Permanent-Magnet Motor Freewheel Angle Control at High Back-EMF
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Solution Overview
Problem
Permanent-magnet motors face challenges in controlling power and efficiency as the rotor speed increases, leading to difficulties in driving current and maintaining constant power due to rising back electromotive force (EMF), which results in inefficiencies and current spikes.
Innovation Solution
The method involves sequentially exciting and freewheeling the motor winding, varying the freewheel angle in response to speed changes, and using a control system with a position sensor and current controller to manage excitation and freewheeling, ensuring constant power and high efficiency across a wide speed range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the rotor speed increases, then the back emf magnitude increases, but it becomes increasingly difficult to drive current and power into the motor
Solution Approach 1:
The patent applies dynamics by making the freewheel angle variable rather than fixed. The control system dynamically adjusts the freewheel angle based on the motor's operating speed and back emf conditions. This allows the motor control to adapt to changing back emf magnitudes at different speeds, enabling current to be driven into the winding over an extended period even at high speeds where back emf is large, thus maintaining power input capability across the speed range.
2Power
If the excitation voltage exceeds the falling back emf, then current spikes arise, but motor control becomes unstable
Solution Approach 1:
The patent applies preliminary action by implementing a controlled freewheeling phase before full excitation. During the freewheeling interval, the winding is allowed to freewheel at a controlled angle, which prepares the current path and prevents sudden current spikes when excitation voltage is applied. This preliminary freewheeling action ensures that when excitation occurs, the current transitions smoothly rather than spiking, maintaining waveform stability.
3Device complexity
If the freewheel angle is fixed, then control simplicity is maintained, but power and efficiency control deteriorates with speed variations
Solution Approach 1:
The patent applies parameter changes by varying the freewheel angle parameter according to motor speed and back emf conditions. Instead of using a fixed freewheel angle, the control system adjusts this parameter dynamically - decreasing the freewheel angle as speed increases to maintain optimal power transfer. This parameter adaptation enables effective power control across the speed range while keeping the control methodology relatively simple, avoiding the need for complex multi-parameter control schemes.
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 efficient motor operation with substantially constant power and efficiency of at least 75% over a 10 krpm operating speed range, preventing current spikes and maintaining performance despite variations in speed and excitation voltage.
Implementation Method 1
As the rotor of a permanent-magnet motor rotates, it induces a back emf in a winding of the motor
Data Source
AI summary
A method of controlling a permanent-magnet motor that includes sequentially exciting and freewheeling a winding of the motor. The method includes varying the angle over which the winding is freewheeled in response to changes in speed of the motor. Additionally, a control system for a permanent-magnet motor, and a product incorporating the control system and motor.