Frequency Converter Shutdown Control for DC-Link Overvoltage
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Solution Overview
Problem
Existing frequency converters for inductive loads, such as ElectroMagnetic Stirrers (EMS) in steel production, face issues with overvoltage generation during sudden stops, which can damage capacitors due to the accumulation of high energy levels in the inductive load, and known solutions like additional hardware units are bulky, expensive, or ineffective.
Innovation Solution
A control method for frequency converters that involves short-circuiting the inductive load after stopping PWM modulation, dissipating accumulated energy through parasitic resistive elements without requiring additional hardware, ensuring an instant and safe stop by controlling the inverter transistors to prevent overvoltage on the DC-Link capacitors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If PWM modulation is suddenly interrupted to stop the frequency converter, then the stopping time is reduced, but overvoltage is generated on the DC-Link capacitors which can damage them
Solution Approach 1:
The control method performs preliminary action by detecting the interruption of PWM modulation and immediately activating a controlled shutdown sequence. The inverter transistors are kept in the on-state for a predetermined time period after PWM interruption to prevent overvoltage, rather than waiting for overvoltage to occur and then responding
Solution Approach 2:
The method converts the harmful effect of stored magnetic energy (which causes overvoltage) into a beneficial controlled dissipation process. By keeping the inverter transistors on after PWM interruption, the magnetic energy is safely dissipated through the parasitic resistance of the inductive load and the on-state resistance of the transistors, preventing capacitor damage
2Reliability
If additional hardware units (dissipative units or capacitor banks) are added to absorb magnetic energy, then overvoltage protection is improved, but device complexity and cost increase
Solution Approach 1:
The system uses its own existing components (inverter transistors and parasitic resistance of the inductive load) to dissipate the magnetic energy and protect against overvoltage. No external dissipative units or additional capacitor banks are required. The inverter transistors serve dual purposes: normal switching and energy dissipation during shutdown
Solution Approach 2:
The inverter transistors perform multiple functions: they serve as switching elements during normal PWM operation and as energy dissipation path during shutdown. The parasitic resistance of the inductive load, normally an unwanted effect, is utilized as a dissipation path for magnetic energy, eliminating the need for dedicated protection hardware
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 method effectively limits overvoltage on DC-Link capacitors, ensuring a non-destructive stop of the converter, is robust against inverter faults, and avoids the need for additional hardware, thus being cost-effective and efficient.
Implementation Method 1
dissipating accumulated energy through parasitic resistive elements
Implementation Method 2
generating the aforesaid three-phase currents Iu, IV, Iw in output from the inverter 804 starting from a rectified voltage present between the first A and the second B terminals by means of pulse width modulation signals
Implementation Method 3
control, by means of pulse width modulation signals, the switching on/off of the electronic power transistors Q11, Q12, Q21, Q22, Q31, Q32 of the inverter 804
Data Source
AI summary
The invention relates to methods for controlling a frequency converter which supplies electric power to an inductive load. The converter comprises a power supply circuit with an input connected to a voltage source and an output connected to a power circuit by means of a first and a second conductive lines; the power supply circuit provides a direct voltage between the first and the second conductive lines to a first input of the power circuit. The power circuit comprises an inverter circuit and an intermediate circuit connected between the conductive lines to be interposed between the power supply circuit and the inverter circuit. The inverter circuit comprises three power transistors connected between the first conductive line and three output terminals, respectively, of the inverter, three additional power transistors connected between the second conductive line and said three other output terminals, respectively.