Brushless Permanent Magnet Motor Control for Lower Rotor Eddy Currents
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Solution Overview
Problem
The operational temperature of rotors in brushless permanent magnet motors is a concern due to high eddy currents induced by time harmonic variation of the stator magnetic field, leading to increased magnet temperature and potential mechanical and magnetic performance issues.
Innovation Solution
A brushless permanent magnet motor controller divides the electrical cycle into two portions, applying different sets of voltage vectors in each portion, using a combination of five-step and seven-step space vector pulse width modulation to reduce peak-to-peak current ripple and eddy currents, enabling accurate rotor position determination without additional sensors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional voltage vector control is applied throughout the entire electrical cycle, then control simplicity is maintained, but rotor temperature rise increases due to high eddy currents
Solution Approach 1:
The electrical cycle is divided into two distinct portions: a first portion (e.g., 60-120 degrees) where voltage vectors are applied with low-side switches turned on to enable current measurement for rotor position determination, and a second portion (remaining 240-300 degrees) where zero-voltage vectors are applied with all low-side switches turned off to minimize eddy currents and reduce rotor temperature. This segmentation allows optimization of both temperature control and position sensing in different cycle segments.
2Measurement precision
If low-side switches are kept on during the first portion of the electrical cycle to enable current measurement, then rotor position determination accuracy is improved, but eddy currents increase leading to higher rotor temperature
Solution Approach 1:
The control strategy applies different voltage vector sequences periodically within the electrical cycle. During the first portion, voltage vectors are applied periodically to generate measurable current for position determination. During the second portion, zero-voltage vectors are applied periodically to minimize eddy currents. This periodic alternation between measurement and temperature control modes resolves the contradiction between measurement accuracy and temperature management.
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 reduces rotor temperature rise and enhances control flexibility by minimizing eddy currents, allowing for precise rotor position estimation using a single current monitor and reducing the need for additional hardware.
Implementation Method 1
The operational temperature of the rotor is principally impacted by the amount of loss generated in the rotor, of which a significant amount is induced by time harmonic variation of the stator magnetic field. The higher the RMS value of higher order phase current ripple, the higher the eddy currents induced in the rotor permanent magnets
Implementation Method 2
a low side switch is always on during the first portion of the electrical cycle, which may enable determination of a current value indicative of current flowing through the phase winding, for example by measuring a voltage across the low side switch
Data Source
AI summary
A brushless permanent magnet motor has a phase winding, an inverter for applying voltage vectors to the phase winding, and a controller for controlling the inverter. The inverter has a plurality of high side switches and a plurality of low side switches. The controller is configured to divide an electrical cycle of the motor into a first portion and a second portion different to the first portion, apply a first set of voltage vectors to the phase winding in the first portion of the electrical cycle, and apply a second set of voltage vectors to the phase winding in the second portion of the electrical cycle. The second set of voltage vectors is different to the first set of voltage vectors. The controller is configured to turn on a low side switch to apply each voltage vector of the first set of voltage vectors, and turn off all low side switches to apply a zero-voltage vector of the second set of voltage vectors.