Electric Machine Winding Timing for Back-EMF Power Control
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Solution Overview
Problem
As the permanent-magnet rotor of an electric machine rotates, it induces a back emf that increases with speed, making it difficult to drive current and power into the machine, leading to inefficient control over power delivery.
Innovation Solution
The method involves sequentially exciting and freewheeling the winding by varying the advance and freewheel angles in response to changes in excitation voltage, allowing for better control over efficiency and power, and using a control system with a position sensor, current controller, and inverter to manage these processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the permanent-magnet rotor rotates at higher speed, then the back emf increases, but it becomes increasingly difficult to drive current and power into the electric machine
Solution Approach 1:
The winding is excited in advance of the zero-crossings of back emf by an advance angle. This preliminary action allows current to be driven into the winding before the back emf reaches its peak, overcoming the increasing difficulty of power delivery at higher speeds.
Solution Approach 2:
The advance angle and freewheel angle are dynamically varied in response to changes in excitation voltage and rotor speed. This dynamic adjustment optimizes the timing of excitation and freewheeling intervals to maintain effective power delivery across varying operating conditions.
2Power
If the excitation voltage decreases, then the power delivery capability reduces, but maintaining constant power requires increasing advance angle and decreasing freewheel angle
Solution Approach 1:
The control system varies the advance angle and freewheel angle in response to changes in excitation voltage. This feedback mechanism automatically adjusts the timing parameters to maintain substantially constant power output despite variations in excitation voltage, achieving ±5% power stability.
Solution Approach 2:
The system changes the timing parameters (advance angle and freewheel angle) in response to changing excitation voltage conditions. By adjusting these parameters dynamically, the system compensates for voltage drops during battery discharge and maintains constant power delivery.
3Loss of energy
If the winding is freewheeled within the region of falling back emf, then efficiency improves and current spikes are avoided, but torque production decreases for a given current level
Solution Approach 1:
The system employs periodic excitation followed by periodic freewheeling intervals. During the freewheeling phase, the winding is disconnected from the excitation source and allowed to freewheel, which improves efficiency and prevents current spikes during the falling back emf region, while the periodic excitation restores torque production.
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 enables efficient power control with a high efficiency ratio of output to input power, maintaining constant power and efficiency across a range of excitation voltages, even as the battery pack discharges, and prevents current spikes, resulting in a smoother current waveform.
Implementation Method 1
As the permanent-magnet rotor of an electric machine rotates, it induces a back emf in a winding of the electric machine
Data Source
AI summary
A method of controlling an electric machine that includes sequentially exciting and freewheeling a winding of the electric machine. The winding is excited in advance of zero-crossings of back emf in the winding by an advance angle, and the winding is freewheeled over a freewheel angle. The method then includes varying the advance angle and the freewheel angle in response to changes in the voltage used to excite the winding. Additionally, a control system for an electric machine, and a product incorporating the control system and electric machine.