Research On Soft Starter Of Induction Motor Used In The Occation Of Starting With Load

The induction motors have been widely used in industry, however, the instantaneous current and torque of the starting process can cause bad influence on both the gird and the motor system. Theory of Soft Start is risen to reduce the impact and to optimize the starting performance. Traditional approaches, such as Back-to-back Connected Thyristor-based Soft Starters, drawdown the starting current by means of decreasing the starting voltage. Despite of its significance in current limiting and in price limiting, the approach above will cause a severe descension of the starting torque. As a result, it cannot be used in over-loading occasions.Inverter Soft Starter have shown the best performance so far in the soft-starting area. But there are still limitations, such as the premium price of the IGBTs, which caused restriction of wide usage. We can see from principle analysis of the inverter, that by reducing the starting frequency along with starting voltage can the Inverter Soft Starters be used in over-loading occasions. Back to the origin, it is known that keeping voltage and frequency to vary synchronously will maintain air-gap flux at a constant level, and this will help getting a higher starting torque.Facing the advantages and disadvantages of both devices, e. g. Thyristor-based Soft Starters and Inverter Soft Starters, we attempt to introduce Thyristors into the Inverter Soft Starters. Although we aim at better performance, difficulties are in our way resulting from the difference between thyristors and IGBTs. Firstly, lower switch frequency of thyristors makes it impossible to use SPWM (Sinusoidal Pulse Width Modulation) to maintain the voltage-frequency rate. Secondly, thyristors are half-controlled devices, leading to another problem of how to turn them off. Additionally, with power motor as the load, the power factor angle changes during starting process, and that aggravates the problem. Taking all the difficulties above in account, this paper proposes a novel Soft-Starter. The novel Soft-Starter try to use thyristors as master device, and a square-wave driven motor, which is variable voltage and variable frequency, as its load.This novel Soft-Starter contains two high-voltage IGBTs and six thyristors to make up a hybrid switch. The six thyristors implement frequency-varying function, while the two IGBTs, locating right on the DC link, help to turn off the thyristors. What's more, the IGBTs can function as a chopper controller. Consequently, the novel Soft-Starter accomplishes the requirement of variable voltage and variable frequency.The novel Soft-Starter adopts current-limit soft-start mode, in which change of voltage and frequency rate follows the curve of voltage-frequency coordination control with voltage-compensation.It is the TMS321LF2407A device that accomplished most functions like software protection, drive-signal output, and sampling. In order to assure the rapidity and efficiency of protection implementation, this paper also give out the over-current protection scheme based on desaturationn detection. And a more efficacious drive circuit is given out to assure the reliability of the thyristors.All the above-mentioned are reflected in the prototype. Experiments proved that this novel Soft-Starter has a better performance than Thyristor-based Soft Starter; meanwhile the price of this novel Soft-Starter is lower than inverter soft starter. Accordingly, we can say the novel Soft-Starter is significantly practical.