Introduction to Three Sorts of Soft Starters

Figure 1: 3 different sizes of soft starters.

Soft starter, in essence, is a reduced-voltage starter that can control automatically. As it can adjust output voltage arbitrarily, plays a role in current closed-loop control, it has more advantages, compared with traditional reduced-voltage modes such as series resistance start, autotransformer start, etc.

1. Classification of Soft Starter

There are a lot of classification methods:
According to the voltage, there are high-voltage soft starter, and low-voltage start.
According to the drive medium, there are solid soft starter, and hydraulic resistance soft starter.
According to the control principle: there are electronic soft starter, and electromagnetic soft starter.
According to the operation mode: there are online soft starter, and bypass soft starter.
According to the load: there are standard soft starter, and heavy load soft starter.

Detailed introductions about the online operation soft starter, bypass operation soft starter (external bypass soft starter), built-in thyristor bypass type online operation soft starter are as follows.

1.1 Online Operation Soft Starter

Figure 2: The wiring diagrams of online and bypass type operation soft starters.

Last century, the brands that produced online operation soft starter were A-B, ABB, Schneider, Siemens, etc. During the application process, it was found that online operation has the following disadvantages:

1.Large power consumption of long-term online operation of the thyristor results in energy waste.
2.The amount of heat dissipation of thyristor is so large that mechanical air cooling is required.
3.Long-term online operation of thyristor poses high harmonic pollution to power grid.
4.The thyristor is far less reliable than the mechanical switch in long-term work as a main switch element.
5.Cost is too expensive for consumers to accept it.

Its advantages:
1.It integrates the starting, protection, and control of motor, and powerful intelligent controller is completely exerted.
2.It is suitable for places where frequent starting is needed for adopting mechanical air cooling.
3.The circuit is simple and easy to maintain and repair.

1.2 Bypass Operation Soft Starter

Figure 3: The appearance of bypass soft starter cabinet.

At the end of last century and the beginning of this century, considering the shortcomings and technical difficulties of online operation, the soft starter manufacturer has directly developed bypass soft starter, which bypasses to the contactor after the motor is activated. Its advantage is that it avoids the shortcomings of online operation of thyristor, especially without mechanical air cooling.

Nevertheless, it has the following disadvantages:
1.Circuit is intricate, and system is less reliable.
2.Powerful intelligent controller is not fully utilized, and some cannot protect the motor.
3.The volume and cost of sets of devices are increased.
4.Difficulty of maintenance and repairment mounts.

A comprehensive comparison shows that bypass operation mode is mostly adopted in the market. Even if the soft starter with the online operation mode is selected, the designer still installs a set of bypass operation contactor to make the soft starter operate in bypass.

1.3 Built-in Thyristor Bypass Type Online Operation Soft Starter

Figure 4: Built-in bypass soft starter.

Built-in thyristor bypass type online operation soft starter, (shortened to built-in bypass soft starter), is a device in which a set of mechanical contacts and thyristor in parallel are set in the inside of online operation soft starter. During the process of soft starting and soft parking, the thyristor is operated, and the mechanical contact is disconnected; when the motor is running normally, the thyristor is turned off, and the mechanical contact is closed. This set of actions is automatically completed by the internal controller.

It also can be called bypass type soft starter, which moves the external contactor into the soft starter and integrates it into one without increasing the size. It has all the advantages of the above two types while dodging their respective disadvantages:

1.Simple circuit.
2.Natural air cooling.
3.The thyristor is only in charge of starting and parking, avoiding power consumption and heat dissipation caused by its online operation.
4.The size is as small as the external bypass type, saving the space of the complete set.
5.Powerful intelligent controller is fully exerted, providing protection and control for motor.

2. Conclusion

Based on analysis above, Built-in bypass type soft starter is widely adopted with its most advantages and least shortcomings. When selecting the soft starter, it should take the function, technical parameters and reliability of the specific product brand into account, and make the selection according to the actual use.

Related Info