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# How to Size a Soft Starter?

Load capacity of soft starter mainly refers to the overload capacity. Due to different working system, actual load capacity of soft starter is also different. For example, the soft starter generally bears a current of 2~4 times of rated current at starting process, and the duration is within 60s. For long-term working systems, the soft starter actually works under short-term overload during starting process. But for short-term working systems, the soft starter actually works under long-term overload during starting process. Thermal capacity of power semiconductor devices (thyristors) is very small, so the overload capacity of devices mainly depends on the overload capacity of power conductor devices and heat dissipation capacity of soft starters.

I. Overload capacity of the soft starter is generally designed according to the following principles
For overload lasting for more than 15min, it shall be designed by continuous load condition.
For overload lasting for less than 15min, the device’s overload capacity is calculated by the method for calculating junction temperature of thyristor devices by transient thermal impedance curve.
It shall be noted that: in general, the larger the thyristor capacity, the larger the radiator size and the larger the radiator fan, the stronger the load carrying capacity of the corresponding soft starter. Of course, the higher the volume and cost of corresponding equipment. For power electronic equipment, it’s not the smaller the volume, the better under the same capacity.

II. How to select soft starter capacity
In principle, soft starter size should be larger than that of the motor being driven. The rated capacity of the soft starter is usually chosen according to two nominal values, one is corresponding motor power, another is maximum allowable working current of soft starter. Following two aspects shall be noted:
1. When it’s nominated by the rated power of the motor being driven, the rated current of products with different voltage grades is different. For example, 75kW soft starter, its voltage grade is AC 380V, then the rated current is 150A. If its voltage grade is AC 690V, the rated current is 100A.
2. If it’s nominated by the maximum allowable working current of soft starter, the rated capacity of products with different voltage grades is different. For example, 160A soft starter, its voltage grade is AC380V, then the rated capacity is 75kW. If the voltage grade is AC 690V, the rated capacity is 132kW.

Capacity selection of soft starter shall be considered comprehensively, such as its load carrying capacity, working system, environment, cooling conditions.It shall be explained here that: as a general principle, the full voltage blocking torque of motor is much higher than the starting torque of load, the much higher, the easier control to the starting process. However, starting torque of motor can’t be improved by simply increasing the capacity of soft starter without increasing the capacity of motor.

III. In some circumstances, capacity of soft starter should be upsized
1. The soft starter, which operates online under full voltage or adopts energy-saving control mode, usually operates under heavy load. Compared with motors of the same grade, the rated current of soft starter has a smaller current margin, so if the motor often operates under heavy load, its operating current will easily exceed the rated current of soft starter, and thus causing overload of soft starter during operation. So the capacity of soft starter shall be upsized.
2. The motor is used for continuously variable or intermittent loads with shorter periods. In this case, the motor is not allowed to overload for a short time, otherwise, the soft starter may overload during operation. So the capacity of soft starter shall be upsized.
3. The motor works at a repeated short-term working system, and the cycle is less than the time interval of starting specified by the manufacturer. In this case, soft start overload may occur at starting process, so the capacity of soft starter shall be upsized.
4. Some loads are too heavy or the grid capacity is too small, when the motor starts, long starting time causes overload tripping of soft starter. So on the premise that the motor is not damaged, the capacity of soft starter can be enlarged appropriately.
5. For loads with special requirements for acceleration time, the length of motor acceleration time is a relative concept related to the inertia. Some loads require shorter acceleration time, and the acceleration current of motor will be larger. The capacity of soft starter can be enlarged appropriately by then.
6. Overcurrent protection may be caused by the load with large impulse current in the transition process, and it can be solved by increasing the capacity of soft starter.