Sensorless BLDC Motor Startup Using BEMF Zero Crossing Detection
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Solution Overview
Problem
Brushless direct current (BLDC) motors face challenges in starting up without position sensors, particularly at low rotational speeds, where reliable determination of back electromotive force (BEMF) zero crossings is difficult, leading to inefficient startup and potential rotor blocking, which is critical in path-dependent applications.
Innovation Solution
The startup process is divided into three phases: initial forced commutation like a stepping motor, dynamic flyback pulse blanking for reliable zero crossing detection, and transition to sensorless operation using BEMF zero crossings for commutation, with pre-commutation and current limitation to prevent blocking and achieve uniform acceleration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the motor is operated without position sensors using BEMF zero crossings for commutation, then the device complexity is reduced, but the reliability deteriorates at low rotational speeds where BEMF zero crossing detection is unreliable
Solution Approach 1:
The startup process is divided into three distinct phases: initial forced commutation phase, transition phase with dynamic flyback pulse blanking, and sensorless BEMF commutation phase. This segmentation allows the system to use different commutation strategies appropriate for each operational stage, ensuring reliability during startup while maintaining simplicity during normal operation.
Solution Approach 2:
The motor is initially commutated without position information using predetermined commutation times like a stepping motor during the startup phase. This preliminary action brings the motor up to a sufficient rotational speed before transitioning to BEMF-based sensorless commutation, ensuring that the rotor is already rotating when reliable position detection begins.
2Reliability
If the motor is started with predetermined commutation times like a stepping motor, then the reliability of startup is improved, but the use of energy increases and efficiency decreases
Solution Approach 1:
The commutation strategy is made dynamic by transitioning from predetermined commutation times during startup to BEMF zero-crossing-based commutation during normal operation. The system adapts its control method based on the operational phase, using efficient sensorless commutation as soon as the motor reaches sufficient speed, thereby minimizing energy waste while ensuring reliable startup.
3Productivity
If the motor accelerates rapidly during startup, then the productivity is improved, but the risk of rotor blocking increases
Solution Approach 1:
The system continuously monitors for BEMF zero crossings and uses this feedback to detect rotor position and motion. By verifying that zero crossings occur in the correct order and at expected intervals, the system can detect rotor blocking or missed commutation steps and take corrective action, enabling safe rapid acceleration.
Solution Approach 2:
The mechanical monitoring system is replaced with an electrical detection system that uses BEMF zero crossings to monitor rotor position and detect blocking. Instead of mechanical sensors, the system uses the electrical characteristics of the motor phases to detect rotor motion status, enabling non-contact monitoring during rapid acceleration.
4Manufacturing precision
If flyback pulses are used for current regulation, then the precision of current control is improved, but the measurement precision of BEMF zero crossings deteriorates due to signal interference
Solution Approach 1:
The harmful flyback pulses are extracted and removed from the BEMF zero crossing detection signal path. By blanking or filtering out these high-voltage switching transients, the system prevents them from interfering with the sensitive zero crossing detection circuitry, allowing both precise current control and accurate position detection to coexist.
Solution Approach 2:
A signal processing intermediary (flyback pulse blanking circuit or software filter) is introduced between the raw phase voltage signals and the zero crossing detection logic. This intermediary removes or attenuates the flyback pulse components while preserving the BEMF zero crossing information, enabling both functions to operate without mutual interference.
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 method ensures rapid and reliable startup with uniform acceleration, preventing rotor blocking and enabling efficient operation without position sensors by using dynamic flyback pulse blanking and pre-commutation, thus improving the motor's efficiency and reliability.
Implementation Method 1
the BEMF (back electromotive force) voltage is used for determining the position. This BEMF voltage represents a voltage induced by the permanent magnetic rotor that, on rotation of the rotor (i.e. not at a standstill), occurs at the stator phases
Data Source
AI summary
A method for starting a multiphase, sensorless commutated, brushless electric motor. The method has three operating phases. A start-up phase in which the motor is operated from a standstill with specified commutation times. An acceleration phase in which the motor is accelerated up to a nominal speed, wherein the commutation times are determined on the basis of the zero crossings of the BEMF voltage of the non-energized stator phase windings. And a stationary operating phase in which the nominal speed is kept constant. The transition from the start-up phase into the acceleration phase takes place when, during the start-up phase, a predetermined number of successive zero crossings of the BEMF voltage in the expected order in the expected motor phases have been identified. The transition from the acceleration phase into the stationary phase takes place once the nominal speed has been reached.