Permanent-Magnet Motor Control for Constant Power at High Speed
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Solution Overview
Problem
As the permanent-magnet rotor of an electric machine rotates, the induced back emf increases with speed, making it difficult to maintain constant power delivery, leading to inefficiencies and control challenges over varying speed and voltage ranges.
Innovation Solution
A control system that sequentially excites and freewheels the winding of the electric machine, adjusting the advance and freewheel angles in response to speed and voltage changes to maintain constant power, thereby ensuring efficient and stable power delivery across a range of speeds and voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the rotor accelerates, then the back emf magnitude 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, overcoming the increasing back emf barrier and enabling power delivery at higher speeds
Solution Approach 2:
The control system dynamically adjusts the advance angle and freewheel angle in response to changes in speed and excitation voltage. This dynamic adaptation allows the system to maintain optimal current drive capability across the entire operating speed range, resolving the contradiction between speed increase and power delivery difficulty
2Power
If the excitation voltage exceeds the falling back emf, then current spikes arise, but this causes instability and potential damage
Solution Approach 1:
The control system freewheels the winding in advance before the back emf falls below the excitation voltage level. This preliminary freewheeling action provides a safe current path before the dangerous condition occurs, preventing current spikes and maintaining reliability
Solution Approach 2:
The control system uses the falling back emf region, which would normally cause harmful current spikes, as an opportunity to implement freewheeling. By switching to freewheel mode in this region, the system converts a potentially harmful condition into a beneficial opportunity to maintain current stability and protect the system
3Loss of energy
If the control system maintains constant power over a wide speed range, then efficiency improves, but the control complexity increases
Solution Approach 1:
The control system changes the parameters of advance angle and freewheel angle to maintain constant power across the operating range. By adjusting these two parameters in response to speed and voltage changes, the system achieves constant efficiency without requiring complex control mechanisms
Solution Approach 2:
The control system uses feedback from speed and voltage measurements to dynamically adjust the advance and freewheel angles. This feedback mechanism enables automatic constant power maintenance across varying operating conditions while keeping the control logic relatively simple
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 solution achieves substantially constant power with minimal variance (+/â5%) and high efficiency (at least 80%) across the operating speed and voltage ranges, preventing current spikes and improving the motor's performance and reliability.
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
An electric system that includes a single-phase permanent-magnet electric machine and a control system for driving the electric machine. The control system sequentially excites and freewheels a winding of the electric machine so as maintain substantially constant power over an operating speed range spanning at least 10 krpm and/or 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 product comprising the electric system.