Working Principle and Starting Modes of Soft Starter

Figure 1: Soft starter.

Soft starter is a motor control device that integrates soft starting, soft parking, and light load and energy saving of motor, and multiple protection functions. Not only does it realize the smooth and shock-free motor starting during the entire starting process, but adjusts the parameters in the process, such as current limiting value, starting time, etc., according to the characteristics of motor load.

1. Working Principle of Soft Starter

Soft starter is mainly composed of three-phase antiparallel thyristor connected in series between power supply and controlled motor and its electronic control circuit.
Different methods can be used to control the conduction angle of three-opposite parallel thyristor, so that the input voltage of the controlled motor changes in accordance with different requirements, and different functions can be realized.

When the motor is activated by soft starter, output voltage of thyristor steps to rise, and the motor gradually accelerates. It is not until the thyristor is full conduction that the motor works at the rated voltage to achieve smooth starting, reduce starting current, and avoid starting overcurrent tripping.

When the motor reaches its rated revolution, the starting process is over. The soft starter automatically replaces the thyristor that has completed its task with a bypass contactor, and provides rated voltage for the normal operation of the motor to reduce the heat loss of the thyristor, prolong the service life of the soft starter, improve its working efficiency, and make the power grid avoid harmonic pollution.

At the same time, soft starter also provides soft parking function. Soft strating is opposite to soft parking, in which the voltage gradually decreases, and the revolution drops to zero to avoid torque impact caused by free parking. The voltage curves of soft starting and soft parking are shown in the figure below.

Figure 2: The inner structure of soft starter.

2. Starting Mode of Soft Starter

2.1 Ramp Type Voltage Rising Soft Starting

This is the easiest way. As is shown in figure 1, there is no current closed-loop control, and only the conduction angle of the thyristor is adjusted to increase it as a certain function of time. The shortcoming is that, due to no current limit, sometimes a large impulse current will be generated to damage the thyristor in the process of starting, which has a great impact on the power grid, and this way of starting is rarely used in practice.

Figure 3: Ramp voltage rising soft starting.

2.2 Ramp Type Constant Current Soft Starting

This starting mode is to gradually increase starting current in the initial stage of motor starting, and when the current reaches the default value (t1-t2 stage), keep it constant until the starting is over. During the starting process, rate of current rise change can be adjusted and set in accordance with motor load. The faster the current rise is, the larger the starting torque is, and the shorter the starting time is. This mode is one that has been applied most, especially suitable for starting of fans and valves load.

2.3 Snap Soft Starting

After starting up, it makes the starting current reach the default value within the fastest time, which is called snap soft starting. By adjusting the default value of starting current, the fast starting can be achieved.

2.4 Impulse Impact Soft Starting

In the initial stage of starting, let the thyristor conduct in a very short time with large current for a period of time, then fall back, and then rise linearly in accordance with the default value, and connect to the constant current to start. This starting method, which is rarely used in general loads, is suitable for heavy loads and needs to overcome large static friction.

Figure 4: Sudden jump soft starting.

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