Brushless DC Motor Control for Low-Speed Cogging Torque
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Solution Overview
Problem
Brushless DC motors face challenges in operating in low rotational regions due to cogging torque, which hinders their ability to drive oil pumps effectively at low oil temperatures, leading to difficulties in oil supply and potential cavitation issues.
Innovation Solution
A control device for brushless DC motors that includes a phase angle detection unit, an energization period calculation unit, and a drive control unit to manage the energization of coils based on detected cogging torque peaks, allowing the motor to operate in low rotational regions by synchronizing driving torque with cogging torque fluctuations and adjusting the duty cycle of the driving signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the driving duty is increased to overcome cogging torque peaks, then the motor can start rotation, but the motor cannot operate in low rotational region below the lower limit speed
Solution Approach 1:
The patent applies dynamics by making the energization period variable rather than fixed. The control device dynamically adjusts the energization period based on the phase angle signal switching timing, allowing the motor to adapt to cogging torque fluctuations. This enables operation at low speeds by extending the energization period when needed to overcome cogging torque peaks, while maintaining normal operation at higher speeds.
Solution Approach 2:
The patent changes the parameter of energization period duration based on operating conditions. By detecting when the phase angle signal switches (indicating cogging torque peak timing) and adjusting the energization period accordingly, the system can provide sufficient driving torque to overcome cogging torque at low speeds while maintaining efficient operation at higher speeds.
2Adaptability or versatility
If the motor operates in low rotational region, then oil pump can be driven at low temperature, but cogging torque hinders rotation
Solution Approach 1:
The patent applies preliminary action by detecting the phase angle signal switching timing in advance, which indicates when cogging torque peaks occur. Based on this detection, the control device pre-adjusts the energization period to ensure sufficient driving torque is applied before the rotor encounters the cogging torque peak, enabling successful rotation even in the low rotational region.
3Device complexity
If fixed energization period is used, then control is simple, but motor cannot overcome cogging torque at low speeds
Solution Approach 1:
The patent applies feedback by using the phase angle signal from hall sensors to detect rotor position and cogging torque peak timing. This feedback information is used to dynamically adjust the energization period, creating a closed-loop control system that ensures reliable operation across the full rotational range while maintaining reasonable control complexity.
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 the brushless DC motor to operate reliably in low rotational regions without being hindered by cogging torque, ensuring effective oil pump operation even at low oil temperatures, thus preventing cavitation and maintaining oil supply.
Implementation Method 1
a inverter circuit provided between the coil of each of the phases of the brushless DC motor and a DC power supply, and energizing the coil of each of the phases by switching a plurality of switching elements according to an input of a driving signal
Implementation Method 2
A hall sensor corresponding to each of the phases is disposed at an interval of 120 degrees in the brushless DC motor, and a phase angle at an interval of 60 degrees is determined based on a combination of detection signals output from each sensor
Data Source
AI summary
The present invention includes a phase angle detection unit generating a phase angle signal switched at a timing at which a cogging torque generated with the rotation of a rotor of a brushless DC motor reaches near a peak on a negative side hindering the rotation of the rotor, an inverter circuit energizing coils of respective phases of the brushless DC motor by switching elements according to an input of a driving signal, an energization period calculation unit calculating an energization period Tw from a target rotation speed set for the brushless DC motor, and a drive control unit energizing the coils sequentially by outputting the driving signal to the respective switching elements for each energization period, gradually increasing a duty of the driving signals to the switching elements at a start of each energization period, and decreasing the duty after the phase angle signal is switched.