Brushless DC Motor Control Device Ripple Current Management
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Solution Overview
Problem
Brushless DC motors controlled in a PWM manner via an inverter circuit using a position-sensorless method face challenges in accurately detecting the rotational angle due to increased ripple current, especially at high speeds, which requires larger capacitors and increased circuit size.
Innovation Solution
A motor control device and method that estimates the ripple current and switches between balanced-drive and unbalanced-drive switching patterns based on the current magnitude, performing balanced-drive switching when the ripple current is low to stabilize rotor position detection and switching noise suppression, and switching to unbalanced-drive when the ripple current increases to reduce ripple current occurrence, thereby improving position detectability and avoiding capacitor capacity increases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If balanced-drive switching is used to suppress ripple current and stabilize rotor position detection, then position detection accuracy is improved, but ripple current increases at high speeds requiring larger capacitors
Solution Approach 1:
The patent dynamically switches between balanced-drive switching and unbalanced-drive switching modes based on operating conditions (speed and load). At low speeds, balanced-drive switching is used for accurate position detection. At high speeds, unbalanced-drive switching is used to suppress ripple current, avoiding the need for larger capacitors.
Solution Approach 2:
The patent changes the switching mode parameter based on operating conditions. By detecting ripple current magnitude and switching between different drive modes, the system optimizes both position detection accuracy and ripple current suppression without requiring increased capacitor capacity.
2Quantity of substance
If unbalanced-drive switching is used to reduce ripple current, then capacitor capacity can be reduced, but rotor position detection accuracy deteriorates
Solution Approach 1:
The system dynamically selects the appropriate switching mode based on real-time operating conditions. Unbalanced-drive switching is applied only when necessary (at high speeds with large ripple current), while balanced-drive switching is used when position detection accuracy is critical (at low speeds), thus optimizing both capacitor size and detection accuracy.
Solution Approach 2:
The switching mode parameter is changed based on detected ripple current magnitude and operating speed. This allows the system to use unbalanced-drive switching for ripple current reduction when needed, while maintaining balanced-drive switching for accurate position detection during low-speed operation.
3Measurement precision
If balanced-drive switching is performed at high speeds, then rotor position can be detected accurately, but ripple current increases requiring larger smoothing capacitors
Solution Approach 1:
The patent implements dynamic switching mode selection based on motor speed and load conditions. At low speeds, balanced-drive switching maintains accurate position detection. At high speeds, unbalanced-drive switching suppresses ripple current, allowing the system to maintain position detection accuracy without increasing capacitor capacity across the full speed range.
4Quantity of substance
If larger capacitor capacity is used to suppress ripple current at high speeds, then ripple current is reduced, but device size increases
Solution Approach 1:
Instead of using a large capacitor to handle all operating conditions, the patent uses dynamic switching mode selection. The unbalanced-drive switching mode suppresses ripple current at high speeds without requiring large capacitor capacity, thus avoiding increased device size while maintaining effective ripple current suppression.
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
A motor control device for controlling a brushless DC motor (7) in a PWM control manner using an inverter circuit (6) according to a position-sensorless method includes: a ripple current estimating device (12); and a control device (13, 14). When the magnitude of the ripple current is less than or equal to a threshold value, the control device performs balanced-drive switching that a first output terminal of the inverter circuit is repeatedly and alternately connected to high and low voltage sides of a drive power supply, and the second output terminal of the inverter circuit is repeatedly and alternately connected to the low and high voltage sides. When the magnitude of the ripple current exceeds the threshold value, the control device performs unbalanced-drive switching that the first output terminal is repeatedly and alternately connected to the high and low voltage sides, and the second output terminal is connected only to the low voltage side.