What is Soft Starter and How does it work?

Well, we all know about basic electrical motor starters. If you are totally beginners in an electrical field then you may consider reading about basis electrical starter. There are various methods to start an electrical motor like “DOL – Direct Online Starter”, “Star-Delta Starter”, “Soft Starter”, “VFD – Variable Frequency Drive”.

All the above-described methods have their own applications and benefits. But, in this blog, we are going to see What is Soft Starter and How it works? 

What is Soft Starter?

A soft starter is a device that is added between the main power source and an electrical motor. The main purpose to use the soft starter is to reduce the initial stress to an electric motor during power-up.

what is soft starter

Soft starter slowly and gradually begins applying a voltage to an electrical motor. This will allow a smooth acceleration of the power instead of a sudden and violent burst of the power to an electrical motor. That will ultimately reduce the initial stress to the motor as well as increases the life-span of electrical equipment.

There is a high inrush electrical current in all the typical starters, whereas a soft starter provides a smooth and steady linear slope of the power. This considerably reduces wear and tear of electrical motor circuits and resulting in a healthier overall machine.

Soft starters are generally used in industrial applications that require high initial torque and large inrush current like crusher, water pump, blowers, etc.

How does Soft Starter work?

what is soft starter

Soft Starter is connected in serial in the three-phase power supply circuit through three pairs of thyristors connected in reverse parallel. It takes the advantage of the electric switch characteristic of the thyristor and controls its voltage.

When the starting voltage reaches its rated value the bypass contactor will pick up and put the electrical motors into operation directly.

An electrical soft starter is one example of a possible soft start solution. There is also another option like VFD.

Some Common Applications for Soft Starter

As above we have seen What is a soft starter and how does it work? till now. After that, another important question When do I need a soft starter?

A soft starter is used where there is a considerable amount of torque and high inrush current is required to start the equipment like,

Water Pump ⇒ In various pump application there may be a risk of power surges. By installing a soft starter and limiting current to the pump motors this risk can be reduced.

Ball Mill ⇒ By using soft starter voltage slop starting, we can reduce the abrasion of gear torque and a working load of maintenance.

Blower Fan ⇒ It is always possible that a sudden start may cause problems, the sudden and jerky start may damage the gear and may impact the life of the electrical motor. By installing a soft starter startup will become smooth and stable.

Compressor ⇒ By using soft starter current limiting mode, we can achieve a smooth starting and reduce the heat production of the motor and prolong its life cycle.

Crusher ⇒ Through soft starting we can achieve a smooth start-up, achieve a gradual start.

Advantages of Soft Starter

Reduced Energy Use

The soft starter gradually builds up the voltage and applied it to the electrical motor, this, in turn, prevents the motor’s initial high starting current. This means that use less energy as compared to convention electrical starter.

Lower Risk of Power Surge

In a conventional starter, the maximum voltage is applied to the motor initially, there is always a possibility of overload on circuits and this may lead to a power surge. While with the soft starter, voltage is gradually applied to the electrical motor. This, in turn, reduces the chance of power surge.

Adjustable Acceleration Time

Almost all the soft starter comes with this option, the user can set the time to build up the maximum voltage as per application. This provides flexibility to the user.

Reduced Risk of Overheating

As the soft starter provides a soft slop for the voltage to build up at the motor. So there is always a gentle surge of electricity is supplied to the motor instead of the initial blast of power, resulting in the reduced risk of overheating.

Improved Operating Efficiency and Extended Life Span

A conventional starter can work fine sometimes but not always. However, for some heavy load applications, a soft starter provides a smooth start-up, reducing the chance of overheating and electrical power surge. Because of all that soft starter greatly improves the efficiency and life span of the overall machine.


In conclusion, we can say that, when an industrial application requires high initial torque and a large inrush current, a soft starter is an ideal option for controlling the system.

I hope now you got the basic idea about What is Soft Starter and how does it work? Thank you for reading this article, if you have any query then feel free to comment down below. You can also find some important articles about PLC below.

You can find the must-read articles about Programmable Logic Controller from the below link:

What is PLC? How does it work?

Logic gates using PLC ladder logic.

Basic bit logic instructions with PLC programming examples.

5 different types of PLC programming languages.

Three different types of electric motor starter with its PLC program.

30+ Most useful PLC communication protocols

How timer works in PLC?

How counter works in PLC?


You can also find the important questions and answers asked in the interview.

25+ Most important questions asked in the PLC interview

Top 15 always asked questions in the SCADA interview

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1 thought on “What is Soft Starter and How does it work?”

  1. Very interesting, good job, and thanks for sharing such a good blog. your article is so convincing that I never stop myself to say something about it. You’re doing a great job. Keep it up.


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