Electric Machine Active Short Control for Transient Current Peaks
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Solution Overview
Problem
Electrified vehicles face high transient currents during inverter-controlled active shorts, which can demagnetize permanent magnets and reduce the efficiency of electric machines.
Innovation Solution
A controller is programmed to control the inverter to actively short the electric machine phases after conditioning, which involves slewing the voltage and current to target values at specific slew rates, based on the rotational speed and time constant of the electric machine, to manage the transient current effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the inverter is controlled to provide an active short circuit of the electric machine terminals to protect the DC link capacitor, then the capacitor is protected from induced voltage, but a high transient current is generated that could affect magnetization of the permanent magnets
Solution Approach 1:
The controller performs preliminary conditioning of the electric machine by controlling electric machine voltage toward zero at a first slew rate or controlling electric machine current to a target short circuit current at a second slew rate before executing the active short circuit. This preliminary action prepares the machine state to minimize transient current when the short circuit is applied, thereby protecting permanent magnets while maintaining capacitor protection.
Solution Approach 2:
The controller dynamically adjusts the timing of the active short circuit based on real-time monitoring of electric machine voltage and current. The short circuit is executed after either a specified elapsed time from the active short request signal, or after the electric machine voltage is below a first threshold or the electric machine current is within a predetermined range of the calculated short circuit current. This dynamic timing optimization ensures minimal transient current while maintaining protection functionality.
2Reliability
If the active short circuit is executed immediately to protect the DC link capacitor, then the capacitor is protected from induced voltage, but the transient current peaks increase causing potential demagnetization
Solution Approach 1:
The controller implements preliminary voltage or current conditioning of the electric machine before executing the active short circuit. By controlling voltage toward zero at a specified slew rate or current to a target value, the machine is prepared in advance to reduce the magnitude of transient current peaks during the subsequent short circuit, thereby preventing demagnetization while maintaining capacitor protection.
Solution Approach 2:
The controller continuously monitors electric machine voltage and current during the conditioning period and uses this feedback to determine the optimal timing for executing the active short circuit. The short circuit is triggered when voltage falls below a threshold or current approaches the target value, ensuring that transient current peaks are minimized while still providing timely protection for the DC link capacitor.
Data Source
AI summary
An electrified vehicle includes a traction battery, an inverter coupled to the traction battery via positive and negative legs of a high voltage bus and operable to convert DC power from the traction battery to three-phase AC power, a capacitor connected across the positive and negative legs of the high voltage bus, an electric machine having three phases coupled to the inverter, and a controller programmed to control electric machine voltage toward zero at a first slew rate or control electric machine current to a target short circuit current at a second slew rate in response to an active short request signal and to control the inverter to actively short the three phases of the electric machine after either (a) a specified elapsed time, or (b) after the electric machine voltage is below a first threshold or the current is within a predetermined range of the target short circuit current.