Increasing Torque Capability of AC Drives via Active Thermal Management of Inverters

Abstract

This work proposes an active thermal management technology that operates ac drive inverters safely at thermal limits to maximize their current and torque capability. For that purpose, the technology exploits two fundamental opportunities: First, it monitors the coolant and junction temperature to adaptively determine the maximal peak current capability that increases at low coolant temperature. Second, it takes advantage of the thermal impedance frequency response function <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Z_{\mathrm{th}}(\text {j}\omega)$</tex-math></inline-formula> that decays with higher frequencies. Thus, larger currents are feasible at higher excitation frequencies without increasing peak junction temperature, because the associated transient losses are absorbed by the thermal capacitance of the power module. This article presents a unique active thermal management algorithm that addresses the two identified opportunities. It utilizes primarily a reduced-order electrothermal real-time model as well as negative temperature coefficient thermistor temperature information. Thus, it can be applied on most drive systems with small software modifications to adaptively maximize its current capability. To make this technology applicable in ac drives, this article also proposes torque control strategies that utilize variable current limits and discusses essential design limitation. With the introduced technology, state-of-the-art drive inverters can operate safely at up to 200% overload current and torque across a wide speed range. This supports the development of highly reliable drives with increased base speed range and power density that save space, cost, and resources.