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Comparison of starting current and rated current of soft start cabinet

Jan 08, 2021

Comparison of starting current and rated current of soft start cabinet

The soft-start control cabinet is the motor control center. The soft start control cabinet is composed of the following parts: input circuit breaker, soft starter (including electric control circuit and three-phase thyristor), soft starter bypass contactor, secondary side control circuit (to complete manual start and other functions selection and Operation, remote start, soft start and direct start), with voltage, current display and fault, running, working status indication and other indicator lights.


After using the soft starter in the soft start cabinet, the starting current is 2-3 times the rated current, which is estimated to be 2.5 times. The working current is the rated current when the motor is fully loaded. The soft starter is only an auxiliary starting. When a high-power motor starts, it can reduce the starting current of the motor and affect the power grid.


The soft starter uses a three-phase parallel thyristor as a voltage regulator, which is connected between the power supply and the motor stator. This circuit is similar to a three-phase fully controlled bridge rectifier circuit. When a 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 completely connected. The motor works on the mechanical characteristics of the rated voltage to achieve a smooth start, reduce the starting current, and avoid starting overcurrent tripping. When the motor reaches the rated speed, the starting process ends, and the soft starter automatically replaces the completed thyristor with a bypass contactor to provide rated voltage for the normal operation of the motor, thereby reducing the heat loss of the thyristor and extending the service life of the soft starter. Improve its work efficiency and avoid grid harmonic pollution. The soft starter also provides a soft stop function. Compared with the soft start process, the voltage gradually decreases and the number of revolutions gradually decreases to zero, avoiding the torque impact caused by free stop.