Research On The Soft Starter With Discrete Frequency Variable Capability

Soft-starter is a novel kind of motor control device which possess multiple functions, such as voltage ramp start, current limit start, pump stop, and protections. Compared with traditional soft start means, soft-starter possesses much more superiority, which includes adjustable output voltage, smoothly start motor without step changes, low surge current and low rush torque, simple operation, narrow space demanding, and etc. As the development of power electronics, the technique of induction motor soft-start is gradually developed. And the functions of softstarter are more and more complicated by the development of softstarting technology. After studied the present background of softstarter and investigated a lot of information, this paper did much effort on the functions of softstarter, and developed a set of softstart system. The content of this paper includes:By the analysis of dynamic model of induction motor with softstarter controlling, this paper get the states equations, and build a simulation model. By the contrast of simulation results and measured results, the validity of simulation model is approved.Based on the simulation model, this paper presented that the expected output voltage should be the amplitude of base frequency, and calculated the base frequency of output voltage. With the results of relationship between output voltage and fire angle with lag angle, this paper realized ramp voltage soft start, current limit start, and soft stop. Besides the basic functions, this paper studied the malfunction detection of induction motor and time reversal protection, realized the motor protection under abnormal conditions.In order to elevate the starting torque of soft-starter and extend its application, this paper studied Discrete Variable Frequency softstart technology, besides those basic functions. This paper analyst the frequency spectrum of output voltage and phase shifts of three phases under the condition of line frequency division. By the result of analysis, this paper concluded the starting torque at different discrete frequency, selected with those starting frequencies, and realized Discrete Variable Frequency softstart. Furthermore, this paper studied the negative effect of Discrete Variable Frequency softstart, and presented some improvement and solutions.At last, this paper introduced the hardware and software of softstarter system, and presented the experiment results. This novel softstarter system not only possesses those merits of traditional softstarter, but also meets the requirements of starting with heavy loads.