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2014-11

Soft starter working principle and operation characteristics

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The starting torque Ma of a three-phase asynchronous motor is directly related to the quadratic of the applied voltage, that is, as long as the voltage on the motor terminals is lowered, these values ​​are affected. The soft start works by controlling the conduction angle of the three opposite parallel thyristors connected in series between the power supply and the controlled motor to raise the terminal voltage of the motor from a preset value to a rated voltage.

    First, the main startup mode of soft start

(1) The voltage double ramp starts. During the startup, the output torque of the motor increases with the increase of voltage. An initial starting voltage Us is provided at startup. The Us is adjustable according to the size of the load, and the Us is adjusted to be greater than the load. Static friction torque produces optimum starting characteristics. At this time, the output voltage rises from Us at a certain slope, and the motor continues to accelerate. When the output voltage reaches the speed-up voltage Ur, the motor also reaches the rated speed. The soft starter automatically detects the speed-up voltage during the start-up process, and when the motor reaches the rated speed, the output voltage reaches the rated voltage.

(2) Current limiting start: It is the soft start mode that limits the starting current of the motor to not exceed the set value during the starting process of the motor. The output voltage rapidly increases from zero, knowing that the output current reaches the preset current limit Im, and then gradually increases the voltage under the condition of maintaining the output current I<Im until the rated voltage, so that the motor speed gradually rises to reach the rated value. Rotating speed. The soft starter used in the cold shearing machine of the rolling mill adopts the current limiting start, which reduces the starting current in the conventional mode during the starting process, and disconnects the soft when the starting voltage reaches the rated voltage. Starter bypass contactor. The advantage of this startup method is that the starting current is small and can be adjusted as needed. The impact on the power grid is small, and its disadvantage is that it is difficult to know the starting voltage drop at the time of starting, and the pressure drop space cannot be fully utilized.

(3) kick start: start the start phase, let the thyristor fall back after all the conduction in a very short time. This time can be adjusted artificially and then linearly rise according to the original set value, enter the constant current start, the start The method is suitable for start-up situations where heavy loads are required and friction is to be overcome.

(4) Soft start application

Asynchronous motors can be used in a variety of applications where speed regulation is not required. They are suitable for all types of fan loads and require soft start and soft stop. For changing load conditions, the motor is in light load operation for a long time, only in short-term or instantaneous full-load operation, the application of soft starter (without bypass contactor) has the effect of light load and energy saving.

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