Traction Motor Flux Reference via Effective Resistance
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Solution Overview
Problem
Existing methods for flux estimation in electric drive motors during braking operations face instability and inaccuracies due to variations in DC link voltage caused by factors like wheel slip and inverter cutout, leading to oscillations and inefficiencies in torque control.
Innovation Solution
A method that calculates a braking factor, resistance scale factor, and power or torque scale factors to adjust the flux estimate, using a combination of feed-forward and feedback terms based on measured torque, motor frequency, and DC link voltage, to accurately set motor torque and maintain stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If DC link voltage is used to estimate flux during braking operation, then flux estimation can be obtained, but substantial errors occur due to wheel slip and resistive grid drop out causing DC link voltage variations
Solution Approach 1:
The patent introduces an intermediary flux estimation process that decouples the direct relationship between DC link voltage and flux calculation. By using measured torque and motor frequency as intermediate variables alongside DC link voltage, the system calculates flux through a multi-parameter formula rather than direct voltage proportionality, thereby filtering out the instability caused by wheel slip and grid drop out
Solution Approach 2:
The patent implements feedback by continuously measuring actual torque and motor frequency during braking operation and using these measurements to adjust the flux estimation calculation in real-time. This feedback mechanism allows the system to compensate for DC link voltage variations by referencing actual motor operating conditions, thereby maintaining accurate flux estimation despite unstable voltage conditions
2Ease of operation
If flux reference is set based on DC link voltage measurement during braking, then flux reference can be obtained, but oscillations and instability occur due to the circular relationship between torque, power, and DC link voltage
Solution Approach 1:
The patent implements feedback by continuously measuring actual torque and motor frequency during braking operation and using these measurements to adjust the flux estimation calculation in real-time. This feedback mechanism allows the system to compensate for DC link voltage variations by referencing actual motor operating conditions, thereby maintaining accurate flux estimation despite unstable voltage conditions
Solution Approach 2:
The patent applies preliminary action by pre-calculating the flux reference using measured torque and frequency data before using it for torque control. This preliminary calculation based on actual operating conditions ensures that the flux reference is already optimized for the current state, preventing oscillations that would occur with reactive adjustments
3Productivity
If flux reference is set based on inverter torque reference in multi-inverter systems, then torque control can be implemented, but inaccuracies occur due to lack of one-to-one relationship between torque and DC link voltage
Solution Approach 1:
The patent applies segmentation by treating each inverter's flux estimation independently using its own measured torque and frequency data, rather than relying on a centralized torque reference that must be distributed across multiple inverters. This segmented approach allows each inverter to calculate its own accurate flux reference based on its actual operating conditions, eliminating the one-to-one relationship problem in multi-inverter systems
Solution Approach 2:
The patent changes the parameters used for flux reference calculation from solely torque-based or voltage-based to a combination of measured torque, motor frequency, and DC link voltage. This parameter change enables accurate flux estimation in multi-inverter systems by incorporating frequency information that reflects the actual electrical-mechanical coupling state of each motor-inverter pair
Data Source
AI summary
A traction motor system calculates motor flux by generating a real time effective resistance of a resistance grid calculated from motor torque and measured voltage on a DC link. Calculating effective resistance avoids solely relying on DC link voltage, which can be influenced by conditions such as wheel slip and drop out of one or more resistance grids. The effective resistance calculation is based on nominal motor values using known power levels and conditions. From these nominal values and the effective resistance, various scaling factors based on actual motor power can be generated and used to adjust a nominal flux reference to more accurately reflect actual motor flux. The scaling factors include power and torque scaling factors and a resistance scaling factor that is active during conditions such as wheel slip.