Rotor Winding Design And Starting Performance Test Research Of Soft-start Brushless Doubly-fed Machine

Brushless doubly-fed machine（BDFM）is a new type of AC variable speed motor.BDFM eliminate brushes and slip rings in structure,and have the advantages of simple structure,reliable operation and good speed regulation performance.This kind of motor realizes double-fed on the stator.It not only has a simple rotor structure,but also has the excellent characteristics of a wound rotor asynchronous motor and a synchronous motor.It can be used as an AC speed regulating motor and used in large-scale water pumps and fan speed control systems.;It can also be used as a variable frequency constant speed generator,which is widely used in wind power,hydropower and marine shaft belt power generation.At present,the rotors of BDFM mainly adopt three structures:special cage,reluctance and winding.The first two rotor structures have the disadvantages of large rotor harmonic content and low conductor utilization.The wire-wound rotor BDFM has flexible wiring,so the harmonic content of the air gap magnetic field can be weakened through a reasonable winding design,and the efficiency and power density of the motor can be improved.But its starting performance is not as good as cage-type BDFM.In order to solve the problems of low starting torque and large starting current of the wound rotor BDFM,this paper proposes a soft starting method,which redesigns the stator and rotor windings based on the winding theory through the flexibility of the wound rotor connection mode.The stator is composed of two sets of windings:one set of windings is power winding（PW）.When starting,the stator mainly produces ₁p pole-pairs fundamental waves,p_a pole-pairs o main harmonics and p₂ pole-pairs subharmonics.The three magnetomotive forces（MMF）start together.The other set of windings is control winding（CW）,which is similar in connection with ordinary BDFM windings;the rotor winding adopts"composite coil group"structure.The soft-start method makes the motor automatically increase the starting resistance,reduce the starting current and increase the starting torque when starting,which improves the starting performance of the motor;When operation,the starting resistance returns to normal and the loss is reduced.The motor has a high operating efficiency.The research content of this article mainly includes the following aspects:Firstly,the origin and development of BDFM are reviewed.The research status of BDFM body design structure,mathematical model,equivalent circuit and control strategy are introduced.Then the development status of the key starting technology of three-phase AC induction motor is summarized,and the advantages and disadvantages of the two starting methods of BDFM are discussed.Finally,the soft start method of BDFM proposed in this paper is introduced.Secondly,the basic structure of soft-start BDFM and the basic working principle of stator winding and rotor winding are introduced.The specific design method of the two sets of stator windings and the rotor windings is discussed emphatically.The stator PW adopts 3Y/△connection,and the CW adopts Y connection.The rotor windings are connected by a"composite coil group".The analysis shows that the design method reduces the starting current and increases the starting torque,which improves the starting performance of the motor During operation,the starting resistance returns to normal.The motor reduces losses and has a higher operating efficiency.At the same time,the PW schemes with different pitches are designed,and the comparative analysis results of MMF harmonics are given.Finally,based on the working principle of the composite coil structure in different states,the starting performance and running performance of the motor are analyzed.Then,based on the two-dimensional transient magnetic field analysis method,the internal magnetic field changes of the motor under no load are analyzed.Two finite element simulation models of soft starting and asynchronous starting are established,and the relevant variables of the start process under the two start modes are calculated.The distribution diagrams of magnetic field lines,air gap magnetic density,magnetic flux density cloud diagram,starting torque,starting current,speed and so on are obtained at the starting time.By comparing the simulation results,it is concluded that the soft-start BDFM has the advantages of reducing the starting current and increasing the starting torque.In addition,the soft-start BDFM has a smooth torque characteristic curve,harder mechanical characteristics,and strong load capacity.It is especially suitable for applications such as belt conveyors to reduce system costs.Finally,eight motor models with the same stator structure and different rotor turn-ratios were established in the finite element method to study the effect of different turns ratio of the composite coil on the motor’s starting performance.The no-load starting torque and starting current are simulated and calculated to get the best turn ratio scheme.Finally,the main parameters,iron core material,stator and rotor slot size,prototype structure diagram and the expanded view of the stator and rotor windings of the soft-start BDFM prototype are given.A 2/4 pole-pairs soft-start BDFM prototype was developed and a test platform was built.Under the two modes of soft starting and asynchronous starting,the no-load and load starting performance test of the prototype is carried out.The test results further show that the starting performance of soft-start BDFM is superior to asynchronous-start BDFM,and it can effectively reduce the starting current and the impact on the power grid.The correctness of the soft-start BDFM design scheme is verified.