Safe Torque Limiting in Field-Oriented Control Drives
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Solution Overview
Problem
Current safety measures for frequency-converter-fed three-phase motors, such as safe speed limiting, do not adequately address the hazard of finger injuries during maintenance, as they do not effectively limit torque, leading to potential injuries when fingers are introduced between moving parts.
Innovation Solution
A method and apparatus for safely limiting torque in frequency-converter-fed three-phase motors operated under field-oriented control, involving the measurement of phase currents, computation of torque-forming current components, crosswise comparison, and generation of a pulse cancellation signal to switch the motor to zero torque when predetermined limits are exceeded, utilizing a two-channel computer structure for safe and reliable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional safety measures (safe speed limiting) are used, then the machine can operate with basic protective functions, but torque is not effectively limited leading to potential finger injuries when fingers are introduced between moving parts
Solution Approach 1:
The patent replaces mechanical torque limiting devices (sliding clutches, weak points, torque sensors) with an electrical control system that computes torque-forming current components and generates pulse cancellation signals to limit torque electronically, thereby eliminating the need for mechanical safety devices
Solution Approach 2:
The system continuously monitors phase currents, computes torque-forming current components based on rotor position angle, compares computed values against predetermined limit values, and dynamically adjusts motor torque through pulse cancellation signals to maintain safe operating levels
2Object-affected harmful factors
If sliding clutches or torque sensors are used for torque limiting, then torque can be limited to safe levels, but the system requires additional mechanical components and longer shutdown times
Solution Approach 1:
The patent eliminates mechanical torque limiting components (sliding clutches, torque sensors) by implementing pure electrical torque control through field-oriented control and pulse cancellation, reducing device complexity while maintaining torque limitation capability
Solution Approach 2:
The system uses computational models to calculate torque-forming current components based on measured phase currents and rotor position, creating a virtual representation of torque that can be monitored and limited without physical torque sensing devices
3Reliability
If a two-channel computer structure is implemented for safe torque limiting, then reliability and fault detection capability are improved, but device complexity increases
Solution Approach 1:
The control system is divided into two independent computational channels that each perform complete torque calculation and monitoring, allowing cross-comparison of results to detect faults while distributing computational load and improving reliability through redundancy
Data Source
AI summary
A method for safely limiting the torque of a frequency-converter-fed three-phase motor operated under field-oriented control (controlled torque) is disclosed. Actual phase current values of the three-phase motor are used to calculate a sum current signal on two channels which is checked for a sum current value of zero. A rotor position angle is determined on the two channels, and torque-forming current components of the two orthogonal field-oriented current components of a stator current space vector are calculated on the two channels as a function of the measured phase currents and the rotor position angle. The three-phase motor is switched on the two channels without applying a torque if the sum value is not equal to zero and/or if the torque-forming current components calculated on the two channels are not equal. This produces a safety function “safe torque limiting” which can be integrated in commercially available frequency converters.