Thyristor Inverter Switching Pattern for Torsional Resonance Mitigation
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Solution Overview
Problem
Thyristor-based inverters in pulse mode can excite torsional natural frequencies of the drive shaft, leading to increased wear and potential failure due to resonance, especially during startup or low-speed operations where stator voltage is insufficient for normal commutation.
Innovation Solution
A method for switching thyristors in a current source converter that adjusts the pulse number and switching pattern based on stator frequency to avoid resonances, using a controller to switch thyristors in a way that reduces higher-order harmonics and minimizes excitation of torsional natural frequencies by altering the pulse number and timing of thyristor switches during normal and low pulse operation modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thyristors are switched in normal pulse operation mode with pulse number equal to the number of half-bridge arms, then commutation is ensured and power conversion is efficient, but higher-order harmonics excite torsional natural frequencies of the drive shaft causing resonance and increased wear
Solution Approach 1:
The patent dynamically adjusts the pulse number based on operating conditions. During normal operation, the pulse number equals the number of half-bridge arms (e.g., 6) for efficient commutation. During low-speed startup when stator voltage is insufficient, the pulse number is reduced (e.g., to 4 or 2) by simultaneously switching multiple thyristors, which changes the harmonic spectrum to avoid exciting torsional natural frequencies while maintaining reliable commutation through forced commutation capability.
Solution Approach 2:
The patent changes the pulse number parameter from the conventional fixed value equal to the number of half-bridge arms to a variable parameter that can be reduced during low-speed operation. This parameter change alters the switching pattern and harmonic content, preventing resonance excitation of the drive shaft while maintaining effective power conversion and commutation reliability across different operating speeds.
2Object-affected harmful factors
If the pulse number is reduced during low-speed operation, then excitation of torsional natural frequencies is reduced, but the switching complexity and control requirements increase
Solution Approach 1:
The control system dynamically selects between normal pulse operation mode and low pulse operation mode based on rotor speed or stator voltage measurements. The controller adjusts the pulse number by simultaneously switching multiple thyristors in low pulse mode, and this dynamic adaptation is achieved through a control algorithm that monitors operating conditions and automatically transitions between modes, managing complexity through intelligent control rather than hardware complexity.
Solution Approach 2:
The same thyristor-based inverter circuit performs multiple functions: in normal pulse mode it operates with pulse number equal to half-bridge arms for efficient commutation, while in low pulse mode it simultaneously switches multiple thyristors to reduce pulse number for low-speed operation. This multi-functionality allows a single device to handle both high-speed efficient operation and low-speed resonance avoidance without requiring separate circuits.
Data Source
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AI summary
An electrical converter (12) with at least two output phases comprises a rectifier (28) and a thyristor-based inverter (34) interconnected by a DC link (30) with an inductor (32), wherein the thyristor-based inverter (34) comprises a half-bridge (40) with at least two half-bridge arms (42a, 42b) for each output phase of the electrical converter (12) and each arm (42a, 42b) being provided by a thyristor (38). A method for switching the electrical converter (12) comprises: cyclically switching the thyristors (38) of the inverter (34), such that at at least one time instant (58), two thyristors (38) of different half-bridge arms (42a, 42b) are switched on simultaneously, such that a pulse number, which determines at how many time instants (58) thyristors (38) of the inverter (34) are switched during one stator voltage period, is lower than the number of half-bridge arms (42a, 42b) of the inverter (34).