Soft starter capacity 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.

Capacity selection of soft starter shall be considered comprehensively, such as its load carrying capacity, working system, environment, cooling conditions.

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.