Design and evaluation of a new type of harmonic-excited synchronous machine for variable-speed applications
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Date
2023
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Abstract
In this work a new type of brushless-excited wound-rotor synchronous machine for variable-speed applications is proposed and analyzed. The excitation of the field winding is achieved using inductive power transmission which normally requires an additional transmission coil on the stator and receiver coil on the rotor. However, the proposed machine does not need any additional windings or ferromagnetic material on either stator or rotor. This is achieved by applying pole phase modulation to the existing coil groups of a conventional wound-rotor synchronous machine with the help of a modified stator power electronic converter and rotor rectifier. Thus, two distinct airgap fields can be produced with independent current components --- one for controlling the torque production and one for inductively exciting the field winding. A prototype of the machine concept is designed, constructed and investigated experimentally on a machine test bench.
The theoretical analysis of the proposed machine is performed, based on the one-dimensional approximation of the airgap flux density. An analytical full-order machine model is derived and simplifications for steady-state are discussed. A field-oriented control algorithm is conceived which can track both the torque-producing currents and the excitation currents. The machine model, its drive electronics and the control algorithm are then investigated and verified using dynamic simulations.
A machine test bench is built and used to identify the designed machine and verify the capability of the machine concept to produce a load torque and excite the field winding inductively. The no-load experiment is used to isolate the effects of the excitation system and find those excitation system settings that grant the best efficiency. The influence of iron losses are identified as a major contributor to the losses. This is why a computationally quick and simple iron loss model is implemented, applied to the machine concept and compared with measurements. The iron losses are attributed to the additional speed-dependent forward and backward rotating fields superimposed on the conventional fields of a wound-rotor synchronous machine.
As a significant amount of DC link voltage is required to inductively excite the field winding, the torque-speed region of the machine concept is investigated analytically, numerically and experimentally. Simple formulas for the reduced maximum torque-speed curve are given and the loss of efficiency and speed range is calculated. An experimental comparison for a reduced DC link voltage is performed to verify the reduction of speed range. Lastly, an extension to the proposed concept which can address both the iron losses and the DC link voltage demand is discussed.
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Keywords
Wound-rotor, Synchronous machines, Brushless excitation, Harmonic excitation
Subjects based on RSWK
Synchronmaschine