A Power Measurement Method for Electric Automotive Variable-Frequency Drive Systems

In recent times, the increasing global interest towards climate control leads to the increased usage of electric vehicles (EVs). However, to achieve affordability and high energy efficiency, a consistent performance analysis is required for EVs. The quality of such performance analysis is dependent on the accuracy and precision of the power measurement. A typical electric automotive drive for an EV consists of a variable-frequency drive (VFD) which produces non-sinusoidal pulse width modulated (PWM) signals. The characteristic of such PWM signals and the operation at variable frequencies results in the introduction of harmonics distortion and zero crossing error. Based on these factors the conventional power measurement methods cannot provide highly accurate and precise measurement results. To address the above-mentioned issues, this article proposes a novel power measurement method, for an electric automotive variable-frequency drive system, based on the lock-in amplifier (LIA) and lock-in amplifier frequency-locked loop (LIA-FLL). It performs accurate frequency tracking and component extraction which leads to accurate power measurement. The performance of the proposed method is validated through experimentation conducted for a 1.5 kW induction motor fed by VFD at three different supply frequencies (a) 50 Hz, (b) 30 Hz, and (c) 70 Hz. A comparative analysis is presented by comparing the measurement results obtained by the proposed method and YOKOGAWA WT1600 (reference digital power measurement instrument). The analysis illustrates that the proposed power measurement method indicates a relative power measurement accuracy of +/- 0.0338% or less and a frequency tracking accuracy of +/- 0.0024% or less under the frequency variation of +/- 20 Hz and high harmonic distortion.