Variable Speed Motor Torque Control via Compensation Mapping
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Solution Overview
Problem
In subsea multiphase pump systems, the long distance between the variable speed drive (VSD) and the motor, coupled with transformers and cables, leads to voltage drops and current losses, making it challenging to maintain proper torque control and mechanical performance of the subsea motor.
Innovation Solution
A method and system involving a first controller for torque control and a second controller for compensation, using a representation of compensation torque as a function of mapped torque and speed parameters, to adjust the motor's operation and estimate motor currents and voltages, thereby minimizing the difference between the torque reference and shaft torque, without relying on subsea sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a long distance power transmission device (transformers and cables) is used between VSD and motor, then power can be transmitted from topside to subsea, but voltage drops and current losses occur resulting in poor torque control
Solution Approach 1:
The invention changes the control parameters by introducing a compensation torque calculated from a pre-determined map that accounts for the specific voltage drop and current loss characteristics of the power transmission device. This allows the VSD to adjust its output parameters dynamically to maintain accurate torque control despite the long transmission distance
Solution Approach 2:
The invention creates a simplified representation (copy) of the complex power transmission device effects through a pre-determined compensation map. This map copies the essential characteristics of voltage drop and current loss without requiring complex real-time measurements or models of the actual transmission components
2Measurement precision
If subsea sensors are installed to monitor motor status, then accurate motor monitoring can be achieved, but system complexity and cost increase
Solution Approach 1:
The system performs self-monitoring by using the existing VSD measurements and calculations to determine motor status parameters such as torque, power, and efficiency. The controller calculates these parameters from electrical measurements already taken for control purposes, eliminating the need for separate monitoring sensors
Solution Approach 2:
The existing electrical measurements and control algorithms in the VSD are made multi-functional by using them both for torque control and for motor status monitoring. The same current and voltage measurements used for control are also used to calculate power, efficiency, and other diagnostic parameters
3Stability of the object's composition
If VSD is positioned topside close to power source, then power supply stability is improved, but the long distance to subsea motor causes significant voltage drop and current loss
Solution Approach 1:
The invention compensates for transmission losses by dynamically adjusting the VSD output parameters based on a pre-determined compensation map. The map provides correction factors that account for the cumulative effect of voltage drop and current loss over the long transmission distance, allowing the VSD to maintain stable and efficient power delivery
Data Source
AI summary
The present invention relates to a method of operating a variable speed motor (1) drivingly connected to a multiphase pump (3) via a shaft (4). A power transmission device (20) for transmission of power to the variable speed motor (1) from a power source (2) is provided. A first controller (30) is provided between the power source (2) and the power transmission device (20) for controlling the variable speed motor (1). A speed parameter (n) representative of a motor speed (nmot) is calculated in the first controller (30). A torque reference (Tref) is received in the first controller (30). A second controller (40) is provided in communication with the first controller (30) for compensation of the effect of the power transmission device (20). The second controller (40) comprises a representation of a compensation torque (ΔTmap) as a function of a mapped torque parameter (Tmap) and a mapped speed parameter (nmap). The second controller (40) is arranged to receive the torque reference (Tref) and the speed parameter (n) from the first controller (30). The compensation torque (ΔTmap) for the speed parameter (n) and torque reference (Tref) is calculated in the second controller (40) based on the said representation. Then, the first controller (30) is arranged to receive the compensation torque (ΔTmap) from the second controller (40) and the first controller is controlling the variable speed motor (1) based on the received compensation torque (ΔTmap), to keep the difference between the torque reference (Tref) and the shaft torque (Tmot) as small as possible.