Research On Magnetic Field Modulation Capacity And Performance Of Brushless Doubly-fed Induction Machine With Different Rotor Structures

Compared with the current mainstream doubly-fed induction generator(DFIG)used in wind power systems,the brushless doubly-fed machine(BDFM)also has the advantages of small frequency converter capacity and low total system cost,but BDFM eliminates the need for brushes and slip rings,improves reliability,and reduces maintenance costs,so it has broad application prospects in the field of wind power generation.In the past ten years,a lot of research results have been achieved in both the motor ontology and the control strategy,and it has increasingly approached the market.There are two sets of windings on the stator of the brushless double-fed machine,which are called power winding and control winding.There is no direct coupling between the two sets of windings,but energy exchange is achieved through the rotor’s magnetic field modulation.The rotor magnetic field modulation effect is a key factor affecting the operating characteristics of the brushless doubly-fed machine,and the magnetic field modulation effects of different rotor structures are also different.The rotor structure of the brushless doubly-fed induction machine in the current research has two types: cage type and winding type.This paper focuses on the following studies on the magnetic field modulation effect,operating characteristics and loss calculation of these two rotor structures.1.Analysis of magnetic field modulation effect of cage and winding rotorsStarting from the internal magnetic field of the brushless doubly-fed machine,a magnetic field analytical method of cage rotor is proposed to analyze the magnetic field modulation mechanism of the cage rotor.The analysis results show that the magnetic field modulation effect of the cage rotor can be improved by changing the number of short-circuit rings and the span.On this basis,the maximum modulation capacity of the cage rotor is calculated;The maximum modulation capacity of the wound rotor is obtained from the mathematical relationship;the finite element simulation model is established to verify the theoretical analysis.2.Analysis of steady-state performance of open-loop operation of brushless doubly-fed machineIn order to qualitatively analyze the open-loop operation performance of the brushless doubly-fed machine,under ideal conditions,the relationship between the stator winding current,power,electromagnetic torque and terminal voltage during open-loop operation is derived through the modulation relationship between the rotor and the stator’s two magnetic fields.Get concise and clear expressions.The finite element simulation verified the correctness of the theory,and compared the running performance of the cage rotor and the wound rotor structure.3.Analysis of electromagnetic torque fluctuation of brushless doubly-fed machineBased on the analysis of the change of the open-loop operating characteristics of the brushless doubly-fed machine with the initial phase angle of the control winding voltage,the variation of the harmonic content in the air-gap composite magnetic field is analyzed,and then the magnitude and the electromagnetic torque of the brushless doubly-fed machine are analyzed.The reason why the fluctuation varies with the initial phase angle of the control winding voltage.The comparison of the magnetic field harmonics and torque fluctuations of the air gap synthesis of cage and wound rotor structures shows that the wound rotor improves the rotor magnetic field harmonics,which can reduce torque fluctuations and improve machine performance.4.Iron consumption analysis of brushless doubly-fed machineThrough the curve fitting,the analytical expression between BH when the iron core is saturated is established,and it is inserted into the existing analytical calculation program of the core loss of the cage rotor brushless doubly-fed machine,thereby reducing the comparison of finite element simulation.The variation trend of the iron consumption with the initial phase Angle of the control winding voltage is analyzed,and the iron loss of the cage rotor and the wound rotor brushless doubly-fed machine is compared by finite element simulation.The results show that the wound rotor weakens the rotor harmonics and has a significant effect on reducing iron loss.