Inductive Rotary Joint Multimode Inverter Soft Start
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Solution Overview
Problem
Inductively-coupled rotary joints in computer tomography scanners face challenges in managing high inrush currents during power switching, leading to stress or overload of electronic components, and require high-power contactors for output power switching, limiting dynamic range.
Innovation Solution
A contactless inductively coupled rotary joint with a hardware safety circuit that gradually increases output power using a full-bridge and half-bridge inverter mode transition, avoiding high inrush currents and enabling improved dynamic range by controlling semiconductor switches and resonance frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the output power is switched on abruptly, then the power coupling is established quickly, but a very high inrush current occurs that stresses or overloads electronic components
Solution Approach 1:
The patent applies preliminary action by pre-charging the filter capacitor through a controlled current path before main power switch closure. The capacitor is charged to a predetermined voltage level in advance, so that when the main switch closes, the inrush current is limited because the voltage difference across the switching element is reduced. This preliminary charging action prevents the harmful inrush current while maintaining reliable power transfer.
2Power
If high-power contactors are used for switching output power, then power switching capability is achieved, but device complexity and size increase
Solution Approach 1:
The patent replaces the mechanical high-power contactor system with an electronic switching solution using semiconductor devices (such as IGBTs or MOSFETs) in conjunction with resonant circuitry. The resonant circuit enables soft switching conditions that allow standard semiconductor devices to handle high power without requiring bulky mechanical contactors. This substitution eliminates the mechanical moving parts and reduces overall system complexity while maintaining the required power switching capability.
Solution Approach 2:
The patent changes the operating parameters of the switching circuit by utilizing resonant frequency operation and controlled impedance matching. By operating at or near the resonant frequency of the LC circuit formed by the filter capacitor and circuit inductance, the patent achieves high power transfer with reduced current stress on switching devices. The impedance is dynamically adjusted through the resonant condition, allowing efficient power switching without high-power contactors.
3Reliability
If the filter capacitor is charged to nominal output voltage without current limiting, then the capacitor is fully charged, but a very high current flows that may lead to significant stress or overload
Solution Approach 1:
The patent introduces an intermediary current-limiting path using a dedicated charging resistor or current-limiting circuit that is temporarily activated during capacitor charging. This intermediary element is placed in series with the capacitor charging path and is switched out of the circuit after the capacitor reaches the predetermined voltage level. The intermediary component mediates between the power source and the capacitor, allowing controlled charging current that prevents inrush current while ensuring complete capacitor charging.
Solution Approach 2:
The patent employs periodic action by using pulse-width modulation (PWM) or cyclic charging cycles to charge the filter capacitor. Instead of continuous direct charging that would cause inrush current, the charging is performed in controlled pulses or cycles with adjustable duty cycle. The charging current is applied periodically with decreasing amplitude as the capacitor voltage approaches the target level, ensuring complete charging while limiting peak current through the periodic nature of the charging process.
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 solution prevents high inrush currents and eliminates the need for high-power contactors, ensuring smooth power-on and significantly enhancing the dynamic range of the rotary joint.
Implementation Method 1
The inverter circuit is basically a full-bridge circuit, also called H-bridge for generating an AC voltage. There are four semiconductor switches and four diodes
Implementation Method 2
The outputs of the inverter may be coupled via a resonance capacitor and an optional transformer and/or a common mode choke to the primary winding of the rotating transformer. These components preferably form a series resonance circuit having a resonance capacitance and a resonance inductance. The resonance capacitance and the resonance inductance determine at least one series resonance frequency.
Implementation Method 3
Energy coupled from the primary winding at the primary side is received by a secondary winding at the secondary side of the rotating transformer
Data Source
AI summary
An inductive power transfer circuit or inductive rotary joint has an inductive rotating coupler with a primary side and a primary winding rotatably arranged against a secondary side and a secondary winding. The secondary side is connected via a rectifier to a load. The stray inductance of the coupler together with a resonance capacitor a series resonance circuit having a series resonance frequency. An inverter in a full bridge circuit is provided for converting a DC input voltage into an AC voltage. The inverter is operable in a full bridge mode to deliver a high power level and in a half bridge mode to deliver a low power level. This results in a broad dynamic range, soft power on and improved safety, as switching between the modes may be controlled by a simple hardware.