Frequency Inverter Precharging Circuit with Phase Capacitors
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Solution Overview
Problem
Conventional frequency converters require complex and costly pre-charging circuits with high power losses and large volume, as well as filter circuits, which increase costs and reduce efficiency during pre-charging and normal operation.
Innovation Solution
A frequency converter design where mains-side phases are directly connected to input-side phases via switches, allowing phase capacitors to act as both current limiters during pre-charging and filters during normal operation, with a control device managing the switches to optimize charging and reduce power losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a pre-charging circuit with resistors is used to limit pre-charging current, then the current limitation function is achieved, but high power losses occur and the resistors are significantly heated
Solution Approach 1:
The patent changes the electrical parameters by using phase capacitors with specific capacitance values to provide current limitation during pre-charging instead of resistors. The capacitive reactance Xc = 1/(2πfC) provides current limitation without the power losses inherent in resistive circuits, as capacitors store and release energy rather than dissipating it as heat.
Solution Approach 2:
The patent eliminates the need for expensive, high-power resistors that generate heat and require cooling. Instead, it uses phase capacitors that are already present in the circuit for filtering purposes, thereby avoiding the need for additional power-dissipating components and their associated cooling requirements.
2Reliability
If resistors are used for pre-charging current limitation, then current limitation is achieved, but the resistors must be designed to be voluminous when capacitance of back-up capacitor is large
Solution Approach 1:
The patent makes the phase capacitors serve dual functions: they act as filter capacitors during normal operation and as current-limiting elements during pre-charging. This eliminates the need for separate pre-charging resistors and reduces the overall component volume, as the same capacitive elements perform both filtering and current limitation tasks.
Solution Approach 2:
The patent merges the pre-charging current limitation function with the existing phase capacitors used for filtering. By combining these functions into a single component set, the circuit eliminates redundant elements (resistors) and reduces overall circuit volume and complexity.
3Loss of energy
If large resistance values are selected for pre-charging, then power losses are reduced, but the time required for preloading increases
Solution Approach 1:
The patent changes from resistive current limitation to capacitive current limitation. The capacitive reactance provides current limitation during pre-charging without the trade-off between power loss and charging time that characterizes resistive circuits. The capacitor charges the back-up capacitor through a controlled current profile determined by its capacitance value, achieving both efficient energy transfer and reasonable charging time.
4Reliability
If separate power semiconductors are used for precharging the back-up capacitor, then current limitation is achieved, but the circuitry becomes complex and cost-intensive
Solution Approach 1:
The patent makes the phase capacitors perform multiple functions: filtering during normal operation and current limitation during pre-charging. This eliminates the need for separate power semiconductor switches and associated control circuitry dedicated to pre-charging, thereby reducing overall circuit complexity and cost.
Solution Approach 2:
The patent combines the pre-charging current limitation function with the existing phase capacitors, merging two functions into one component set. This eliminates redundant circuitry including separate switches, diodes, and control logic that would be required if dedicated pre-charging semiconductors were used.
5Reliability
If both pre-charge circuit and filter circuit are implemented, then both current limitation and filtering functions are achieved, but the overall volume is large and costs are high
Solution Approach 1:
The patent makes the phase capacitors serve dual purposes: as filter capacitors during normal operation and as pre-charging current-limiting elements. This eliminates the need for separate pre-charging circuit components, reducing overall circuit volume and cost while maintaining both filtering and current limitation functions.
Solution Approach 2:
The patent merges the pre-charging current limitation function with the filter circuit's phase capacitors. By combining these functions into the same components, the circuit eliminates redundant elements and reduces overall volume, achieving both current limitation and filtering without requiring separate dedicated circuits for each function.
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 design simplifies the upstream circuit, reduces power losses, and lowers costs by enabling efficient current limitation and filtering with the same capacitors, allowing for rapid pre-charging and effective filtering while minimizing volume and expense.
Implementation Method 1
The input-side phases are connected via a series circuit to mains-side phases of a multi-phase supply network, wherein the mains-side phases are each connected to one of the input-side phases within the upstream circuit via a respective phase capacitor
Implementation Method 2
the mains-side phases within the upstream circuit are also connected directly to another of the input-side phases via a respective switch, so that the mains-side phases are short-circuited to the input-side phases when the switches are closed
Data Source
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AI summary
The invention relates to a frequency converter with a rectifier (1) on the input side and at least one support capacitor (3) arranged downstream of the rectifier (1). The rectifier (1) has multiple half-bridges (6), via which a respective input-side phase (U, V, W) of the rectifier (1) feeds the support capacitor (3). The input-side phases (U, V, W) are connected to grid-side phases (L1, L2, L3) of a multiphase supply grid (5) via a preceeding circuit*** (4). Each grid-side phase (L1, L2, L3) is connected to one of the input-side phases (U, V, W) within the preceeding circuit (4) via a respective phase capacitor (9). Each grid-side phase (L1, L2, L3) is additionally directly connected to another input-side phase (U, V, W) within the preceeding circuit (4) via a respective switch (10) such that the grid-side phases (L1, L2, L3) are short-circuited with the input-side phases (U, V, W) when the switches (10) are closed, and each phase capacitor (9) connects two grid-side phases (L1, L2, L3) or two input-side phases (U, V, W) together. The frequency converter has a controller (11) which keeps the switches (10) open when pre-charging the support capacitor (3) and closes the switches when a specified charge state (Z1) of the support capacitor (3) is reached.