Active Rectifier and Resonant Inverter for Stable Induction Heating
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Solution Overview
Problem
Induction heating and melting systems face challenges with intermittent short circuits and rapid impedance changes, leading to starting problems and increased complexity due to the use of SCR-based rectifiers, which result in poor power factor, voltage notching, and high harmonic distortion. Additionally, voltage source inverter systems suffer from increased switching losses with frequency modulation control.
Innovation Solution
A power conversion system utilizing an active rectifier with IGBT transistors, a DC link circuit, and an inverter with sinusoidal pulse width modulation (SPWM) control to maintain a constant rectifier phase angle, reducing switching losses and harmonic distortion, and eliminating the need for starting circuits by using transistors that can be switched continuously.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If SCR-based rectifiers with phase control are used to control DC output voltage, then power control is achieved, but power factor deteriorates and voltage notching increases
Solution Approach 1:
The patent changes the control parameter from phase angle control to frequency control. The rectifier operates at a fixed phase angle (improving power factor) while power control is achieved by varying the switching frequency of the inverter, which changes the impedance of the resonant tank circuit and thus the power delivered to the load.
Solution Approach 2:
The patent introduces a resonant tank circuit as an intermediary between the rectifier and the induction heating coil. This tank circuit allows the system to control power through frequency modulation while maintaining a fixed rectifier phase angle, thereby decoupling power control from power factor degradation and voltage notching.
2Power
If inverter frequency modulation is used to control power, then power control is achieved, but switching losses increase
Solution Approach 1:
The patent employs periodic switching of the inverter at optimized frequency intervals. By modulating the switching frequency periodically and synchronizing it with the resonant frequency of the tank circuit, the system achieves power control while minimizing switching losses through resonant operation.
Solution Approach 2:
The patent changes the operating frequency parameter of the inverter to control power output. By varying the switching frequency relative to the resonant frequency of the tank circuit, power control is achieved while maintaining efficient operation and minimizing switching losses through resonant coupling.
3Reliability
If pony circuit with SCRs is used to initiate oscillation, then starting is enabled, but circuit complexity increases
Solution Approach 1:
The patent removes the pony circuit and its associated SCRs from the system. The simplified circuit uses only the main inverter switches, which can be turned on immediately without requiring a separate starting circuit, thereby eliminating the complexity of the pony circuit while maintaining reliable starting capability.
Solution Approach 2:
The inverter circuit is designed to be self-starting using the main inverter switches alone. The circuit can initiate oscillation and begin operation without external assistance from a pony circuit, as the main switches are capable of providing the necessary initial current and voltage conditions for resonant operation.
4Power
If inverter switches operate at higher phase angles to control power, then power control is achieved, but current handling capability decreases
Solution Approach 1:
The patent changes the control parameter from phase angle to frequency. By operating the inverter switches at a fixed, optimized phase angle and controlling power through frequency modulation, the system achieves power control while maintaining maximum current handling capability and avoiding the de-rating that occurs at higher phase angles.
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 system achieves stable operation with improved power factor, reduced voltage notching, and lower harmonic distortion, allowing for increased current draw without de-rating the inverter, and eliminates the need for complex starting circuits.
Implementation Method 1
a rectifier controller to control the rectifier transistors at a generally constant angle between triggering of the rectifier transistors relative to an AC input phase voltage using sinusoidal pulse width modulation (SPWM) with modulation index (MI) control
Implementation Method 2
Induction heating and melting systems use rectifiers and inverters to convert line frequency power to controllable high frequency power to drive induction heating coils
Data Source
AI summary
An induction heating or melting system and a power conversion system thereof has an induction heating coil, an active rectifier having rectifier transistors, a DC link circuit coupled to an output of the active rectifier, an inverter having inverter transistors and an input coupled to the DC link circuit, a resonant tank circuit coupled to an output of the inverter and having the induction heating coil, a rectifier controller configured to control the rectifier transistors at a generally constant angle between triggering of the rectifier transistors relative to an AC input phase voltage using sinusoidal pulse width modulation (SPWM) with modulation index (MI) control to control a system output power, an inverter controller, and an input filter coupled to an input of the active rectifier.