Power Electronics Devices Of Soft Starter
Each two thyristors parallel connected reversely of six thyristors or three MTX thyristor power modules to compose a three-phase voltage regulator, and connect it between the power supply and the motor stator.
For instance of three-phase fully controlled bridge rectifier circuit, when using a soft starter to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully conducted. The motor works on the mechanical characteristics of the rated voltage to achieve smooth start, reduce the starting current, and avoid starting over current trip. When the motor reaches the rated speed, the starting process ends, and the soft starter automatically replaces the thyristor with a bypass contactor to provide rated voltage for the normal operation of the motor to reduce the thermal loss of the thyristor and extend the service life of the soft starter, improve working efficiency and avoid harmonic pollution to the power grid.
The soft starter can also provide a soft stop function which is opposite to the soft start process. The voltage gradually decreases and the speed gradually decreases to zero to avoid torque shock caused by free stop. When using soft start, the starting current is generally 2~3 times of the rated current, the grid voltage fluctuation rate is generally within 10%, and the impact on other equipment is very small.
In daily maintenance, pay attention to check the environmental conditions of soft start frequently. Pay attention to check whether there are objects around the soft starter that hinder its ventilation and heat dissipation, and ensure that there is enough space around the soft starter (more than 150mm), regularly check whether the distribution line terminals are loose, regularly clean the dust to avoid affecting the heat dissipation and prevent the thyristor from being damaged due to excessive temperature rise. At the same time, it can avoid leakage and short circuit accidents caused by dust.
The thyristor worked as AC voltage regulation in soft starter. To low voltage application (input voltage:380V), the forward and reverse repetitive rated voltage (VDRM, VRRM) is recommended to choose 1200V. To medium voltage application (input voltage:660V), the forward and reverse repetitive rated voltage (VDRM, VRRM) is recommended to choose 2200V or above. To high voltage application (input voltage≥1100V), the forward and reverse repetitive rated voltage (VDRM, VRRM) is recommended to choose 3500V or above. To 6KV or 10KV soft starter, the high voltage need thyristors be parallel connected reversely then series connect. 6KV soft starter need 6 thyristors(2 thyristors parallel connected reversely in 3 groups). 10KV soft starter need 10 thyristors (2 thyristors parallel connected reversely in 5 groups). Such the withstand voltage on each thyristor will be 2000V and the forward/reverse un-repetitive rated voltage - VDSM and VRSM of thyristor selected should be more than 6500V.
Runau Electronics is expert in providing the thyristor and power module for soft start especially in high current (more than 3000A) and high voltage (more than 6000V) application. The reliable quality and strong capability enable the minimum investment to realize the maximum performance. The lower conductive voltage drop, stronger over-current capability, higher impact and voltage resistance, all above mentioned properties of Runau thyristor satisfied the universal requirements of soft starter comprehensive application perfectly.
Thyristor Selection Recommend Reference
(KW / In:380V)
Soft Starter Used Thyristor (VDRM /VRRM)
Motor Rated Current Ie (A)
Soft Starter Used Thyristor ITAV (A)