VFSG Start Control With Shared Motor Controllers and Back-EMF Management
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Solution Overview
Problem
Existing engine start systems for gas turbine engines, particularly in aircraft, face challenges in efficiently providing excitation power to variable frequency starter generators (VFSGs) during both start and generate modes, leading to increased back EMF issues.
Innovation Solution
The engine start system incorporates two VFSGs, each with an exciter and stator, along with motor controllers and a switching system. This configuration allows for operational modes where motor controllers provide controlled voltage and frequency to exciters and stators, optimizing power generation and reduction of back EMF.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If three phase AC power is applied to the VFSG exciter coils during start mode, then the VFSG can be energized at zero to low speeds, but during generate mode the back EMF increases
Solution Approach 1:
The system dynamically switches between two different excitation configurations: during start mode, three-phase AC power is applied to the exciter coils to enable zero-to-low speed operation; during generate mode, the excitation is changed to prevent back EMF increase. This dynamic reconfiguration of the excitation system allows the VFSG to optimize performance for each operational phase.
Solution Approach 2:
The invention changes the excitation parameters (voltage, frequency, phase configuration) based on operational mode. During start mode, the exciter receives three-phase AC power at specific voltage and frequency ranges; during generate mode, the excitation parameters are adjusted to maintain controlled back EMF levels, thereby resolving the contradiction between starting capability and back EMF management.
2Device complexity
If a single motor controller is used for each VFSG, then the system is simpler, but the ability to optimize power distribution and reduce back EMF is limited
Solution Approach 1:
The motor controllers are designed with multi-functionality, capable of operating in different configurations (direct excitation or cross-excitation) depending on operational mode. This universal controller design allows the same hardware to perform multiple functions: providing excitation power during start mode and managing back EMF during generate mode, without requiring additional dedicated controllers.
Solution Approach 2:
The switching system acts as an intermediary that routes power between motor controllers and VFSG components (exciters or stators) based on operational mode. This intermediary switching mechanism enables flexible power distribution, allowing one motor controller to supply power to another VFSG's exciter when needed, thereby optimizing back EMF control without increasing overall system complexity.
3Power
If the exciter coils are directly connected to the aircraft AC bus, then the VFSG receives sufficient voltage for starting, but the frequency variation from 360 Hz to 800 Hz causes operational issues
Solution Approach 1:
The motor controller serves as an intermediary between the aircraft AC bus and the exciter coils. Instead of direct connection, the motor controller conditions the power, converting the variable frequency AC bus power (360-800 Hz) into controlled excitation power with stable frequency characteristics suitable for the exciter, thereby resolving the frequency stability issue while maintaining adequate voltage levels.
Solution Approach 2:
The system transforms the excitation parameters by using the motor controller to regulate voltage and frequency delivered to the exciter coils. The controller adjusts these parameters dynamically based on operational mode, ensuring stable frequency delivery to the exciter even when the aircraft AC bus frequency varies, thus resolving the contradiction between power delivery and frequency stability.
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
The system achieves improved control over the start sequence, enhanced start torque capability, and increased efficiency by optimizing excitation power distribution across different operational modes, thereby reducing back EMF and improving overall system performance.
Implementation Method 1
three phase AC power is applied to the VFSG exciter coils to energize the main field
Implementation Method 2
the controller generally converts external power into a form suitable for rotating the gas turbine engine rotor using the starter/generator
Implementation Method 3
the starter/generator operates in a manner so that it generates power
Data Source
Figure 1A
Figure 1B
Figure 2
AI summary
An engine start system for an aircraft includes a first variable frequency start generator (VFSG) (110-a) including a first exciter (120-a) and a first stator, a second VFSG (110-b) including a second exciter (120-b) and a second stator, a first motor controller (115-a) and a second motor controller (115-b), and a switching system (132). The switching system is configured to operatively connect the first motor controller to the first stator or the second exciter and operatively connect the second motor controller to the first exciter or the second stator. In a first mode of operation, the first motor controller is operatively connected to the second exciter, the second motor controller is operatively connected to the second stator, the first VFSG generates power, the first motor controller provides a first controlled voltage and frequency to the second exciter, and the second motor controller provides a second controlled voltage and frequency to the second stator.