Soft start of the motor and several starting methods.

Update:03 Dec 2018

The soft starter connected in series between the power […]

The soft starter connected in series between the power supply and the controlled motor is used to control the conduction angle of the internal thyristor, so that the input voltage of the motor gradually rises from zero with a preset function relationship until the end of the start, and the full voltage of the motor is given. Soft start, during the soft start process, the motor starting torque is gradually increased, and the rotational speed is gradually increased. Soft start generally has the following starting methods.

(1) Ramp boost soft start. This starting mode is the simplest, does not have current closed-loop control, only adjusts the conduction angle of the thyristor, so that it has a certain functional relationship with time. The disadvantage is that, due to the unlimited flow, during the starting process of the motor, sometimes a large inrush current is generated to damage the thyristor, which has a great influence on the power grid, and is rarely used in practice.

(2) Slope constant current soft start. This starting method is to gradually increase the starting current in the initial stage of the motor starting, and keep constant when the current reaches the preset value (t1 to t2 phase) until the starting is completed. During the starting process, the rate at which the current rises and changes can be adjusted according to the motor load. When the current rise rate is large, the starting torque is large and the starting time is short.

The starting method is the most widely used starting method, and is especially suitable for the starting of fans and pumps.

(3) Step start. When the power is turned on, the starting current is quickly reached the set value in the shortest time, that is, the step start. The quick start effect can be achieved by adjusting the starting current setting.

(4) Pulse shock starting. In the start-up phase, let the thyristor turn on for a period of time after a large current is turned on for a short period of time, and then rise linearly according to the original set value, and connect to the constant current start.
The starting method is less applicable in general loads and is suitable for starting situations where heavy loads are required to overcome large static friction.