Active Rectifier SPWM Control for Induction Heating Inverters
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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 converters, which result in poor power factor, voltage notching, and high total harmonic distortion. Additionally, existing systems suffer from increased switching losses and de-rating due to frequency modulation control in VSI systems.
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 and modulation index, reducing switching losses and eliminating the need for starting circuits, while maintaining a high power factor and minimizing harmonic distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If SCR-based rectifier phase control is used to control DC output voltage, then power control is achieved, but power factor deteriorates and voltage notching occurs
Solution Approach 1:
The patent changes the control parameter from phase angle α to modulation index M of SPWM signals. By varying M while keeping the switching frequency constant, the system achieves DC voltage control without the harmful effects of phase control, maintaining unity power factor and eliminating voltage notching
Solution Approach 2:
The patent replaces the mechanical/analogue phase control method with digital SPWM control. The active rectifier uses microprocessor-based SPWM generation to control switching, substituting the traditional phase-angle-based SCR control and eliminating its associated problems with power factor and voltage distortion
2Power
If inverter frequency modulation is used to control power, then power control is achieved, but switching losses increase and device de-rating is required
Solution Approach 1:
The patent changes the control parameter from switching frequency to modulation index M. By controlling the amplitude of the SPWM reference signal while maintaining constant switching frequency, the system achieves power control without increasing switching losses, eliminating the need for device de-rating
Solution Approach 2:
The patent implements dynamic control of the modulation index M based on the detected load impedance. The system continuously adjusts M to maintain optimal operating conditions, allowing full utilization of device current capacity while controlling output power, avoiding the static frequency modulation approach
3Reliability
If pony circuit with second set of SCRs is added to initiate oscillation, then starting problem is solved, but device complexity and cost increase
Solution Approach 1:
The patent enables the inverter to self-start by using the active rectifier's SPWM control to generate the necessary oscillating current. The system uses its own existing components (rectifier transistors and DC link) to initiate operation without requiring external pony circuits or additional SCR sets
Solution Approach 2:
The patent makes the active rectifier serve dual functions: it not only performs rectification but also provides the oscillation initiation function traditionally requiring a separate pony circuit. The rectifier transistors are controlled to generate both the rectified DC and the starting oscillation current for the inverter
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 effectively addresses starting issues, maintains a high power factor, reduces switching losses, and increases current handling capacity without de-rating, thereby improving efficiency and reducing complexity and costs in induction heating and melting applications.
Implementation Method 1
a rectifier controller which controls 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
a DC link circuit having a DC link reactor coupled to an output of the active rectifier
Implementation Method 3
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
Implementation Method 4
a resonant tank circuit coupled to an output of the inverter and having an induction heating coil
Implementation Method 5
The induction heating coil turns are often installed in harsh electrical and mechanical surroundings
Data Source
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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.