Electric Machine Winding Control for High-Speed Back-EMF
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Solution Overview
Problem
Permanent-magnet electric machines face challenges in controlling power and efficiency due to increasing back emf as speed increases, leading to difficulties in driving current and maintaining consistent power delivery.
Innovation Solution
The method involves sequentially exciting and freewheeling the winding of the electric machine, with the advance angle and freewheel angle varied in response to changes in speed, allowing for earlier current drive and extended current delivery, thereby maintaining constant power and efficiency across a range of speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the electric machine operates at higher speed, then the back emf increases, but it becomes increasingly difficult to drive current and power into the machine
Solution Approach 1:
The winding is excited in advance of the zero-crossings of back emf by an advance angle. This preliminary excitation ensures that current is driven into the winding before the back emf reaches its peak, overcoming the increasing back emf at higher speeds and maintaining effective power delivery.
Solution Approach 2:
The advance angle and freewheel angle are dynamically varied in response to changes in speed. As speed increases, the advance angle is increased and the freewheel angle is decreased, allowing the control strategy to adapt to the changing back emf conditions and maintain constant power delivery across a wide speed range.
2Power
If the excitation voltage exceeds the falling back emf, then current spikes arise, but power control becomes difficult
Solution Approach 1:
The winding is freewheeled over a freewheel angle to prepare for the falling back emf region. This preliminary freewheeling action allows current to decay smoothly before the back emf drops, preventing sudden current spikes when the excitation voltage exceeds the falling back emf.
Solution Approach 2:
The advance angle and freewheel angle are varied in response to changes in speed, creating a feedback control system. This feedback mechanism adjusts the excitation timing and duration based on the actual operating conditions, maintaining stable current waveforms and reliable power control across varying speeds.
3Device complexity
If the winding is excited in synchrony with rotor position signal changes, then control is simple, but power delivery becomes difficult at higher speeds
Solution Approach 1:
Instead of exciting the winding in synchrony with rotor position signal changes, the winding is excited in advance of the zero-crossings of back emf by an advance angle. This preliminary excitation timing ensures effective power delivery at higher speeds while maintaining relatively simple control through fixed timing relationships.
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 ensures substantially constant power and high efficiency (at least 75%) over a wide speed range, from 60 to 80 krpm, by adjusting the advance and freewheel angles based on speed and voltage, preventing current spikes and improving motor performance.
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
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AI summary
A method of controlling an electric machine, the method comprising: sequentially exciting and freewheeling a winding of the electric machine, wherein the winding is excited in advance of zero-crossings of back emf by an advance angle, and the winding is freewheeled over a freewheel angle; and varying the advance angle and the freewheel angle in response to changes in speed of the electric machine. Additionally, a control system for an electric machine, and a product incorporating the control system and electric machine.