Asynchronous Motor Voltage Control via Cos Phi Feedback
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Solution Overview
Problem
Existing methods for optimizing the efficiency of asynchronous machines, particularly in the partial load range, require engine-specific measurements and adjustments, leading to high effort and limited dynamics, and often rely on field-oriented control which is not suitable for all applications.
Innovation Solution
A method that uses a cos ϕ control with a speed or rotating field frequency-dependent setpoint, determined deterministically from the machine's equivalent circuit diagram data, to adjust the motor voltage for optimal copper loss reduction, eliminating the need for field orientation and enabling efficient operation across various load conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If field-oriented control or characteristic curve-based control is used to maintain constant flux at rated value, then dynamic behavior and stalling stability are improved, but copper losses increase in the partial load range
Solution Approach 1:
The patent applies dynamics by making the flux adaptive rather than constant. The control system dynamically adjusts the flux level based on the actual load torque, transitioning from a fixed rated flux to a variable flux that decreases in the partial load range. This resolves the contradiction by maintaining sufficient flux for stability while reducing excess flux that causes copper losses.
Solution Approach 2:
The patent changes the flux parameter from a constant rated value to a variable value that depends on load conditions. By implementing load-dependent flux control, the system optimizes the balance between maintaining stability (requiring sufficient flux) and minimizing copper losses (requiring reduced flux in partial load), directly addressing the technical contradiction.
2Loss of energy
If model-based approaches or algorithm-based approaches are used to minimize losses, then copper losses are reduced, but measurement effort and complexity increase
Solution Approach 1:
The patent applies self-service by using readily available measurements (stator currents and voltages) that are already present in typical control systems to calculate the load torque and determine the optimal flux level. The system serves itself by utilizing existing data without requiring additional sensors or complex external measurement equipment, thus reducing device complexity while achieving loss minimization.
Solution Approach 2:
The patent introduces an intermediary calculation approach where the load torque is derived from standard electrical measurements through established relationships. This intermediary step transforms easily measurable quantities (currents and voltages) into the necessary control parameter (load torque) without requiring direct mechanical torque measurement, thereby simplifying the overall system complexity.
3Productivity
If iterative flux adjustment methods are used to find optimal operating points, then efficiency is optimized, but dynamics are limited and torque oscillations occur
Solution Approach 1:
The patent applies preliminary action by calculating the optimal flux level in advance based on the current load torque, rather than iteratively adjusting it during operation. The control system determines the appropriate flux level proactively using the measured load conditions and applies it directly, avoiding the trial-and-error iterative process that causes delays and oscillations, thus improving dynamics while maintaining efficiency optimization.
Data Source
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AI summary
The invention relates to a method for controlling an asynchronous machine with or without rotational speed feedback in an open-loop or closed-loop manner, wherein the asynchronous motor is controlled by a control unit (converter or servo converter). First, a voltage vector rotating at a rotating-field frequency is specified and applied, wherein both the rotating-field frequency and the voltage vector are determined by means of an open-loop or closed-loop control based on an arbitrarily designed voltage-frequency characteristic curve and the voltage vector has been adapted by means of a cos f closed-loop control. Desired values of cos f are calculated on the basis of nameplate data and equivalent circuit data for loss-optimal or loss-optimized operation of the asynchronous machine in order to calculate cos fdesired value, which leads to loss-optimized operation. Thereafter, a value for cos factual is determined on the basis of the applied voltage vector and a current vector or on the basis of the real power and apparent power. Then the difference between cos fdesired value and cos factual is calculated and a closed-loop control to cos f is fed said initial difference. A control value of the closed-loop controller for cos f is calculated and limited and the voltage vector is adapted by means of the control value of the closed-loop control to cos f.