Variable Frequency Start And Speed Control Of Medium Voltage Synchronous Motor Based On CSI

For its outstanding advantages applied to synchronous motor soft start andspeed regulation, self-controlled with variable frequency technology has become aresearch hotspot in recent years, and widely used in medium voltage drives. Butbecause of the technology difficulty and high research investment, the domesticproduction hasn’t been appeared now, and most of the self-controlled soft-startdevices are imported from abroad. Analysis and research about self-controlledsoft-start and speed regulation of synchronous motor are made deeply in this paper,and the main research contents include: the design of digital control system, thefull-frequency Starting Control strategy without position sensors, speed regulationwith variable frequency and excitation control strategy, phase-locked andgrid-connection without impact.The topological structure and working principle of self-controlled soft-startdevice is introduced first in this paper, and then the sensor-less control strategy ofsoft start is given form low-speed to high-speed stages. In the low-speed stage,synchronous motor starts with the method of pulses self-given, when the rotorposition needn’t to be detected, and the current-break commutate mode is adapted toensure the motor commutation; load commutate mode is adapted in middle andhigh-speed stage, in which the rotor position is detected by capturing thezero-crossing points of back EMF.The control strategy of closed-loop speed control is designed in this paper. Inthe low-speed stage, the current closed-loop is adapted, when the dual closed-loop isadapted in high-speed stage, with that the motor speed control and soft stop arerealized. Excitation current holds constant initially, and it is control by closed-loopin high-speed stage to adjust the amplitude of EMF. The motor enters thephase-locked stage when accelerate to95%of rated speed, and then the PLLcoordinates with dual closed-loop to lock phase, and with the amplitude modulationby excitation closed-loop control, the grid-connection without impact can berealized.In this paper a self-controlled soft-start platform is designed aiming at10kVvoltage level synchronous motor. Three TMS320F2812DSPs are chosen as maincontrol chips for rectifier, inverter and excitation system, and the three partscommunicate with each other via the CAN bus. In the150kW Prototype motorexperiment, the full-frequency soft start without position sensors is realized, theimpacts of the switching points are very small, and the entire starting proce ss isstable and reliable. With the dual closed-loop control, the motor system has the functions of speed control and soft stop, the regulating process is smooth, fast, andwithout step losing. At the end of grid connection experiment, the amplitude of theEMF equal to grid, the phase difference is less than2°, and the frequency differenceis less than0.25Hz, the motor connect to the grid without impact.