Soft Starter Motor Voltage Synthesis for Torque Control
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Solution Overview
Problem
Reduced-voltage soft starters (RVSSs) face challenges in accurately controlling motor torque and compensating for switching effects without direct sensing of motor voltage, which can lead to inefficiencies and potential damage from inrush currents and torque shocks.
Innovation Solution
The method involves sensing line voltage and motor current to generate voltage and current signals, processing these signals using ABC-dq0 and dq0-ABC transformations to estimate motor torque, and compensating for switching effects by notching line-to-neutral voltage signals based on phase current detection, allowing for precise torque control and voltage synthesis.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct motor voltage sensing is used, then torque control precision is improved, but device complexity and cost increase
Solution Approach 1:
The patent uses line voltage and motor current as intermediary measurements to indirectly determine motor voltage and torque. Instead of directly sensing motor voltage, the system measures line voltage at the input of the soft starter and uses current signals to compute the voltage drop across the switching circuit, thereby deriving the actual motor voltage. This intermediary approach avoids the complexity of direct motor terminal voltage sensing while maintaining adequate torque control precision.
2Object-affected harmful factors
If switching circuit is used to reduce voltage, then inrush current is limited, but torque control accuracy deteriorates due to switching effects
Solution Approach 1:
The patent applies preliminary compensation for switching effects by detecting when phase currents are within detection windows around zero current levels and preemptively notching the load-side line-to-neutral voltage signals. This preliminary action occurs before the switching effects can significantly degrade torque control accuracy, allowing the system to maintain precise torque control while still using the switching circuit to limit inrush currents during motor startup.
3Object-affected harmful factors
If RMS voltage ramping is used, then starting current is controlled, but starting torque control is insufficient
Solution Approach 1:
The patent implements feedback control by continuously monitoring motor current and using it to generate a torque estimate that serves as a feedback signal for a torque control loop. The system senses line voltage to produce a first voltage signal, senses motor current to generate a current signal, and processes these signals to produce a second voltage signal representative of actual motor voltage. This feedback mechanism enables precise torque control during startup while maintaining current limitation benefits of RMS voltage ramping.
Data Source
AI summary
A reduced voltage soft starter (RVSS) includes a switching circuit configured to selectively couple an AC power source to a motor and a control circuit configured to sense a line voltage provided to the switching circuit to produce a first voltage signal, to sense a current of the motor to generate a current signal and to generate a second voltage signal representative of a voltage of the motor responsive to the first voltage signal and the current signal. The control circuit may be further configured to generate an estimate of a torque of the motor responsive to the second voltage signal and to control the torque of the motor responsive to the torque estimate.