Soft Start Of High Power Synchronous Motor

Synchronous motor is popular in high-power electric drive because its power factor is adjustable and output torque is not sensitive to the power grid voltage fluctuation. Although synchronous motors have many advantages, its biggest drawback is difficult to start. Large and medium-sized high-voltage synchronous motor uses asynchronous start-up method, which causes a great impulse current in the system. On the one hand, the impact current is harmful for motor itself; on the other hand, it drops voltage of power grids, accompanying with causing other equipment in power grids to work abnormally. In recent years, due to the development of power electronics technology, the soft-start technology of synchronous motor has the rapid development.At present, the soft-start of high-power synchronous motor is using static frequency converter method mostly. In this method, the control system is complex because it uses the thyristor as power devices in inverse Converter, and the thyristor's automatic cutting off have to rely on the back-EMF of the synchronous motor. If the inverse converter fails in commutating, motor starting fails. For the thyristor can not turn off on its own, this paper presents a synchronous motor start-up method which is called the speed open-loop constant proportion of voltage to frequency control method using IGBT inverter as power devices.Firstly, a detailed analysis of the starting characteristics of synchronous motor is given, in accordance with its work principle. The curve of motor phase current and electromagnetic torque during the start-up is obtained stemming form the simulation research on asynchronous motor starting methods. For the problem that asynchronous motor starting generates a great current impact, this paper presents speed open-loop constant proportion of voltage to frequency control method. Secondly, based on the study of open-loop control synchronous motor soft start principle, the relationship between stator rotating magnetic field speed and rotor speed is deduced under the condition that synchronous motor can start itself just relying on synchronous electromagnetic torque. By doing power frequency and low-frequency start-up simulation on the same motor, the conclusion that synchronous motor can start is proved that the synchronous motor can start itself just relying on synchronous electromagnetic torque under the low-frequency. And the rule stator voltage frequency changing with time is confirmed through computing the time rotor cost on achieving corresponding synchronous speed. Then, based on the use of voltage-type AC-DC-AC inverter as synchronous motor variable frequency power, the paper designs a constant voltage frequency ratio soft-start control method, which is realized through changing the inverter output frequency and voltage step by step to make the rotor speed to follow the stator rotating magnetic field speed and to increase to rating value. The synchronous motor starting simulation is processed on the MATLAB / SIMULINK after building the mathematic model of open-loop constant voltage frequency ratio soft-start control, which analyzes both no-load start-up and load on start-up process. The simulation results show the feasibility of constant voltage frequency ratio soft-start control method.This article also gives a theoretical analysis on conditions of synchronous motor parallel in power grid after successful start-up and introduces the appropriate parallel in project. The parallel in process simulation on MATLAB / SIMULINK for the synchronous motor reveals the change curves of the power grid instantaneous voltage, synchronous motor phase current etc. and verifies the feasibility of parallel in project.In conclusion, the dissertation concludes the whole work, analyses the findings and achievements, and brings forward further research on high-power synchronous motor.