When designing the inductive component of the core material, the engineer must accurately measure the properties of the material. The dynamic magnetic hysteresis circuit (or B-H curve) of the core contains valuable information about magnetic core loss and other magnetic parameters. Unfortunately, commercial magnetic circuit analyzers are very expensive, not suitable for small-scale research laboratories and manufacturers. This design instance describes a virtual instrument that uses a desktop or notebook computer that combines an analog data acquisition card and the National Instruments LabVIEW software (above 7.1). When using, the software extracts B-H loop information, core loss, and other magnetic parameters, and measurement costs can be accepted.
Figure 1 shows a test equipment for a core device. The device T1 includes a magnetic core material sample and a winding equal to two turns. A precision current detecting resistor R1 serves as a sample of excitation current, which generates a magnetic field in the magnetic core. The voltage drop on R1 is proportional to the excitation current and the magnetic field h. The network generated by the resistor R2 and the capacitor C1 pair the voltage generated in the secondary winding. The voltage of the C1 is proportional to the magnetic flux density B in the magnetic core. In fact, the value of R2 should be much larger than the impedance of the capacitor C1 at the operating frequency. (According to the instrument of the textbook, it is recommended to suggest 100: 1.)
The tolerance and characteristics of components will affect the accuracy of the measurement. R1 should be used without feelings, rated power, 1 Ω resistance of 1% tolerance; C1 should be low leakage, low-media absorption, polyester or polypropylene film capacitance, tolerances should be strict. Data acquisition and view uses a dedicated virtual instrument that is composed of a PCI-6024E data acquisition card and LabVIEW by National Instruments. The software uses Ni's Express VI (virtual instrument) technology, greatly simplified the acquisition and processing of user design data. The application only uses two data acquisition analog input channels: CHANNEL 0 collects magnetic field reading (h), and is displayed on the X-axis of the XY diagram with units per meter, while Channel 1 captures the magnetic flux density B, Tesla (Tesla) is displayed on the Y-axis.
At low frequencies, the magnetic core of the magnetic core consumes the main position, and at higher frequencies, the eddy current loss is more pronounced. The magnetic core loss can be calculated using a wattmeter algorithm, but it is also possible to write its own arithmetic expression to the formula node of the VI block diagram. LabVIEW can also save data and output the result into a Microsoft Excel data sheet format, or is output to other programs for further analysis.
You can determine the inductive value by other parts of the eight differential analog input channels of the data acquisition card. At this point, it is necessary to measure the voltage on the primary winding of the device and calculate its root root value. The ratio of the rimming current measured by the R1 determines the size of the winding scale impedance value XL. Then, the electric inductance amount is calculated by the following formula: L = XL / 2πf, wherein f means the frequency of the applied excitation voltage.
Figure 2 is a hysteresis curve of a 3B7 mixed ferrite core, initial and secondary windings are 100, measured by 60 Hz. For comparison, Figure 3 shows a 60 Hz hysteresis curve of a 100W power transformer on a ring magnetic core composed of a particle oriented silicon steel sheet. The loop of the ring core indicates a stronger hysteresis, which is the characteristics of the saturated core power converter to use. In order to apply 60 Hz excitation, a primary winding of the device can be driven by a buck (insulation) transformer, which can be powered by an adjustable autopilot transformer, such as Variac, Genrad (www.ietlabs.com). When the B-H curve is obtained, the primary voltage is gradually increased until the upper and lower part of the hysteresis circuit is flat, indicating that the core is saturated. If you use accurate words, you don't need to correct it. However, when evaluating the core material, it may be necessary to use different turns to test the best hanging value of the windings.
At 60 Hz test, R2 in the integral network uses a resistor of 267 kΩ tolerance, and C1 uses a 1MF polyester media capacitor. According to the number of turns and the current required to obtain a useful output voltage, the AC excitation voltage is usually sufficient to complete the test. In a higher frequency magnetic core measurement, a signal generator is connected to the power amplifier and change the value of each component in the RC integrator to make it function properly at the desired frequency. Although the device does not use the simulation output of the data acquisition card, this output can be used as a sinusoidal source of the power amplifier.
Please check that you are ready to use the electrical specifications of the capture card to avoid exceeding the peak / peak difference voltage of the card and the common mode input voltage. If the excitation voltage is close or exceeds the nominal value of the card, it is to be increased by a 10: 1 voltage limit and add a gain multiplier having a factor of 10 in the software to compensate for the loss of the attenuator.
Editor in charge: GT, read full text
Our other product:
