Research On Starting Characteristics Of Brushless Doubly-fed Machines Based On Pole Changing Winding

The brushless doubly-fed motor is a new type of AC motor that eliminates brushes and slip rings.The stator power winding of the brushless doubly-fed motor is connected with the power frequency power supply,and the control winding changes the excitation current and frequency through the frequency converter to realize the adjustment of the rotor speed.Through the special structure of the rotor,the rotor is coupled to the stator power winding and the control winding respectively to generate two different pole numbers and the opposite magnetic field.Because of its simple structure,high reliability,adjustable power factor,etc.,it can realize synchronous,asynchronous,double-fed and other operating modes,and has broad application prospects in the fields of variable frequency speed control system and variable speed constant frequency power generation.However,as an asynchronous motor that operates synchronously,the brushless doubly-fed motor also has a large starting current and a small starting torque,which cannot meet the application requiring heavy-duty starting.However,unlike ordinary asynchronous motors,the structural characteristics of the brushless doubly-fed motor are that the rotor windings are self-closing and no slip ring brushes are connected externally.The purpose of increasing the starting torque,reducing the starting current and high efficiency operation can not be achieved by serially cutting or cutting off the starting resistor in the rotor circuit by means of a brush.At the same time,the brushless doubly-fed motor has two sets of windings with different pole numbers,which is also different from the single-pole asynchronous motor.In this paper,the wound rotor double-fed motor is taken as the research object,and the theoretical research and experimental discussion of this new type of motor are carried out.The purpose of the research is to apply the rotor structure designed with the principle of "tooth harmonic" and "composite coil" to the brushless doubly-fed motor to improve the starting performance of the brushless doubly-fed motor.The main contents are as follows:Firstly,on the basis of reviewing the development history of brushless doubly-fed motors at home and abroad,the operating principle and energy flow mode of brushless doubly-fed motors are summarized.The problems and defects of the brushless doubly-fed motor are analyzed,which leads to the variable-pole starting type brushless doubly-fed motor proposed in this paper,and the principle of "composite coil" used in the rotor winding is elaborated.The principle of pole-changing is introduced into the design of stator windings,and the theoretical derivation and proof of the stator power winding and control winding arrangement are carried out.Secondly,the self-inductance,mutual inductance and resistance calculation formula of the brushless doubly-fed motor winding are given.The general equivalent circuit in the double-feed mode is obtained by looking up the data and the equivalent circuit in the variable pole starting mode is derived.Then it is in Matlab.The starting current and starting torque in the pole-changing mode and the doubly-fed mode are analyzed,and the turns ratio of the multi-turn and the less-turn of the composite coil is adjusted by the analysis result feedback.Then,the mathematical model of the brushless doubly-fed mode and the pole-changing mode in the stationary coordinate system is established,and the no-load and load characteristics of the two modes are simulated in Matlab,and the motor is obtained during the starting process.Transient simulation results.Then,the finite element method is used to simulate the no-load and load simulation of the asynchronous start mode and the variable-pole start mode of the brushless doubly-fed motor respectively,and the motor saturation degree and load capacity of the two modes are analyzed.Finally,a 36/24-slot 4kW brushless doubly-fed motor with theoretical research is used to test the steady current and speed of the doubly-fed asynchronous start mode and the variable-pole start mode under different load conditions.The current curves of the two modes at no load and full load are given and the distortion of the waveform is analyzed.