Brushless Compressor Motor Braking Using Back-EMF Voltage Control
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Solution Overview
Problem
Existing compressor braking methods for refrigeration devices with brushless motors, such as PMSM and BLDC motors, require complex pump designs and risk hitting the compressor shell due to unknown braking forces and potential resonance frequencies during freewheeling.
Innovation Solution
A method involving determining induced voltages and supplying a braking current with decreasing frequency based on these voltages to control the speed reduction, ensuring the rotor is aligned with the stator field, even at low speeds where rotor position determination is unreliable, thus preventing resonance and simplifying the pump design.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the compressor is allowed to freewheel during braking, then the braking process is simple, but the compressor may hit resonant frequencies and strike the compressor shell
Solution Approach 1:
The patent replaces the mechanical freewheeling braking method with an electromagnetic braking system. The control unit energizes the motor windings with a braking current that generates a magnetic field opposing the rotor motion, providing controlled electromagnetic braking force instead of uncontrolled mechanical freewheeling, thus avoiding resonant frequencies and compressor shell contact.
Solution Approach 2:
The patent changes the electrical parameters of the motor during braking by applying a specifically calculated braking current to the windings. The braking current magnitude is determined based on the back EMF voltage and resistance values, transforming the motor from motor mode to generator/brake mode, enabling controlled deceleration that avoids harmful resonances.
2Speed
If a short circuit braking method is used below speed threshold, then braking is achieved quickly, but a complex and robust pump design is required
Solution Approach 1:
The patent replaces the mechanical short-circuit braking method with electromagnetic braking. Instead of short-circuiting the windings to create a purely resistive brake, the control unit applies a controlled braking current that generates electromagnetic braking force, providing smoother and more controllable deceleration that reduces mechanical stress on the pump components.
Solution Approach 2:
The patent uses feedback from the back EMF voltage measurement to determine the appropriate braking current. The control unit continuously monitors the induced voltage in the windings and adjusts the braking current accordingly, creating a closed-loop control system that optimizes braking performance while minimizing mechanical stress on the pump.
3Measurement precision
If rotor position determination is used for speed control, then precise speed control is achieved above threshold, but control becomes unreliable below the speed threshold
Solution Approach 1:
The patent uses the motor's own back EMF voltage, which is naturally generated during rotation, to determine the braking current. This self-service approach eliminates the need for external sensors or complex position determination systems, providing reliable braking control across the entire speed range including very low speeds where traditional position sensing fails.
Solution Approach 2:
The patent replaces the sensor-based rotor position determination system with an electromagnetic field-based braking control system. By using the back EMF voltage to directly control the braking current, the system achieves reliable low-speed control without depending on mechanical or sensor-based position detection that becomes unreliable at low speeds.
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 allows for controlled and reliable braking of the compressor, avoiding resonance frequencies and reducing the complexity of the pump design, ensuring safe operation and minimizing the risk of the compressor hitting the shell.
Implementation Method 1
below the target speed, the speed control cannot be used because the rotor position can no longer be reliably determined by measuring induced voltages due to back EMF in the stator windings
Implementation Method 2
When current flows through the windings, they generate a magnetic field that exerts a torque on the permanent magnet, thus setting the rotor in motion
Data Source
Figure 1~3
AI summary
The invention relates to a method for braking a compressor of a refrigeration device, of an air conditioning device, or of a heat pump, wherein the compressor comprises a brushless motor (100) having windings (130U, 130V, 130W) and comprises a controller (160) for braking the motor. According to the invention, the controller (160) is designed to brake (306) the brushless motor (100) by means of a braking current in a controlled manner starting from an operating rotational speed (302), wherein the braking current during the controlled braking is dependent on induced voltages determined before the controlled braking. The method for braking contains the following steps: a) rotating (201) the motor at an operating rotational speed; b) receiving (202) a signal for braking; c) determining (203) voltages induced in the windings; d) supplying (204) a braking current having a decreasing frequency to the windings, wherein the braking current during the braking is dependent on the previously determined induced voltages.