异步电动机软起动的研究，是近年来人们对电动机起动研究的一个重要方面。由于传统的软起动是将晶闸管触发角的大小按一定的规律去改变，以此去减小电动机的输入电压，将电动机的起动电流以及起动转矩同时降低，从而无法实现重载起动。虽然变频器对于电动机的起动效果好，但是由于其成本投入大，仅用于电动机的起动是不经济的。针对以上出现的问题，本文利用晶闸管斩波技术，采用了一种新方案，即分级变频软起动技术。在不增加成本的情况下，通过对工频半波的通断控制，达到改变电压和频率的目的，满足电动机对起动电流和起动转矩的要求。 本文以分级变频理论为依据，通过对各级子频率进行数学分析找出子频率与原始信号相位角之间的关系。并且通过对理论以及仿真的研究，计算出了能使电动机达到最大电磁转矩的最优触发角和最佳的相位角组合。本文的分级变频软起动形式提出分四级起动，并且能够保证在各个子频率下都可以是最优的正序电压组合和实现启动转矩最大化。通过对电动机在软起动调压过程中产生电流振荡的分析，并且为了使在起动中电磁转矩和电流脉动得到抑制，我们提出了电动机分级变频软起动控制器中在二分频到工频期间实施电动机功率因数角闭环控制的观点。 通过分级变频软起动器的建模和仿真实验，并与传统软起动方式进行对比，验证了分级变频软起动在重载起动方面所具有的优势。根据理论的分析和仿真结果，研制了分级变频软起动器实验样机，实验结果表明，分级变频软起动器可以减小起动电流，增大起动转矩，而且其控制效果好，验证了设计电路的正确性和可实施性。

Soft Start of three-phase asynchronous motor is an important research topic. The traditional soft is by changing the thyristor firing angle to reduce the size of the input voltage of the motor so that the motor starting current is reduced, but limits the size of the starting torque, heavy-duty starter can not be achieved. Starter motor with inverter has invested too much cost. So for these contradictions, we use thyristors chopping technique, we propose a new scheme, namely the classification frequency soft start technology, the software we have different power frequency half-wave-off combined control, in order to achieve VVVF, so that we can not increase the cost to meet the original motor for starting current and torque requirements. In this paper , a hierarchical frequency as the theoretical basis, we use mathematical analysis methods for the classification of sub-frequency inverter system in-depth study to determine the phase angle between sub-level frequency and the original signal. Through theoretical and simulation analysis, we can find the best combination of phase angle and various sub-optimal firing angle frequency of maximum electromagnetic torque. In this paper, the traditional motor soft starter main circuit structure has been improved, and made four starting points classification frequency regulator in the form of soft start, to ensure that the sub-optimal frequency levels can be achieved positive sequence voltage combinations, in order to determine the starting torque moments maximized. This paper also analyzes the causes of the current oscillations generated by the motor soft start regulating the process and made a classification variable frequency motor soft start controller imply the motor power factor angle view of closed- loop control during the second frequency to the power frequency, the method can make the electromagnetic torque and current ripple significantly suppressed in the process of starting. By classification variable frequency soft starter modeling and simulation, comparing with the tradition soft start, we validated that the classification variable frequency soft starter has the advantage in terms of heavy-duty starter. According to the results of theoretical analysis and simulation, we develop a classification variable frequency soft sarter prototype. The experiment results show that the classification variable frequency soft starter can reduce the starting current and in crease the starting torque, and the control effect is very good. According to this, we verify the correctness of the design of the circuit and feasibility.