Research Of Sensorless Control On Direct Drive Drilling Brushless DC Motor

Drilling is the key link of petroleum mining, its efficiency needs to be promoted.The traditional screw drilling tool and surface-driving electric drilling tool has such defects as huge power consumption, low mining efficiency, complicated transmission mechanism etc. Direct-drive downhole multipole permanent magnet brushless DC motor(BLDCM)has become the domestic and overseas’ petroleum industry experts and scholars research hotspot because of its high efficiency, high torque density, high starting torque, flexible control method, when using the low speed, high torque motor,and the frequency conversion part works with motor in downhole, under the high temperature, the most vulnerable gear reduction part can be avoided and the cable numbers are decreased. But the traditional BLDCM operates with a rotor pole position detection sensor make the structure complex, volume enlarged, reliability decreased, especially torque motor needs a high precision on sensor installation,which limits its using and higher costs. Therefore, to research and apply the sensorless control method on the direct-drive downhole BLDC Drilling Motor(DM) is significant.In this paper, for the characteristics of low rotor speed, under high temperature,high pressure and complicated load condition of the DM, the following research and analysis were carried out:Firstly, several sensorless control methods and its characteristics, research status and background of this subject are introduced, the DM’s mathematical model was established aiming at DM operate at low rotor speed, a winding flux linkage function method which has a good performance at low speed is considered. The relevant information of rotor can be acquired by analyzing first order differential equation from motor’s conducted phase, and the speed-independent expression is derivated after mathematical processing, this expression only relate to motor’s line voltage,resistance, inductance. In order to reduce expression’s dependence on parameters,calculation amount, calculation error, the resistance term and inductance term were eliminated, only the line voltage is used to calculate commutation instant, Z functionmethod was presented. To ensure motor starts without reverse rotating with loaded,several sensorless starting methods were analyzed and the pulse injection method is used. In the meantime, to keep rotor speed at the given speed when sudden load applied, a speed closed-loop control is used. By establish the DM’s simulation model in simulink, the rotor position detecting accuracy and parameters variation of using Z function method and its control strategy at low rotor speed with sudden load applied is preliminary verified.Eventually, set the DM as the main controlled object, based on Z function method,the motor experimental platform is built, several indexes such as the feasibility and commutation instant capture accuracy when using Z function method were tested and verified at the 60 rpm and 400 rpm. The results show that Z method calculates motor’s commutation instant by using conducted line to line voltage, it could be used on the DM, both the winding flux linkage function method’s dependence on motor and DSP calculation are decreased...