Characteristics and principle of soft start for electric motor

Characteristics of soft start

The starting performance of asynchronous motor is mainly composed of two indexes, starting current multiplier and starting torque multiplier. Soft starter starts with changing the supply voltage added to the motor to reduce starting current and starting torque. The traditional motor start way autotransformer, Y/ delta decompression, their common feature is the simple control circuit, the starting torque is not adjustable and two times the impact of current, impact torque on the load. The soft start can effectively reduce the starting current of the motor, and its starting current is only 50% of the hard starting current of the standard motor, which is 20% of the hard starting current of the high efficiency motor. Soft start limiting characteristics can effectively limit the inrush current, avoid unnecessary impact torque and current distribution on the impact of the network, to effectively reduce the line contactor switch and false triggering action; the frequent start stop motor, can effectively control the motor temperature rise, greatly prolong the life of the motor.

The principle of soft start

It is widely used and soft starter is the soft start of thyristor. Soft start principle of thyristor in three-phase power supply and motor series three-phase thyristor tube joint, using the principle of phase control thyristor, changing the trigger angle of thyristor, when starting the motor terminal voltage with thyristor conduction angle gradually increased from zero, you can adjust the output voltage of thyristor the voltage regulating circuit, the motor speed increases gradually until it reaches the starting torque requirements and meet the end of the boot process; the output of the soft starter is boosting a smoothing process (and with current limiting function), until the thyristor wholeconduction motor under the rated voltage; the bypass contactor is switched on (to avoid the formation of motor harmonic pollution on the grid in the operation of the thyristor is prolonged life), motor in steady state; the first cut off the bypass contactor when parking, and then by the soft starter in the conduction angle of the thyristor is gradually reduced Small, the three-phase power supply voltage gradually reduced, motor speed from large gradually reduce to zero, stop the process of completion.