Sensorless Synchronous Motor Starting via VFD Control
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Solution Overview
Problem
Synchronous motors with brushless DC exciters face difficulties in starting without a speed sensor, as they require synchronization before applying significant load, and replacing the DC exciter with an AC exciter can be mechanically challenging in hard-to-access locations.
Innovation Solution
A method where a synchronous machine with a brushless DC exciter is started as an induction motor and then transitioned to synchronous motor operation without a speed sensor, using a VFD to apply magnetizing current and field current to achieve synchronization and ramp up to desired speed.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a synchronous motor with a brushless DC exciter is started without a speed sensor, then the device complexity is reduced, but the motor cannot be properly synchronized and may slip a pole causing loss of speed control
Solution Approach 1:
The patent replaces the mechanical/electrical sensing system (speed sensor) with a control-based synchronization method. The VFD controller uses current sensing and control algorithms to detect rotor position and achieve synchronization without a dedicated speed sensor, thereby reducing device complexity while maintaining reliability through software-based monitoring.
Solution Approach 2:
The patent introduces an intermediary synchronization method where the VFD controller acts as a mediator between the motor drive and the rotor position. By using current measurements and control algorithms as an intermediary mechanism, the system can infer rotor position and achieve synchronization without direct speed sensing, resolving the contradiction between reduced complexity and maintained reliability.
2Ease of operation
If the DC exciter is replaced with an AC exciter to enable starting without a speed sensor, then the starting capability is improved, but the ease of repair deteriorates due to hard-to-access locations
Solution Approach 1:
Instead of changing the exciter type from DC to AC to enable sensorless starting, the patent inverts the approach by keeping the DC exciter and developing a control method that enables sensorless operation. This inversion allows the system to achieve improved starting capability while maintaining the original exciter configuration, thus preserving ease of repair and avoiding mechanical modifications in hard-to-access locations.
3Power
If a VFD is used to start a synchronous motor with a brushless DC exciter, then the power control is improved, but the ease of operation deteriorates due to difficulty in achieving synchronization without a speed sensor
Solution Approach 1:
The patent implements feedback control where the VFD controller continuously monitors stator currents and uses this feedback to estimate rotor position and adjust the drive parameters for successful synchronization. This feedback mechanism automates the synchronization process, improving ease of operation despite the added power control capability, by allowing the system to self-adjust without manual intervention or speed sensors.
Solution Approach 2:
The patent enables the system to perform synchronization automatically through self-service control algorithms. The VFD controller uses embedded algorithms to detect rotor position, determine synchronization timing, and execute the synchronization sequence without external speed sensor input or manual operation, thereby improving ease of operation while maintaining advanced power control capabilities.
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 successful starting and synchronization of synchronous motors with brushless DC exciters without a speed sensor, overcoming the mechanical challenges of replacing exciters and ensuring stable speed control.
Implementation Method 1
The motor may be started as an induction motor by applying a magnetizing current to build flux through the stator
Implementation Method 2
a field current is then applied to the motor through the DC exciter
Implementation Method 3
Once this transition is completed, the drive may ramp up to the desired speed demand
Data Source
AI summary
A starting method and system for a motor where the motor may be started as an induction motor by applying a magnetizing current to build flux through the stator, with the field current set at the maximum permissible exciter stator current (i.e., the current that will cause rated no-load current in the main field at the transition speed). The motor stator currents will be maintained at a value that allows the motor to generate sufficient breakaway torque to overcome any stiction. At a specific transition speed or after a period of time, the drive will initiate a transition from induction motor control to synchronous motor control by removing the initial magnetizing current, and a field current is then applied to the motor through the DC exciter. Once this transition is completed, the drive may ramp up to the desired speed demand.