Background: A multifunctional power factor device based on the FFT algorithm has
been designed. The digital panel can display power factor, voltage, current, frequency, phase angle,
active power and reactive power.
Objective: To solve the influence of the harmonic interference and the aperiodic component on the
accuracy of power factor measurement.
Methods: In this paper, by combining a fixed-point 256-point FFT algorithm and taking STM32 as
the core microcontroller, the hardware circuit and the software program are designed respectively.
The hardware circuit is tested and analyzed in practice. Among them, the hardware circuit mainly
includes the main circuit design, STM32 control circuit design, EMI and second-order RC filter
circuit design, sampling circuit design, and signal conditioning circuit design. The software program
mainly includes the main program, AD conversion subroutine, voltage and current acquisition
subroutine, LCD display subroutine and twirl factor array. FFT algorithm is achieved by the
table look-up method.
Results: Finally, the hardware circuit is built and the software program is debugged to test the designed
device. The experimental results show that the designed power factor instrument meets the
task requirements under the different types of loads.
Conclusion: After processing and analyzing the measurement results, it can be concluded that: under
the pure resistive load, the maximal relative error of electrical parameters is 4.49%; and under
the resistive inductive load, the maximal relative error of electrical parameters is 2.86%. Both results
meet the design requirements.