Soft start means that the voltage of the motor is gradually increased from zero to the rated voltage, so that the starting current of the motor during the starting process is changed from uncontrollable overload impulse current to controllable. And the size of the starting current can be adjusted as needed. There is no impact torque in the whole process of motor startup, but smooth startup and operation.
Hazards of direct start
1. Cause grid voltage fluctuations
When the AC motor is directly started at full voltage, the starting current will reach 4 to 7 times the rated current. When the capacity of the motor is relatively large, the starting current will cause a sharp drop in the grid voltage, affecting the normal operation of other equipment in the same grid.
During soft start, the starting current is generally 2 to 3 times the rated current, and the grid voltage fluctuation rate is generally within 10%, which has very little impact on other equipment.
2. Impact on the grid
The impact on the power grid is mainly manifested in two aspects:
1) The impact of the large current directly started by the super-large motor on the power grid is almost similar to the impact of a three-phase short circuit on the power grid, which often causes power oscillation and makes the power grid unstable.
2) The starting current contains a large number of high-order harmonics, which will cause high-frequency resonance with the power grid circuit parameters, resulting in faults such as malfunction of relay protection and automatic control failure.
The starting current is greatly reduced during soft start, and the above effects can be completely eliminated.
3. Damage the motor insulation
1) The Joule heat generated by the large current acts on the outer insulation of the wire repeatedly, which accelerates the aging of the insulation and reduces the service life.
2) The mechanical force generated by the high current causes the wires to rub against each other, reducing the insulation life.
3) The jitter of the contacts when the high-voltage switch is closed will generate an operating overvoltage on the stator winding of the motor, sometimes reaching more than 5 times the applied voltage. Such a high overvoltage will cause great damage to the motor insulation.
During soft starting, the maximum current is reduced by about half, and the instantaneous heat generation is only about 1/4 of that of straight starting, and the insulation life will be greatly extended; during soft starting, the motor terminal voltage can be adjusted from zero, which can completely avoid overvoltage damage.
4. The damage of electric power to the motor
The large current produces a great impact on the stator coil and rotor squirrel cage bar of the motor, which will cause failures such as loose clamping, coil deformation, and squirrel cage bar breakage.
During soft start, the impact force is greatly reduced due to the small maximum current.
5. Damage to mechanical equipment
The starting torque of full-pressure direct starting is about twice the rated torque. Such a large torque is suddenly added to the static mechanical equipment, which will accelerate gear wear and even tooth beating, accelerate belt wear and even break the belt, and accelerate fan blades. Fatigue or even broken blades and so on.
The torque of soft start will not exceed the rated torque, and the above drawbacks can be completely overcome.
The three-phase asynchronous motor is the most commonly used in engineering. Due to its motor starting characteristics, these motors are directly connected to the power supply system to start (hard start), which will generate a surge (impulse) current as high as 5 to 7 times the rated current of the motor, making Overload of power supply system and switchgear connected in series. On the other hand, direct start also produces higher peak torque. This kind of impact will not only impact the driving motor, but also damage the mechanical device, and also affect the normal operation of other electrical equipment connected to the same power grid.
The birth of squirrel-cage asynchronous motor electronic soft starter solves this problem. It can not only change the starting characteristics of the motor to protect the drag system, but also ensure the reliable starting of the motor and reduce the starting impact. Therefore, with the rapid development of power electronics technology, intelligent soft starters will be more widely used.
The soft starter uses a three-phase parallel thyristor as a voltage regulator, which is connected between the power supply and the motor stator. Such a circuit is like a three-phase fully-controlled bridge rectifier circuit. When the soft starter is used to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on, and the motor works on the mechanical characteristics of the rated voltage to achieve smooth starting, reduce starting current, and avoid starting overcurrent tripping. When the motor reaches the rated speed, the starting process is over, and the soft starter automatically replaces the thyristor that has completed the task with a bypass contactor to provide the rated voltage for the normal operation of the motor to reduce the heat loss of the thyristor and prolong the service life of the soft starter. Improve its work efficiency, and make the power grid avoid harmonic pollution. The soft starter also provides a soft stop function. The soft stop is the opposite of the soft start process. The voltage gradually decreases and the speed gradually drops to zero to avoid the torque shock caused by the free stop.
1. No inrush current. When the soft starter starts the motor, by gradually increasing the conduction angle of the thyristor, the starting current of the motor increases linearly from zero to the set value.
2. Constant current start. The soft starter can introduce current closed-loop control, so that the motor can maintain a constant current during the starting process and ensure the smooth start of the motor.
3. According to the load condition and the power grid relay protection characteristics, it can be freely adjusted from the stepless to the best starting current.
4. Reduce the starting current of the motor, reduce the power distribution capacity, and avoid investment in capacity expansion.
5. Reduce the starting mechanical stress and prolong the service life of the motor and related equipment.
6. The starting parameters can be adjusted according to the load to achieve the best starting effect.
7. A variety of startup modes and protection functions are available, which are easy to improve the process and protect the equipment.
8. The unique external control terminal can easily realize remote control or automatic control.
9. All-digital open user-operated display keyboard, flexible and easy to operate and set.
10. Highly integrated microprocessor control system with reliable performance.
11. The high-current non-contact AC switch has stepless voltage regulation, wide voltage regulation range and strong overload capacity.