In this example, we will show how to use the Teledyne LeCroy MDA810 Motor Drive Analyzer to analyze efficiencies of a small motor and drive from the power output of the DC battery to the motor shaft. The MDA measures speed and torque from available signals without requiring an expensive dynamometer setup, which makes it very useful to a design engineer assessing drive and motor performance at their bench.
Voltage and Current Acquisitions
Figure 1 shows a short (100-ms) capture of signals displayed with Teledyne LeCroy’s Q-Scape Mosaic multi-tab display mode. The named tabs contain the DC battery voltage and current output (top left), the drive output voltage and current outputs (top right), and the mechanical analog torque load cell output signals and the Quadrature Encoder Interface (QEI) A, B, and Z sensor signals (bottom left). Below the acquisition waveforms is the Numerics table that displays mean values of various DC, drive output, and mechanical motor values, including efficiencies. For this acquisition, the MDA is sampling at 25 MS/s using 2 MS of acquisition memory.
The Motor Drive Analysis summary tab shown in Figure 2 provides a summary of the acquired waveforms used in the DC Bus, Drive Output, and Mechanical setups.
QEI speed sensors are common in drive designs and directly interface to the MDA for speed and angle calculation. The MDA also interfaces many other types of analog and digital speed sensors. This example also shows use of a torque load cell, but the MDA derives torque data using a formulaic relationship to current and a known torque constant, or from data embedded in a Controlled Area Network (CAN) serial data signal. Thus, the MDA can calculate motor mechanical power without needing analog/digital tachometer and torque load cell sensor signals from a conventional dynamometer. Figure 3 shows the speed (left) and torque (right) sensor, serial data, and inferred calculation methods supported in the MDA.
Figure 4 shows a larger view of the Numerics table shown in Figure 1. The MDA calculates these values on a per-cycle basis, and all of the calculated values average for each measurement and each source. The “Bus” row is the DC battery, the “Σrst” row is the drive output, and the “Mechanical” row is the motor output. Note that there are two different efficiency measurement shown in the table – ηstage and ηtotal – that represent the stage-by-stage efficiency and the cumulative efficiency, respectively.
Displaying Per-cycle and Per-phase Power Behaviors
Figure 5 displays per-cycle Waveforms of various power, speed, angle, torque, and efficiency measurements. The Sync signal defines the measurement cycle for all of these Waveforms, and the Sync source signal is Channel 3 (as shown in the setup summary in Figure 3). These Waveforms display one measurement value per cycle with the vertical scale representing the measurement unit and the horizontal scale time-correlated to the other waveforms acquired in Figure 1. The three Power Waveforms shown in the top grid scale identically, as are the two Efficiency Waveforms in the bottom grid.
We could perform additional math on any of the waveforms above to understand their behaviors better.
In Figure 5, it appears that the torque is varying in a narrow band of 34 to 40 mN•m. We can change the Sync to a shorter period by selecting digital line D0 as the Sync source signal, and comparing this to the Torque Waveform calculated over the Sync period defined by C3, as shown in Figure 6. An FFT of the Torque Waveform displays in the bottom grid, and the frequency content of this torque signal (and the torque ripple) shows in the FFT.
The Teledyne LeCroy Motor Drive Analyzer contains a powerful set of algorithms that permit complete understanding of the static and dynamic operation of a motor drive. Additionally, it interfaces to a wide variety of analog and digital speed sensors built into motor-drive designs or motors. Or, it can infer torque and speed from other measured signals (e.g., torque from measured current or embedded in a CAN serial data signal, or speed from the period of a measured voltage or current). The instrument calculates mechanical power from speed and torque. While this example shows a short acquisition under steady-state operating conditions, we use the same principles with very long acquisitions during dynamic drive and motor operation.