PMSM Field-Weakening Control via Stator Current Phase Shift
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Solution Overview
Problem
Existing methods for field-weakening in permanent-magnet synchronous induction machines are inadequate for highly dynamic applications, particularly in low-inertia servomotors, leading to inefficient reactive power consumption, oversized converters, and limitations in speed range due to voltage and current limitations.
Innovation Solution
A method that involves a phase shift of the stator current vector to reduce the stator current magnitude, allowing the current regulators to operate within the voltage headroom, with the current vector rotating along an elliptical trajectory in the d-q coordinate system, avoiding transient torque drops and maintaining control during dynamic operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the stator voltage is increased proportionally with frequency to maintain magnetic flux and torque, then the magnetic flux and torque remain constant, but the voltage induced by the rotor eventually exceeds the available stator voltage, limiting further speed increase
Solution Approach 1:
The patent changes the control parameter from maintaining constant magnetic flux to allowing magnetic flux to decrease deliberately. By reducing the stator voltage increase rate below the frequency increase rate, the magnetic flux is reduced, which in turn reduces the induced rotor voltage, enabling operation at higher speeds beyond the base speed range
Solution Approach 2:
Instead of increasing stator voltage to maintain flux (conventional approach), the patent inverts the approach by deliberately reducing flux through controlled voltage reduction. This field-weakening strategy allows the motor to operate in the constant power region at speeds above base speed
2Speed
If field-weakening is achieved by increasing stator current to counteract permanent magnet field, then higher speeds can be reached, but the reactive power requirement increases significantly
Solution Approach 1:
The patent applies partial field-weakening by controlling the stator voltage increase rate to be less than the frequency increase rate, rather than attempting to maintain constant flux. This partial reduction in magnetic flux is sufficient to enable high-speed operation while minimizing the reactive power penalty associated with strong field-weakening currents
3Device complexity
If simple speed control adjusts frequency and voltage proportionally, then the control is simple, but the feedback on excitation field changes reduces regulation quality
Solution Approach 1:
The patent employs feedback control by continuously monitoring the actual magnetic flux (derived from stator voltage and frequency) and adjusting the voltage-frequency relationship accordingly. This ensures that the field-weakening process maintains optimal regulation quality while enabling extended speed range operation
4Reliability
If converters are sized to supply both shaft power and reactive power for inductances, then the machine can operate, but the converter becomes oversized particularly in low-inertia servomotors
Solution Approach 1:
By changing the operating strategy to accept reduced magnetic flux at high speeds, the patent reduces the reactive power demand on the converter. This allows the converter to be sized more appropriately for the shaft power requirement rather than being oversized to accommodate excessive reactive power for field-weakening
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
Enables stable operation and increased speed in field-weakened mode without torque drops or oscillations, allowing for more efficient reactive power consumption and the use of less powerful converters, suitable for highly dynamic servo applications.
Implementation Method 1
The field-weakening effect is achieved in the case of the permanent-magnet synchronous induction machine by means of a stator field that counteracts the field of the permanent magnets
Implementation Method 2
They consist of a stator that carries the rotating field winding and a rotor that carries permanent magnets for magnetic excitation. A voltage is therefore induced in the stator when the rotor rotates
Implementation Method 3
If the machine is operated as a motor, the rotor is loaded and rotates with the rotary field of the stator, with the stator and rotor being offset from one another by a pole wheel angle
Data Source
Figure 1
Figure 2
Figure 3~4
AI summary
The method involves adjusting amount of stator current that flows during field impairing operation of a rotary field machine (9), to be smaller than maximum stator current, during non-field impairing operation of the rotary field machine. The field impairment takes place by a stator magnetic field that acts against field of permanent magnets. The stator magnetic field is caused by phase shifting (theta i) of the stator current. The stator voltage of the rotary field machine is pulse-width modulated using a modulator (7). An independent claim is also included for a converter for operation of the permanent magnet excited synchronous rotary field machine.