Sensorless PM Brushless Motor Speed Detection During Zero Vector Braking
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Solution Overview
Problem
Existing methods for detecting the speed of a Permanent Magnet Brushless Motor during rotation, particularly in applications like washing machines, face challenges when the motor applies a Zero Vector, as there is no current flowing in the DC bus and no sensors are present, making it difficult to determine the motor speed during braking.
Innovation Solution
The solution involves detecting the voltage across a switch of a switching stage connected to a phase of the motor during the Zero Vector, determining the current direction, and calculating the motor speed based on transitions from one current direction to another, using a high voltage gate driver integrated circuit (HVIC) with a bootstrap power supply to compare voltages and output signals indicating current sign and frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If sensorless control is used to eliminate sensors, then device complexity and cost are reduced, but speed detection capability during Zero Vector phase is lost
Solution Approach 1:
The patent uses the bootstrap capacitor voltage as an intermediary signal to indirectly detect motor speed. Instead of directly measuring current or speed, the voltage across the bootstrap capacitor serves as a mediator that reflects the motor's operating state during Zero Vector phase, enabling speed detection without direct sensors.
Solution Approach 2:
The patent replaces physical sensors with an electrical measurement approach. By monitoring the voltage across the bootstrap capacitor and comparing it with a reference voltage, the system substitutes mechanical/sensor-based detection with an electrical field-based measurement method.
2Ease of operation
If Zero Vector is applied for braking, then motor control is improved, but current sensing becomes impossible due to no current flowing in DC bus
Solution Approach 1:
The bootstrap capacitor voltage serves as an intermediary that indirectly indicates current flow status in the motor phases during Zero Vector braking. By monitoring this voltage and comparing it with a reference, the system can infer current direction and frequency without direct current sensing.
Solution Approach 2:
The system implements feedback by continuously monitoring the bootstrap capacitor voltage and using it to determine motor speed and current transitions. This feedback mechanism allows the controller to adjust braking performance based on real-time motor state information.
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 allows for accurate detection of motor speed during Zero Vector conditions, enabling safe stopping of the washing machine drum and determining when it is safe to open the door, by determining the current sign and frequency, thus overcoming the limitations of sensorless speed detection.
Implementation Method 1
a comparator circuit to periodically compare the voltage between the bootstrap power supply and low-side driver power supply with a threshold and to output a signal
Data Source
AI summary
A method of determining the speed of a spinning sensorless brushless motor driven by an inverter when the phase terminals of the motor are shorted including determining a voltage related to the voltage developed in a phase leg of the inverter; from the determined voltage, determining the direction of current and providing a first signal determining transitions between current flowing in each of two directions; and from the first signal, determining the frequency of the current and thus the motor speed.