Shaft Torque Sensing via Dual Stator-Rotor Assemblies
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Solution Overview
Problem
The low torsional stiffness of the shaft in electric submersible pumps (ESP) leads to easy twisting during operation, making it difficult to accurately sense rotor position and control the motor for higher power density and efficiency.
Innovation Solution
A system and method for measuring shaft torque in a downhole permanent magnet motor by adding two sensor rotor-stator sections at either end of the shaft and applying Fourier transform to the resultant voltages, using existing power cables to communicate rotational position without additional sensing wires.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the shaft diameter is made smaller to reduce motor size, then the motor achieves higher power density, but the torsional stiffness of the shaft decreases making it difficult to accurately sense rotor position
Solution Approach 1:
The motor is divided into multiple independent rotor-stator assemblies (first, main, and second assemblies) positioned at different locations along the shaft. Each assembly independently measures rotor position, and by comparing measurements from multiple locations, the system can detect and compensate for shaft twist, thereby maintaining accurate rotor position sensing despite the shaft's low torsional stiffness
Solution Approach 2:
Multiple rotor-stator assemblies act as intermediaries to indirectly measure shaft twist. Instead of directly measuring the difficult-to-detect shaft twist with a single sensor, the system uses multiple measurement points along the shaft to infer twist through comparative analysis, enabling accurate rotor position determination
2Measurement precision
If multiple rotor-stator assemblies are added to measure shaft twist, then rotor position sensing accuracy improves, but the device complexity increases
Solution Approach 1:
The additional rotor-stator assemblies serve dual functions: they generate torque contribution to drive the pump while simultaneously providing rotor position measurement data. This multi-functionality reduces the need for separate sensing components and justifies the added complexity through performance benefits
Solution Approach 2:
The sensing function is merged with the motor drive function by using the same rotor-stator assemblies for both torque generation and position measurement. The stator windings and rotor structures serve both propulsion and sensing purposes, integrating multiple functions into unified components
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
Enables precise control of the motor for improved efficiency and reliability by directly measuring torque, eliminating the need for additional wires and providing accurate rotor position feedback.
Implementation Method 1
A permanent magnet motor of an ESP is a synchronous motor and has embedded magnets in a rotor. The speed at which the rotor turns is in lockstep with a rotational speed of a magnetic field created by a stator.
Implementation Method 2
the torsional stiffness of the shaft of the motor is low and will easily twist during normal operation of the ESP... induces a twist on the motor shaft, which can be used to determine torque
Data Source
AI summary
The present disclosure relates to methods and systems for determining the torque applied to a long shaft of a motor. A three-phase motor has a shaft, a main rotor-stator assembly and two sense stator-rotor assemblies. A control circuit applies three phase power to cables connected to the assemblies. At a measurement circuit, time-varying voltages on the cables are measured, which include the voltage induced due to the two sense rotor-stator assemblies. The sense assemblies differ from the main motor assembly to induce additional, small voltages into the electrical circuit. A transformation is applied to these signals to determine the main frequencies, amplitudes, and phases of the different signals. Using the phase differences among the voltages induced by the first and second sense rotor-stator assemblies and the main rotor-stator assembly, the torque on the shaft may be determined.