Inverter Control Unit Active Short Circuit Braking Torque
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Solution Overview
Problem
Inverters with DC link capacitors experience undesired braking torque during recharging due to interference between safety functions and the DC voltage source, leading to operational instability.
Innovation Solution
A control unit that monitors current flow between the input and switching elements, maintaining an active short circuit state if certain conditions are not met to prevent recharging when the DC link capacitor voltage is below a threshold, thereby avoiding braking torque and ensuring robust operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the control unit triggers recharging of the DC link capacitor when voltage drops below threshold, then the DC link capacitor is recharged, but undesired braking torque occurs due to interference with safety functions
Solution Approach 1:
The control unit continuously monitors the current flowing between the input and switching elements, using this feedback to determine whether to enable recharging. When the monitored current indicates a risk of braking torque (negative current with absolute value above threshold), the control unit prevents recharging despite low voltage conditions, thus eliminating the harmful effect while maintaining voltage stability through alternative means.
2Reliability
If the control unit maintains active short circuit state to prevent braking torque, then operational safety is improved, but recharging of DC link capacitor is delayed
Solution Approach 1:
The control unit dynamically adjusts the active short circuit state based on real-time current monitoring. When current conditions are safe (not negative or below threshold), the control unit releases the active short circuit state to allow recharging. When dangerous current conditions arise, the control unit re-activates the short circuit state to prevent braking torque, thus optimizing the balance between safety and recharging efficiency.
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
The control unit prevents undesired braking torque by maintaining the active short circuit state when recharging conditions are not fulfilled, ensuring a more robust and safe operational mode for the inverter.
Implementation Method 1
a DC link capacitor (6) for smoothing a voltage supplied by a DC voltage source (4)
Implementation Method 2
energy may be recovered from the electric machine in order to recharge the smoothing capacitor
Data Source
Figure 1
Figure 2
AI summary
Control unit (17) for controlling an inverter (3) with a DC link capacitor (6), with a plurality of output phases (20) and with a plurality of switching elements (13, 14) being arranged in half bridges (10) and being connected to the DC link capacitor (6) and the output phases (20), wherein in a failure mode the control unit is configured to control the switching elements (13, 14) to adopt an active short circuit state, in which the output phases (20) are short-circuited, wherein the control unit is configured to control the switching elements (13, 14) to be transferred from the active short circuit state into a freewheeling state, in which the switching elements (13, 14) of one half bridge are turned off, if a first condition, according to which a voltage measuring signal (26) representing a voltage dropping over the DC link capacitor (6) is indicative that the voltage between a positive potential (11) of the DC link capacitor and a negative potential (12) of the DC link capacitor (6) is below a predefined positive voltage threshold, and a second condition, according to which a current measuring signal (28) representing a current (29) flowing between an input (8) of the inverter (3) and the switching elements (13, 14) is indicative that the current (29) in a direction from the positive potential (11) to the negative potential (12) is above or equal to a predefined current threshold being below zero, are fulfilled, and to maintain and/or adopt the active short circuit state, if the first condition is fulfilled and the second condition is not fulfilled.