Permanent-Magnet Motor Control for Back-EMF Power Stability
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Solution Overview
Problem
Controlling a permanent-magnet electric machine becomes challenging due to increasing back emf as the rotor accelerates, making it difficult to drive current and power effectively, especially with varying excitation voltages and speeds.
Innovation Solution
The method involves sequentially exciting and freewheeling the stator winding by varying the advance and freewheel angles in response to changes in excitation voltage, ensuring constant power and efficiency across a range of voltages, and adjusting these angles based on speed and voltage levels using lookup tables and correction values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the rotor accelerates, 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 excitation ensures that current is established in the winding before the back emf reaches its peak, allowing power to be delivered effectively even as the rotor accelerates and back emf increases.
Solution Approach 2:
The advance angle and freewheel angle are varied dynamically in response to changes in excitation voltage and rotor speed. As speed increases, the advance angle is increased and the freewheel angle is decreased, allowing the control strategy to adapt to changing back emf conditions and maintain effective power delivery across the operating range.
2Power
If the excitation voltage decreases, then the power delivery capability reduces, but maintaining constant power requires adjusting control parameters
Solution Approach 1:
The control system varies the advance angle and freewheel angle in response to changes in excitation voltage. By monitoring voltage levels and adjusting the timing parameters accordingly, the system maintains constant power delivery across a broad voltage range (from 80% to 120% of nominal voltage) without requiring complex hardware modifications.
Solution Approach 2:
The method changes the temporal parameters (advance angle and freewheel angle) of the excitation waveform in response to voltage variations. When excitation voltage decreases, the advance angle is increased and freewheel angle is decreased to compensate, maintaining the same power output despite the lower voltage input.
3Power
If the winding is excited in advance of back emf zero-crossings, then power delivery is improved, but current spikes may arise when excitation voltage exceeds falling back emf
Solution Approach 1:
The winding is excited in advance of the zero-crossings of back emf by an advance angle, which improves power delivery by establishing current before back emf peaks. However, this early excitation can cause current spikes when the excitation voltage exceeds the falling back emf.
Solution Approach 2:
The freewheeling phase, introduced to manage the harmful effect of current spikes, is converted into a beneficial feature. By freewheeling the winding over a freewheel angle in the region of falling back emf, the system not only avoids current spikes but also achieves smoother current waveforms and improved efficiency, as the falling back emf naturally assists in current decay during this phase.
4Power
If the freewheel angle is decreased, then current is driven into the winding for a longer period increasing power, but the efficiency may be reduced in certain operating regions
Solution Approach 1:
The freewheel angle is varied dynamically in response to changes in excitation voltage and operating conditions. By optimizing the freewheel angle for different voltage and speed ranges, the system achieves a balance between extending current conduction for higher power and maintaining efficiency by freewheeling during regions of falling back emf where less torque is required.
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 allows for consistent power delivery and high efficiency (at least 75%) over a broad range of excitation voltages and speeds, preventing current spikes and maintaining smooth current waveforms, even as the battery discharges.
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 a single-phase permanent-magnet electric machine, the method comprising: sequentially exciting and freewheeling a winding of the electric machine, wherein the winding is excited by an excitation voltage in advance of zero-crossings of back emf in the winding 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 the excitation voltage. Additionally, a control system for an electric machine, and a vacuum cleaner incorporating the control system and electric machine.