Inverter Control Device for Managing DC Link Voltage During Load Shedding
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Solution Overview
Problem
In electric vehicles with asynchronous machines, load shedding leads to high currents due to low leakage inductances, potentially damaging power switching elements and requiring oversized, expensive DC link capacitors to prevent overvoltage, which limits recuperation torque and efficiency.
Innovation Solution
A control device that evaluates the disconnection of the DC voltage source and alternates between DC braking and freewheeling switching patterns to manage energy stored in inductors, canceling out voltage extremes and avoiding capacitor overload, allowing the use of standard capacitors and maintaining efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If immediate active short circuit is triggered to convert stored energy into heat, then overvoltage is prevented, but very high currents flow through power switching elements causing damage
Solution Approach 1:
The control device alternates between DC braking and freewheeling switching patterns in periodic cycles. During DC braking, energy is converted to heat; during freewheeling, energy circulates without dissipation. This periodic alternation prevents both overvoltage and excessive currents by distributing the energy conversion over time cycles rather than immediate short circuit.
2Reliability
If DC link capacitor is significantly oversized to withstand overvoltage, then component damage is prevented, but cost and size increase
Solution Approach 1:
By implementing periodic alternation between DC braking and freewheeling, the control device limits voltage excursions to normal operating ranges, eliminating the need for oversized capacitors designed for extreme overvoltage conditions.
Solution Approach 2:
The control device converts the potentially harmful stored energy in inductors into useful braking torque during DC braking phases, while using freewheeling phases to recover and recycle this energy, thereby preventing overvoltage without requiring larger capacitors.
3Reliability
If recuperation torque is limited to avoid harmful overvoltage, then component safety is ensured, but efficiency and operating range are reduced
Solution Approach 1:
The control device enables full recuperation torque by implementing periodic alternation between DC braking and freewheeling modes. This allows the system to handle high recuperation energies that would otherwise cause overvoltage, maintaining both component safety and maximum operating efficiency.
Solution Approach 2:
The control device dynamically changes switching parameters between two distinct patterns (DC braking and freewheeling) based on real-time evaluation of the DC link voltage state, allowing full recuperation torque while preventing overvoltage through adaptive control.
4Reliability
If DC braking is continuously applied to convert energy into heat, then overvoltage is prevented, but harmful negative voltage and excessive heat generation occur
Solution Approach 1:
The control device alternates between DC braking (energy to heat) and freewheeling (energy circulation) modes. This periodic action allows heat generation during braking phases to be balanced by energy recovery during freewheeling phases, preventing excessive temperature rise while maintaining overvoltage protection.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution prevents damage to power switching elements, eliminates the need for oversized capacitors, and maintains recuperation torque without efficiency losses, ensuring safe operation and efficient energy management during load shedding.
Implementation Method 1
a first switching pattern, which effects DC braking
Implementation Method 2
energy stored in inductors of the electric machine
Data Source
AI summary
A control device for an inverter has a DC voltage input and a power unit with three half-bridges each formed by two power switching elements, the control device being arranged to driving the power switching elements in a normal operating mode for converting a DC voltage applied to the DC voltage input into a polyphase AC current provided at an AC current output. The control device is adapted to evaluate a signal state of a signal indicating a disconnection of a DC voltage source from the DC voltage input and to control the power switching elements in dependence on a result of the evaluation for alternately adopting a first switching pattern causing DC braking and a second switching pattern causing freewheeling.