Torque Estimation for Asynchronous Machines Using Electrical Slip Pulsation
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Solution Overview
Problem
Existing torque estimation methods for asynchronous electric machines in electric vehicles, particularly railway vehicles, are inaccurate due to non-bijective correspondence between current, speed, and torque values, which are influenced by motor flux and temperature, leading to potential safety issues during electrical braking.
Innovation Solution
A method utilizing an I-Ω estimator that transforms phase currents into direct and quadrature currents in a rotating coordinate system, calculates torque without a look-up table, and activates friction brakes based on sign and absolute value differences between commanded and estimated torque, ensuring reliable torque estimation and safety.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a look-up table approach is used for torque estimation, then the torque values can be stored and retrieved for current and speed pairs, but the estimation accuracy deteriorates because there is not a bijective correspondence between current, speed values and torque due to motor flux and parameter changes with temperature and saturation
Solution Approach 1:
The patent transforms the torque estimation problem from using current and speed as independent variables to using current and electrical slip pulsation as variables. This parameter transformation accounts for motor flux and saturation effects, as electrical slip pulsation inherently captures the relationship between stator and rotor frequencies that reflects magnetic circuit conditions. The estimator uses the formula Te = (3/2) * (P/2) * (Lm/Iq) * Is^2 * s, where s is the electrical slip pulsation, making the estimation accurate across varying temperature and saturation conditions.
2Speed
If the look-up table is only used for speeds greater than 16 km per hour, then the torque estimation may work at higher speeds, but the reliability deteriorates because torque check failures cannot be avoided at lower speeds where the look-up table is not applicable
Solution Approach 1:
The patent creates a universal torque estimator that functions accurately across the entire operating speed range, from zero to high speeds. The I-Ω estimator based on electrical slip pulsation does not require speed thresholds or different estimation methods for different speed ranges. The electrical slip pulsation can be accurately measured at all speeds, making the estimator universally applicable and reliable for torque check functions at any operating condition.
3Reliability
If friction brakes are activated based on torque mismatch greater than 30%, then safety can be ensured, but the operational reliability deteriorates due to false activations when the look-up table approach gives wrong torque estimation
Solution Approach 1:
The patent replaces the mechanical look-up table-based torque estimation with an analytical I-Ω estimator that uses electrical measurements (current and electrical slip pulsation) to calculate torque. This substitution eliminates the fundamental cause of estimation errors in the look-up table method, providing accurate torque values that enable reliable safety monitoring without causing false friction brake activations, thus maintaining operational smoothness while ensuring safety.
Data Source
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AI summary
The present invention relates to a method for estimating a torque of an asynchronous electric machine (7) of an electric vehicle, in particular a railway vehicle (1), comprising: receiving at least a measurement of a first phase current (i1) and a second phase current (i2) of the asynchronous electric machine (7); receiving a rotor rotational speed (ωm) of the asynchronous electric machine (7); calculating an estimated torque based on the rotor rotational speed of the electric machine and the first and second phase current.