Single-Phase PM Motor Control for Constant Power at High Speed
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Solution Overview
Problem
Existing electric systems with permanent-magnet rotors face challenges in maintaining constant power delivery as the rotor accelerates, due to increasing back emf, making it difficult to control power efficiently across varying speeds and voltages.
Innovation Solution
A control system that sequentially excites and freewheels the stator winding of a single-phase permanent-magnet electric machine, adjusting the advance and freewheel angles in response to speed and voltage changes to maintain constant power, ensuring efficiency and smooth current waveform.
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 control system excites the winding in advance of the zero-crossings of back emf by an advance angle. This preliminary action allows current to be established in the winding before the back emf reaches its peak opposing value, enabling power to be driven into the machine even at high speeds where back emf would otherwise prevent current flow.
Solution Approach 2:
The control system dynamically varies the advance angle and/or freewheel angle in response to changes in speed and excitation voltage. This dynamic adjustment optimizes the timing of excitation and freewheeling intervals to maintain substantially constant power delivery across a wide operating range from 60 krpm to 100 krpm and voltage ranges where minimum voltage is less than 80% of maximum voltage.
2Power
If the excitation voltage exceeds the falling back emf, then current spikes arise, but the winding needs to be excited to maintain power
Solution Approach 1:
The control system excites the winding in advance of the zero-crossings of back emf by an advance angle. This timing ensures that excitation occurs when back emf is rising and has not yet fallen to levels that would cause current spikes, while still maintaining power delivery by establishing current before the optimal torque-producing region.
Solution Approach 2:
The control system implements freewheeling where the winding is disconnected from the excitation voltage source but current continues to flow through the winding. This continuous action maintains current flow and power delivery while preventing the abrupt current spikes that would occur if excitation voltage were applied when back emf has fallen too low.
3Ease of operation
If the winding is excited in synchrony with rotor position signal changes, then control is simple, but power delivery becomes difficult at high speeds due to back emf
Solution Approach 1:
Instead of exciting the winding in synchrony with rotor position signal changes, the control system excites the winding in advance of the zero-crossings of back emf by an advance angle. This preliminary excitation ensures current is established before back emf opposes further current flow, enabling effective power delivery at high speeds while maintaining relatively simple control logic.
Solution Approach 2:
The control system changes the timing parameter (advance angle) of winding excitation relative to the rotor position signal. By varying this parameter in response to speed and voltage changes, the system maintains optimal power delivery across the operating range from 60 krpm to 100 krpm without requiring complex control mechanisms.
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
The system achieves constant power and high efficiency across a wide speed and voltage range, preventing current spikes and maintaining efficient operation even at high speeds, with an efficiency ratio of at least 80%.
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
An electric system comprising a single-phase permanent-magnet electric machine and a control system for driving the electric machine, wherein the control system sequentially excites and freewheels a winding of the electric machine so as maintain substantially constant power over at least one of an operating speed range spanning at least 10 krpm and an excitation voltage range extending between a minimum voltage and a maximum voltage, the minimum voltage being less than 80% of the maximum voltage. Additionally, a vacuum cleaner comprising the electric system.