Soft-Start Control Using Current Derivative Measurement
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Solution Overview
Problem
Existing soft-start systems for electric motors, particularly those with nominal voltages above 500 V or 5 kV, face reliability issues due to transient voltage and current stresses on thyristors, leading to imprecise control and increased costs from complex voltage measurement requirements and phase deviation in current transformers.
Innovation Solution
A method using a Rogowski sensor to measure the derivative of the current, allowing precise determination of the switching instant for thyristors, thereby reducing transient currents and simplifying control, and employing a 'delta-gamma' control strategy to minimize voltage across the switch during closure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If voltage measurements are performed across thyristor terminals to detect voltage minima for gamma control, then switching precision is improved, but device complexity and cost increase due to sophisticated signal processing and insulation requirements
Solution Approach 1:
The patent introduces an intermediary sensor (current transformer or Rogowski coil) that indirectly measures the voltage minima information through current derivative detection, avoiding direct voltage measurement across the thyristor terminals. This intermediary approach simplifies the measurement system while maintaining the precision needed for gamma control.
Solution Approach 2:
The patent replaces the direct voltage measurement system with a current-based measurement system using magnetic field sensing (current transformers or Rogowski coils). This substitution eliminates the need for high-voltage insulation and complex voltage sensing circuitry, reducing device complexity while achieving the same control precision.
2Ease of operation
If conventional current transformers are used to detect zero crossing, then current measurement is achieved, but measurement precision deteriorates due to phase deviation and offset errors
Solution Approach 1:
The patent changes the measurement parameter from direct current magnitude to current derivative (rate of change). By measuring di/dt instead of i, the system eliminates phase deviation and offset errors inherent in conventional current transformers, achieving precise zero-crossing and voltage minima detection without the measurement errors that plague traditional approaches.
3Stress or pressure
If multiple thyristors are connected in series for high voltage applications (≥5 kV), then voltage handling capability is improved, but measurement practicality deteriorates due to the complexity of taking voltage measurements across series-connected thyristors
Solution Approach 1:
The patent uses an intermediary current measurement approach that works equally well for single or series-connected thyristors. By measuring the current derivative through a common conductor, the system avoids the need to physically access or measure across each individual thyristor terminal in a series string, making high-voltage applications as practical as low-voltage ones.
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 enhances the precision of motor startup control, reduces production losses, and lowers system costs by simplifying signal processing and eliminating the need for complex voltage measurements, while maintaining precise synchronization and minimizing transient currents.
Implementation Method 1
A method uses a Rogowski sensor to measure the derivative of the current
Data Source
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AI summary
The invention relates to a method for controlling a switch (T1, T2) that drives a power supply line to an electric motor (7) from an alternating voltage source (1), comprising determining the switch closing time from a measurement of the derivative of the current flowing on the power supply line. The invention also extends to a starting system and a computer program product capable of implementing this method.