of measurement periods.
Once it is verified that measurement periods are properly determined, the sync signal is usually turned off.
Figure 6 shows the same acquisition as Figure 5, but also including the control signal for rotational direction reversal and various calculated Numerics and Statistics table values and other waveforms. The acquired signals are the voltages (C1 and C2), currents (C5 and C6), and control (C4)
In this example, the values of greatest interest are the motor’s VRMS, IRMS, real power, apparent power, reactive power, power factor, and phase angle, and the mean values of these measurement parameters for the full acquisition are shown in the Numerics table, much like what a power analyzer would display. The P(Σrst) and S(Σrst) waveforms (overlaid on top of each other, in the bottom right grid) are per-cycle “synthesized” Waveforms that plot the per-cycle values (shown in the Statistics table) versus time, time-correlated to the original acquisition waveforms. They are created by touching or clicking a Numerics table cell value. These per-cycle Waveforms clearly indicate the dynamic power behaviors of the motor drive output and motor, something that would not be obvious by only viewing a Numerics table mean value. Viewing the real power and apparent power per-cycle waveforms at the directional transitions of the motor provides valuable insight into the power consumption during each direction change, which is important in this application because this motor is part of a hand-held tool, so power consumption needs to be minimized because high power loss can result in user discomfort.
To take a closer look at the area of interest during one of the transitions, we can use the MDA810’s powerful Zoom+Gate feature. The Zoom+Gate provides a simple means of zooming all input sources, detailed waveforms, and sync signals together, and positioning the zoom window on any portion of the trace. The common zoom window then acts as a measurement gate for the Numerics and Statistics tables. Figure 7 shows the result of a Zoom+Gate around the area of interest – the transition from one rotational direction to the other, which is one complete cyclic period as shown by the DrvOutSyncZ sync signal, the per-cycle Waveforms, and the Statistics table.
The power consumption during the rotational change is reasonable for the motor under test – 3.894 Watts.
One may further analyze the motor’s power consumption by determining the heat loss during operation. We can do so by setting up the MDA810 to measure the power values using the Harmonic Filter settings at both Full-Spectrum and Fundamental settings at the same time. After comparing these results, we can calculate the heat loss in the winding as the difference between the Full Spectrum and Fundamental calculated real power results. The Harmonic Filter setup defines the filter that will be applied to the input waveforms to perform the power calculations. It can be defined in both the AC Input and Drive Output.
For this example, the AC Input Harmonic Filter is set up for Full Spectrum and the Drive Output is set up for Fundamental only. The voltage and current inputs are set up to be the same channels in both setup dialogs. As a result, the only difference in the power calculations are the harmonics being measured. Figure 8 shows the same acquisition as in Figure 7, but with the Zoom+Gate set for one motor rotational operating cycle. The Σabc parameters in the Numerics table represent the full-spectrum power values, while the Σrst parameters represent the fundamental only. Using cursors, we can measure the time of the motor operating cycle and then calculate Joules from the power parameters. The formula to convert watts to Joules is as follows:
$$E_{(J)} = P_{(W)} * t_{(s)}$$
The power value shown in the Numerics table for Σabc is 3.668 W. The length of time measured using cursors is 907.68 ms. This value may be found as the ΔX value located in the bottom right corner of Figure 8. Converting to Joules using the formula, the energy consumed for Σabc is 3.33 Joules. Use the same method to with Σrst, we calculate the energy consumed as 2.51 Joules. By subtracting these numbers, we arrive at 0.82 Joules. The 0.82 Joules value represents the heat loss in the winding for this tool during the time it rotates in one direction before switching directions.
Let’s look at an example where the power plots indicate some issue with the motor operation. In this next example, we test the same type of motor with the same two-wattmeter wiring configuration. The display also shows motor position (C3: encoder position, C7: sensorless position), control (C4), speed (C8), and power (P(Σrst) and S(Σrst)) signals (Figure 9). Note that in the clockwise rotation the per-cycle P(Σrst) and S(Σrst) power waveforms are smooth and consistent. In the counterclockwise rotation, however, there is an oscillation in the signal. This can be observed in the power waveforms in the upper right corner of Figure 9.
After observing this vibration, further investigation discovered an issue with the commutation of the motor. This issue was in turn traced back to a sensorless control issue. The power waveforms, after correcting the sensorless control issue, appears in Figure 10.
Conclusion
The ability to view motor drive output waveforms and measure a motor’s dynamic operating characteristics, including dynamic power values, and correlate the dynamic behaviors to the control system activity can provide valuable insight into the overall performance of a motor drive system. Using a powerful tool with dynamic measurement capability, like the MDA810, brings a deeper understanding of what transpires during these events when compared to the static measurement capability of a power analyzer. The ability of the MDA810 to perform these dynamic power measurements along with complete embedded control testing with correlation of control signals to power events provides unparalleled debugging and analysis for complete motor testing.
