🔌 Everything You Need to Know About Soft Starters for Electric Motors

In the world of modern industrial maintenance, motor starting has evolved far beyond traditional direct-on-line (DOL) or star-delta starters. Today, electronic motor control systems like the soft starter offer smart and efficient ways to start and protect electric motors.

But what exactly is a soft starter? How does it work? When should you use it? And how do you choose and install one properly?

In this detailed and engaging technical article, we’ll walk you through everything you need to know to master this essential piece of industrial technology.

⚙️ What Is a Soft Starter?

A soft starter is an electronic motor starter designed to control the voltage applied to an electric motor during startup. Instead of supplying full voltage instantly (which causes high inrush current), the soft starter gradually increases the voltage, allowing the motor to ramp up smoothly.

This prevents the motor and connected mechanical equipment from experiencing sudden shocks, and reduces the electrical and mechanical stress caused by hard starts.

🧠 How Does a Soft Starter Work?

A soft starter uses thyristors (SCRs) or similar power electronics to regulate the voltage sent to the motor.

When the start command is given, the device allows only a portion of the full voltage to reach the motor windings. Over a set period (usually a few seconds), it increases the voltage gradually until the motor reaches full speed. Afterward, the device either disconnects or continues to monitor the motor depending on the configuration.

This soft starting process helps avoid the typical high inrush current that occurs in standard motor startup, making it a key part of motor starting solutions.

🚀 Why Use a Soft Starter?

You might wonder: why not just stick with traditional motor starting methods like DOL or star-delta? Here’s why soft starters are increasingly preferred:

They reduce inrush current by up to 60%, preventing sudden voltage drops in your electrical system.

They extend the life of mechanical components by minimizing startup shock (belts, pumps, gearboxes, etc.).

They help maintain electrical stability across your network.

They provide a safer, more controlled startup process for motors and operators.

🧩 When Should You Use a Soft Starter?

A soft starter is not needed in every case, but it's strongly recommended when:

Your motor is larger than 5.5 kW (7.5 HP).

Your electrical system is sensitive to voltage dips.

Your application involves mechanical systems that need smooth starts, such as pumps, fans, compressors, or conveyor belts.

You want to reduce maintenance costs caused by mechanical stress during hard starts.

📐 How to Choose the Right Soft Starter for Your Motor

To choose the right soft starter, you need to gather four key pieces of data from the motor nameplate:

1. Motor power (in kW or HP)

2. Supply voltage (230V, 400V, or 690V)

3. Rated current (Amps)

4. Type of load – is it light (like a fan) or heavy (like a compressor)?

With this information, choose a soft starter that supports the same voltage and handles a current equal to or slightly higher than the motor’s rated current.

📌 Example:

For a 22 kW motor running at 400V and drawing 42A, select a soft starter rated for at least 45A and compatible with 400V three-phase input.

Always ensure compatibility with your motor starting requirements and verify that the soft starter’s performance curve matches your application.

🔧 How to Install a Soft Starter (Basic Wiring Overview)

Installing a soft starter is not overly complex but requires attention to detail and electrical safety protocols.

Basic Steps to Install a Soft Starter:

1. Disconnect all power before beginning any installation work.

2. Connect the three-phase power supply (L1, L2, L3) to the soft starter input terminals.

3. Connect the soft starter output terminals to the motor terminals (U, V, W).

4. Connect the grounding (earth) properly.

5. If your soft starter includes control terminals, wire your START and STOP buttons as indicated in the manual.

6. Optional: connect it to a PLC or motor protection relay for added monitoring and safety.

Don’t forget to install a motor protection circuit breaker before the soft starter to protect against short circuits or overloads.

🛠️ Is a Soft Starter the Same as a VFD?

Not at all  this is a common misunderstanding.

A soft starter only controls voltage during startup. Once the motor reaches full speed, the soft starter either bypasses or stays in monitoring mode. It does not change motor speed.

A VFD (Variable Frequency Drive), on the other hand, controls the frequency and voltage, allowing full speed control of the motor throughout its operation. VFDs are ideal when your application requires variable speed or precise torque control.

✍️ Final Thoughts

If you work in industrial maintenance, electrical installation, or automation, having a deep understanding of soft starters is essential. It’s a powerful tool that improves safety, efficiency, and longevity in electric motor systems.

Choosing the right soft starter and installing it correctly can save your facility thousands of dollars in downtime and repairs  and give your systems the smooth, reliable startups they deserve.