Why Do Some Compressors Have A Soft Start?

- Jun 01, 2019-

Why do some compressors have a soft start?


We know that the directly start current of the motor is 4 to 7 times of the rated current. If the power of the motor is relatively small, then the impact on the grid is not very large. We allow it to start directly. Generally, the motor power is below 7.5KM. Directly launched.


Why do some compressors have a soft start?


Why is the motor soft start?


But for large motors, the power is particularly large, so it is not allowed to start directly. If his rated current is 100A, then the current can reach several hundred amps at the moment of starting. The impact of such a large current on the grid is very The large ones will affect the normal operation of other loads carried by the same power grid. In this case, we can often see that, for example, when a large load starts, the light bulb will be darkened, etc., and we sometimes have this situation at home. For example, when the TV is turned on, the light bulb sometimes suddenly darkens, and also because the starting current of the load is too large, and the insulation of the motor winding is also extremely disadvantageous.


Therefore, for large motors, soft start is generally required. The so-called soft start is that the voltage we add to the initial stage is lower than the rated voltage.


Common soft starters include soft starter start-up and autotransformer step-down start. The autotransformer step-down start is applied to a voltage of 60% to 80% of the rated voltage at the beginning of the motor start-up. Let it move first. After turning it, let him switch to the rated voltage to work. This is a simple soft start process, and the soft starter is more advanced than the autotransformer step-down start, the principle is the same, soft start When the voltage of the motor is gradually increased from 0 to the rated voltage, his starting process is smoother, the starting effect is better, and the impact on the grid and the damage to the winding is also minimal, which is better than the auto-transformer step-down starting. This is the advantage of soft starters in industrial applications.


The most commonly used in today's transmission engineering is the three-phase AC induction motor. In many cases, these motors are directly connected to the power system due to their starting characteristics. If you start directly online, it will generate up to 6 times the rated current of the motor. This current overloads the power supply system and the switching devices in series. If it is started directly, it will also produce a higher peak torque, which will not only impact the drive motor, but also damage the mechanical device. For example: auxiliary power transmission unit (V-belt, gear). In order to reduce the direct current, a starting auxiliary device such as a starting reactor or an autotransformer should be used, but conventional methods such as starting a reactor or an autotransformer can only gradually reduce the voltage, and the soft starter smoothly raises the terminal. The voltage can be achieved without impact starting.


Therefore, the power supply system and the motor can be optimally protected.


1. Reduce the starting current of the motor, reduce the distribution capacity, and avoid the investment of capacity expansion;


2. Reduce the starting stress and extend the service life of the motor and related equipment;


3. Smooth start and soft stop avoid the surge problem and water hammer effect of traditional starting equipment;


4, a variety of starting modes and a wide range of current, voltage and other settings, can adapt to a variety of load conditions;


6. It can smoothly start/stop the motor without stepping, avoiding mechanical and electrical shocks caused by starting the motor with traditional starting methods such as direct start, star/delta start, auto-decompression start-up, etc.


7, can effectively reduce the starting current and distribution capacity, to avoid capacity expansion investment.




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