Dynamic Braking Current Phase Alignment with Back-EMF
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Solution Overview
Problem
Existing dynamic braking systems in electric motors, such as those used in permanent magnet AC motors, are inefficient at high speeds due to current flowing almost entirely in quadrature to the back EMF, resulting in reduced stopping torque and potential damage to the motor or drive system.
Innovation Solution
A dynamic braking system employing a current regulator that sets the q-axis voltage to zero, ensuring the braking current is in phase with the motor's internal EMF, maximizing stopping torque per ampere and implementing current limiting to control the braking current amplitude, while optionally adding damping to prevent motor ringing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a three phase short is applied across the motor windings for dynamic braking, then the motor can be stopped regardless of rotor position, but the braking current flows almost entirely in quadrature to the back EMF at higher speeds, resulting in reduced stopping torque
Solution Approach 1:
The patent applies dynamics by making the braking method adaptive to rotor position. Instead of using a fixed three-phase short circuit approach, the system dynamically selects between two braking methods: using transistors connected to a single supply bus when rotor position is unknown (providing position-independent braking), and using transistors connected to opposite supply buses when rotor position is known (maximizing stopping torque by aligning current with back EMF). This dynamic adaptation resolves the contradiction between ease of operation and braking effectiveness.
Solution Approach 2:
The patent changes the electrical parameters of the braking system based on rotor position information. When rotor position is known, the system changes the configuration by connecting transistors to opposite supply buses, which alters the current flow path to be in phase with back EMF, thereby maximizing stopping torque. This parameter change resolves the technical contradiction by transforming the braking current from quadrature (ineffective) to in-phase (effective) with back EMF.
2Productivity
If the braking current amplitude is increased to improve stopping performance, then the motor stops faster, but the peak current may cause damage to the power transistors or demagnetize the motor
Solution Approach 1:
The patent implements feedback by using rotor position information to control the braking strategy. The control system receives feedback about rotor position and adjusts the braking current path accordingly. When rotor position is known, the system uses this feedback to configure transistors for maximum torque braking with controlled current paths, preventing excessive peak currents while maintaining fast stopping performance. This feedback mechanism resolves the contradiction between stopping speed and component protection.
Solution Approach 2:
The patent uses the rotor position sensor and control system as an intermediary between the braking command and the power electronics. This intermediary processes the braking request, determines the appropriate transistor configuration based on rotor position, and controls the current flow paths to maximize braking torque while preventing damaging peak currents. The intermediary protects the system by intelligently managing the braking process.
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
This approach significantly enhances the motor's braking efficiency, allowing for faster and more reliable stopping, reducing the risk of damage, and ensuring that all braking current contributes to stopping torque, with improved damping to prevent oscillations.
Implementation Method 1
In a dynamic braking mode, the motor usually operates as a generator and either dissipates energy into the motor windings, or dissipates energy into a resistive load
Implementation Method 2
A back-biased diode, or flyback diode, is commonly connected across each of the emitter-collector circuits of the switching transistors to bypass transients from the switching control of the inductive motor load
Implementation Method 3
Dynamic braking is achieved by simultaneously rendering conductive the three transistors connected to a positive bus, or the three transistors connected to a negative bus. When three such transistors are simultaneously rendered conductive, current flows from one or more motor windings to one of the supply busses through one or two of the conductive transistors
Data Source
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AI summary
A system and method are provided for improved dynamic braking in AC motors with an electronic drive, and more particularly to using a current regulation circuit to control the current supplied to the motor to be in phase with the internal EMF voltage of the motor such that the braking torque of the current is maximized per ampere of dynamic braking current when needed to stop the motor in case of a control failure or emergency. A current regulator produces a voltage command to the motor based on the current command input. The motor is still controlled by a d-q current regulator and the q-axis (torque axis) voltage is dnven to zero while the d-axis (non-torque axis) is left in current control with a zero current command. This way the motor internal voltage drive current in the terminals of the motor but the current is in phase with the internal voltage of the motor.