A Study On Rotor Encoderless Control Method For Brushless Doubly Fed Motors

Traditional high voltage and large power AC driving system is usually realized by induction motor combining with high-voltage inverter.However,all the energy flowing to the motor should pass through the inverter,which enlarge the capacity of the inverter and increase the manufacturer cost.The AC driving system based on the doubly fed induction motor can lower the capacity of the inverter.With the advantages of low cost and high power density,it can be applied on the speed adjust system with turbine and pump loads which require less speed range.Brushless Doubly Fed Machine(BDFM)remove the electric brushes and rings of the rotor windings.It has higher reliability and lower manufacturing and maintenance cost.This paper propose a speed adjust method with and without rotor encoder.The mathematical models,equivalent circuit and vector diagram are analyzed based on the windings structure of BDFM.The harmonics and common-mode voltage characteristics of multi-phase BDFM is also analysis.The relationship of two current vectors of stator power winding and control winding is illustrated.A close-loop speed control method with and without rotor encoder is proposed in this paper on the basis of the vector diagrams.The proposed method can operate on sub-synchronous,synchronous and super-synchronous speed with good steady and transient performance.It can realize power factor correction and speed control at the same time with less dependence on the parameters of the machine and calculation amount.A model of the machine is built on Matlab/Simulink software and simulation is being done.Simulation results verified the validation of the proposed control method.A test bench of a 30 kW BDFM is also built and experiments are carried on.Experiment results confirmed the effectiveness of the control method.