Variable Speed Drive Modulation for Torsional Resonance Control
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Solution Overview
Problem
Variable speed drive systems in LNG plants face issues with torsional resonance vibrations due to inter-harmonic currents, which can cause mechanical damage, and existing solutions require torque sensors for detection and damping.
Innovation Solution
A method and system that utilize a frequency converter to generate inter-harmonic currents with fluctuating frequencies, independent of torsional excitation, to prevent excessive torsional resonance buildup, eliminating the need for torque sensors by causing fluctuations in inter-harmonic frequencies through electric modulation circuitry.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a frequency converter generates inter-harmonic currents to power the electric motor, then the motor can operate at variable speeds, but inter-harmonic currents excite torsional resonance vibrations in the mechanical assembly
Solution Approach 1:
The patent applies periodic action by intentionally modulating the inter-harmonic frequency components at a low frequency (0.1-10 Hz) to create periodic variations that prevent sustained resonance buildup. The modulation periodically shifts the inter-harmonic frequencies away from critical torsional resonance frequencies, thereby reducing vibration amplitudes while maintaining variable speed operation capability.
Solution Approach 2:
The patent changes the frequency parameter of inter-harmonic currents by introducing deliberate low-frequency modulation. This parameter change transforms the static inter-harmonic frequency into a dynamically varying frequency, ensuring that the inter-harmonic components do not consistently excite torsional resonance modes of the mechanical assembly.
2Reliability
If torque sensors are installed to detect and dampen torsional vibrations, then mechanical damage can be prevented, but device complexity and cost increase
Solution Approach 1:
The patent converts the harmful inter-harmonic frequency components into a beneficial control mechanism. By deliberately modulating these inter-harmonic frequencies, the system uses the same frequency components that cause resonance to now serve as a means of preventing resonance, eliminating the need for external sensors while maintaining protection from mechanical damage.
Solution Approach 2:
The system performs self-protection by using its own frequency converter and inter-harmonic current generation capability to detect and dampen torsional vibrations. The frequency modulator uses feedback from vibration sensors already present in the system, and the frequency converter automatically adjusts inter-harmonic frequencies to prevent resonance, making the system self-regulating without requiring additional torque sensors or complex external control systems.
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
This approach effectively reduces torsional resonance excitations, allowing the motor to operate at various speeds without sensor measurement, minimizing mechanical damage and maintaining system stability.
Implementation Method 1
converting AC power obtained from the AC power source at a source frequency to converted power at a variable drive frequency that is unequal to the source frequency
Implementation Method 2
the converted power comprises inter-harmonic currents having additional frequency components at inter-harmonic frequencies
Implementation Method 3
causing fluctuations in the inter-harmonic frequencies independently from any torsional excitation in the mechanical assembly and the AC power source
Implementation Method 4
powering the electric motor with the converted power; driving a mechanical assembly comprising at least a drive shaft and a mechanical load with the electric motor
Implementation Method 5
shaft assemblies in strings of prime movers and loads may exhibit weakly dampened mechanical resonances. These mechanical resonances are often referred to as 'torsional modes' or 'torsional resonances'
Implementation Method 6
The torque associated with a torsional resonance vibration can be large enough to cause mechanical damage
Data Source
AI summary
A variable speed drive system employing an electric motor and a frequency converter arranged between an AC power source and the electric motor is operated. The frequency converter functions to convert AC power obtained from the AC power source at a source frequency to converted power at a variable drive frequency. A mechanical assembly can be coupled to the electric motor. Electric modulation circuitry is provided interacting with the frequency converter. It is arranged to impose fluctuations, independently from any torsional excitation in the mechanical assembly and the AC power source, in the inter-harmonic frequencies of inter-harmonic currents generated in the frequency converter.