Wound Field Synchronous Machine Exciter Phase Disconnection
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Solution Overview
Problem
Wound field synchronous dynamoelectric machines face challenges in maintaining optimal torque levels across a wide range of rotational speeds due to increased back electromotive force (EMF) at higher speeds, which can prevent successful starting operations, especially when using an existing AC power distribution system for both starting and generating modes.
Innovation Solution
A suitable impedance is connected between one phase of the multiphase exciter stator winding, disconnecting it from the AC source above a predetermined rotational speed, inducing a phase-shifted current in the disconnected phase to manage excitation current and limit back EMF, ensuring sufficient torque at high rotational speeds without affecting low-speed performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the WFS dynamoelectric machine uses an existing AC power distribution system for both starting and generating modes, then device complexity is reduced, but back EMF increases at higher rotational speeds preventing successful starting operations
Solution Approach 1:
The exciter stator winding is segmented into multiple independent phases (first phase, second phase, third phase) that can be selectively connected or disconnected from the AC power source. This segmentation allows the system to optimize performance for different operating conditions by controlling which phases are active during starting versus generating modes.
Solution Approach 2:
The system dynamically reconfigures the exciter stator winding connections based on operational mode. During starting mode, all phases are connected to provide maximum excitation current. During generating mode above a threshold speed, certain phases are disconnected to limit back EMF. This dynamic adaptation resolves the contradiction between device simplicity and back EMF management.
2Use of energy by moving object
If all phases of the exciter stator winding remain connected during high-speed generating operation, then excitation current is maintained, but back EMF increases excessively limiting torque
Solution Approach 1:
Instead of maintaining full excitation through all phases at high speeds, the system applies partial action by disconnecting certain phases (second and third phases) during generating mode. This partial excitation is sufficient to maintain adequate torque while preventing excessive back EMF that would limit power output.
Solution Approach 2:
The system changes the electrical parameters of the exciter stator winding by selectively disconnecting phases based on rotational speed. Below a threshold speed, all phases are connected for maximum excitation. Above the threshold, certain phases are disconnected to adjust the excitation level and back EMF characteristics, optimizing torque and power output for generating operation.
3Ease of operation
If the machine operates as a starter motor from standstill, then starting function is achieved, but back EMF at higher speeds prevents successful starting operations
Solution Approach 1:
The system prepares for the transition from starting to generating mode by monitoring rotational speed and pre-configuring the phase disconnection strategy. Before back EMF becomes excessive, the control system disconnects appropriate phases to prevent torque limitation, ensuring smooth transition and successful starting operation through the entire speed range.
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 solution allows the WFS dynamoelectric machine to maintain sufficient torque at higher rotational speeds by controlling back EMF, ensuring reliable starting and generating operations without adding complexity, bulk, or weight.
Implementation Method 1
A prime mover (not shown), such as an aeronautical gas turbine engine, rotates the rotor assembly 18 to cause the exciter rotor winding 10 to cut through the stationary exciter magnetic field flux established by the exciter stator winding 8. Since the exciter rotor winding 10 comprises a multiphase winding, typically three phase as shown in FIG. 1, it thereby generates multiphase AC excitation power.
Implementation Method 2
The rotating rectifier assembly 12 receives this multiphase AC excitation power and rectifies it to provide DC excitation power.
Implementation Method 3
The main rotor winding 14 receives this DC excitation power from the rotating rectifier assembly 12 to generate a rotating main magnetic field.
Implementation Method 4
The rotating magnetic field flux cuts through the main stator winding 16, causing it to generate main generator output power on terminals A, B and C.
Implementation Method 5
However, if the exciter stator winding 8 receives AC from an AC power source, the exciter stator winding 8 may produce an alternating exciter magnetic field flux that cuts the exciter rotor winding 10 even when it is stationary so that it produces AC excitation power even when the rotor assembly is at standstill.
Implementation Method 6
The main stator winding 16 may then receive a multiphase AC control signal from a power source (not shown), such as a motor controller, on terminals A, B and C that generates a rotating main stator magnetic field that interacts with the main rotor magnetic field to rotate the rotor assembly 18, thereby allowing the WFS dynamoelectric machine 2 to serve as a starter motor.
Implementation Method 7
A suitable impedance is connected between one phase of the multiphase exciter stator winding, disconnecting it from the AC source above a predetermined rotational speed, inducing a phase-shifted current in the disconnected phase to manage excitation current and limit back EMF
Data Source
AI summary
A wound field synchronous (WFS) dynamoelectric machine comprises: a rotor assembly; an exciter generator section comprising a stationary multiphase exciter stator winding for receiving excitation power from a multiphase alternating current (AC) source to generate an exciter stator magnetic field, a multiphase exciter rotor winding in the rotor assembly for generating multiphase AC excitation power as it cuts through the exciter stator magnetic field and a rotating rectifier assembly in the rotor assembly for converting the multiphase AC excitation power to direct current (DC) excitation power; a synchronous machine section comprising a DC main rotor winding in the rotor assembly for generating a main rotor magnetic field and a stationary multiphase main stator winding for receiving a multiphase AC control signal to generate a rotating main stator magnetic field that interacts with the main rotor magnetic field to rotate the rotor assembly; and an impedance that selectively connects one phase of the exciter stator winding to another phase of the exciter stator winding that disconnects from the multiphase AC current source above a predetermined rotational speed of the rotor assembly.