Soft Starter Firing Control for Low-Loss Asynchronous Motor Starts
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Solution Overview
Problem
Conventional soft starters for asynchronous motors experience significant losses during start-up due to thyristor operation, leading to limited start cycles and increased cooling requirements, which restrict their application and efficiency.
Innovation Solution
A method employing a model-predictive approach to generate firing signals for soft starters, which forecasts the transient electrical and mechanical motor behavior to optimize thyristor firing opportunities, reducing continuous actuation and relying on steady-state assumptions, thereby minimizing losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional soft starters use thyristors to control motor starting, then the starting current and torque can be reduced, but significant losses occur during start-up leading to limited start cycles and increased cooling requirements
Solution Approach 1:
The patent applies periodic action by using pulse-width modulation (PWM) to switch thyristors on and off at high frequency during the starting process. Instead of continuous thyristor conduction, the system applies periodic voltage pulses to the motor, achieving both current control and reduced losses through the switching action that minimizes dissipative effects in the thyristors.
Solution Approach 2:
The patent replaces the conventional mechanical/continuous control approach with an electronic switching approach. Instead of using thyristors in continuous conduction mode with phase control, the system uses modern semiconductor switches (IGBTs or MOSFETs) in PWM mode, substituting the older mechanical-like continuous adjustment with electronic pulse switching that reduces losses.
2Ease of operation
If thyristors operate continuously during start-up, then motor starting control is achieved, but cooling requirements increase and device complexity increases
Solution Approach 1:
By using periodic PWM switching instead of continuous thyristor operation, the system achieves motor starting control while significantly reducing the thermal load. The periodic nature of the switching allows for better heat dissipation cycles and reduces the overall cooling requirements compared to continuous operation.
Solution Approach 2:
The patent changes the operational parameters of the power electronic devices from continuous conduction mode to discontinuous PWM switching mode. This parameter change transforms the thermal characteristics, reducing peak temperatures and overall heat generation, thereby simplifying the cooling system requirements while maintaining effective starting control.
3Device complexity
If conventional control methods are used, then the structure for generating control pulses remains simple, but losses in thyristors increase and efficiency decreases
Solution Approach 1:
The patent substitutes the conventional phase-angle control method with PWM-based switching control. While the control pulse structure becomes slightly more complex, the energy losses in the power devices are dramatically reduced because modern switches operate in saturation/cutoff regions rather than in the linear region where conventional thyristors dissipate significant power.
Solution Approach 2:
The control strategy changes from continuous phase-angle modulation to discontinuous PWM pulse generation. This parameter change in the control method allows for more efficient power device operation, reducing conduction losses while the control structure complexity increases only marginally due to the digital nature of PWM generation.
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 reduces losses in both the soft starter and the induction motor during start-up, allowing for more frequent start cycles, reduced cooling needs, and the potential for smaller starter designs while saving energy.
Implementation Method 1
a soft starter (1) for starting an asynchronous motor (4), which soft starter is connected between a three-phase electrical grid (5) and the ASM (4), having one antiparallel-connected pair of thyristors (2) per phase a, b, c
Implementation Method 2
asynchronous motor 4... also called induction motors
Data Source
AI summary
A method for starting and stopping an asynchronous motor by way of a soft starter. The method includes the following steps: determining ignition options of one or more thyristors of the soft starter that are possible at a future calculation time; predicting the motor behavior for the determined ignition options, if an ignition of one or more thyristors of the soft starter is carried out; based on the predicted motor behavior, deciding whether an ignition option is to be selected and which is to be selected; and generating one or more ignition signals for one or more thyristors, if the decision for an ignition option has been made.