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Can the motor of the online soft start cabinet start frequently?

Nov 05, 2020

Can the motor of the online soft start cabinet start frequently?



Starting principle of online soft starter cabinet:


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 a three-phase fully controlled bridge rectifier circuit. When using the online soft starter to start the motor, the output voltage of the thyristor gradually increases, and the motor gradually accelerates until the thyristor is fully turned on. The motor works on the mechanical characteristics of the rated voltage to achieve smooth starting, reduce the starting current, and avoid starting overcurrent tripping . When the motor reaches the rated speed, the start-up process ends, and the online soft start cabinet automatically replaces the completed thyristor 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 extend the online soft start cabinet The service life of the power grid improves its work efficiency, and the power grid avoids harmonic pollution. The online soft start cabinet also provides the 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 torque impact caused by free stop.


Nowadays, many motors use online soft-start cabinets as their starting protection devices. Can the motor be started frequently when using online soft-start cabinets?



This is obviously not possible!


Motors are divided into different systems according to operating conditions:


1. For motors with S1 duty system (continuous duty system), no matter whether it adopts soft start or not, it is not suitable for frequent start. Usually a maximum of 24 starts per hour is allowed;


2. S1 continuous working system:


The running time under constant load is sufficient to achieve thermal stability;


3. S2 short-time work system:


Run for a given time under constant load, this time is not enough to reach thermal stability, and then the energy will be stopped for enough time to cool the motor again to within 2K of the temperature of the cooling medium;




4. S3 intermittent work system:


Run according to a series of the same work cycle, each cycle includes a period of constant load operation time and a period of power-off and shutdown time. The starting current of each cycle in this working system will not have a significant impact on the temperature rise;


At the same time, frequent starting will cause the SCR of the online soft start cabinet to overheat, and because there is no rest time, frequent start will cause the SCR breakdown, which will damage the online soft start cabinet.


The start cycle of the soft starter is limited, that is, how many times are allowed to start in a long time. Moreover, the soft starter has a short-time working system and cannot run for a long time.


Furthermore, frequent starting is not good for the motor and will reduce the service life of the motor.