A plug ‘n play Precharger – FSAE Electric

A smart precharge module that saves the day – requires minimal configuration

Download the design files and source code.

Capacitive precharge is no joke – do it wrong and you could blow up your fancy, expensive motor controllers. Timer-based precharge systems are extremely effective but unable to diagnose and respond to potentially destructive wiring faults. This post is about my prototype precharger, designed for the University of Newcastle FSAE Electric race car 2019/2020. The video below introduces the Tractive System, need for a precharge, failure-modes and how this prototype works.

This precharge module measures the accumulator and Tractive System voltages to ensure adequate precharge (eg. 95%) before closing the Accumulator Isolation Relays. This behaviour is irrespective of accumulator voltage and state-of-charge – The downstream voltage should always be 95% the upstream, regardless. There is also fault-diagnosis logic baked-in – an apparently too-slow or too-fast precharge can indicate insidious and destructive wiring faults. Picking these faults up during precharge, before bulk current flows, can really save your bacon.

There should be room for some size optimisation – I’d recommend re-speccing the DC contactor to something less conservative, and replacing the V-F converters with a dedicated microcontroller.

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