NI CompactRIO and LabVIEW FPGA Measure Magnetization Characteristics of the SR Machine

Author(s):
Keunsoo Ha - Virginia Tech University

Industry:
University/Education, Machines/Mechanics

Products:
CompactRIO, PXI/CompactPCI, LabVIEW FPGA Module, LabVIEW

The Challenge:
Developing an algorithm to measure and eliminate the eddy current losses and resistance changes due to heating and measurement of the magnetization characteristics of the switched reluctance (SR) motor.

The Solution:
Using National Instruments LabVIEW 7.1, the LabVIEW FPGA Module, PXI-7831R, several CompactRIO modules, and the interface circuit for sensing the phase current and attenuating the phase voltage to provide an automatic sensor calibration; an automatic, user-friendly experimental environment; and a graphical output of the flux-linkage curve for viewing and storage.

A Double Salient Machine with Highly Nonlinear Characteristics
The design and performance evaluation and high-performance control of the SR machine requires detailed knowledge of the flux-linkage characteristics of the stator windings for the different rotor positions at different current excitation currents. Even though we could understand the operation of the SR qualitatively, we had difficulty modeling and simulating it by analytical methods due to the magnetic nonlinearity. The double saliency of the machine and its methods of the excitation produce current, torque, and flux-linkage waveforms, and all of them are highly nonsinusoidal in both space and time. Generally, we could calculate the magnetization curves without experimental measurement of the machine waveforms by using finite element methods. However, the accuracy of such methods varies according to the choice of the elements used. Not only are the elements computationally complex, but modeling such effects as manufacturing irregularities, 3D fields, and nonuniform iron structures is also difficult. Therefore, we had to develop the experimental environment for measuring the flux-linkage characteristics of the SR machine and an accurate knowledge of the machine magnetic flux-linkage characteristics to predict the machine performance for research, design, and education purposes.

Instrument Integration to Measure the Flux-Linkage Characteristics
The systems for measuring the flux linkage of the SR machine consisted of a two-phase SR machine with four stators, six rotors, a rotor clamping and disk, several NI CompactRIO devices, an interface drive circuit between AC power supply and the CompactRIO devices, LabVIEW 7.1, the LabVIEW FPGA Module 1.1, and the PXI-7831R RIO device. We used a single-phase transformer to connect the AC power supply to the interface circuit. We adjusted the applied AC power supply at every measuring rotor position so that the maximum reactive peak current reached during the test did not exceed approximately 8 A. In the test, we measured the RMS values for the phase current and the phase voltage of the stator winding using the LEM current sensor with a second-order low-pass filter and amplifier and the voltage attenuator. We interfaced these with cRIO-9215. The cRIO-9411 with a J-type thermocouple measured the actual resistance variation due to the temperature changes. We calculated the eddy current losses by subtracting the copper losses from the active input power. We used the encoder to view the variation of the rotor position in test and the real rotor position in locking the rotor. The cRIO-9411 measured the encoder pulses, which were converted to the rotor position through the LabVIEW programming downloaded in the FPGA module. We used the PXI-7831R fully combined with LabVIEW software to collect data such as phase voltage, phase current, rotor position, and temperature of the stator winding. We used the SR machine clamping and disk to fix the rotor position to a given angle and lock the disk coupled with the machine shaft through an attached clamp.

Measurement of the Flux Linkage
We selected “RIO0::INSTR” for the VISA Resource Name input to use the Open FPGA VI reference function. In running the host VI, the Open FPGA VI reference checked the device to determine if the FPGA VI was on the device, and it downloaded the latest compiled version of the FPGA VI. Using an encoder, we represented the rotor position as the degree of angle, as well as the relative position and incremental position. The temperature of the stator resistance was measured by a J-type thermocouple, and the temperature changes eventually affected by the actual resistance. We used the cRIO-9215 support files library in the host VI to convert and calibrate Binary and uncalibrated values for cRIO-9215 in the FPGA VI. The phase current and voltage were sampled at  200 ms from the cRIO-9215. When calculating RMS values of the phase current and the phase voltage, we selected sample lengths three times larger than the number of samples in one period to increase the reliability of the calculated RMS values. The flux-linkage values versus reactive currents were represented and automatically stored onto the computer disk.

Using the flexible and powerful LabVIEW software and CompactRIO modules, we built a successful solution for measuring the full magnetization characteristics of the SR machine. We implemented an automatic sensor calibration; automatic, user-friendly experimental environment; and a graphical output of the flux-linkage curve for viewing and storage. For accurate experimental data measurement, we eliminated the eddy current and resistance changes due to the heating in the test. Using the variable AC power supply instead of chopping the DC power supply, we could simplify the measurement system more and avoid using the chopper to adjust the DC power supply.

For more information, contact:
Keunsoo Ha
Virginia Tech
Elec & Comp Engineering
340 Whittemore
Blacksburg, VA 24061
Tel: (540) 231-6058
E-mail: ksha@vt.edu

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