How to Choose the Right Soft Starter for Your Motor Application

Introduction

Motor starting methods have evolved into a critical element of modern industrial automation. Whether in pulp mills, water treatment facilities, or conveyor-driven assembly lines, soft starters have become the go-to solution for balancing electrical efficiency with mechanical longevity.

The global motor soft starter market was valued at $2.4 billion in 2022, and is projected to reach $4.5 billion by 2032, growing at a CAGR of 6.6% from 2023 to 2032. (source). This growth reflects increasing demand for energy efficiency, reduced equipment wear, and smarter motor control across industries.

Electric motors consume 60–70% of total industrial plant electricity, often necessitating a reliable motor starter. When these motors are started directly—via Direct-On-Line (DOL) or star-delta starters—excessive inrush current and mechanical shock can lead to equipment failures. Soft starters mitigate these risks by gradually ramping up voltage and torque, thus minimizing heat generation, reducing maintenance, and extending equipment lifespan.

While Variable Frequency Drives (VFDs) offer full-speed control, they are often unnecessary for fixed-speed applications. For those, soft starters present a simpler, more cost-effective alternative.

What Is a Soft Starter & How Does It Work?

1. Definition and Purpose

A soft starter is an electronic device that allows motors to start smoothly by gradually increasing the voltage supply. This prevents the mechanical shock and high inrush current that occurs with direct-on-line starts. The soft ramp-up in voltage helps reduce torque spikes, minimizes heat, and protects mechanical components.

Soft starters are most useful in applications where motors run at a fixed speed but need a controlled start and stop. Industries using pumps, fans, conveyors, and compressors commonly benefit from soft starters.

2. How It Works

Soft starters operate using solid-state devices such as Silicon Controlled Rectifiers (SCRs), which modulate the motor voltage during startup.

• SCRs progressively open, controlling the voltage to the motor is crucial for effective soft start functionality.
• Voltage is ramped up gradually in a soft start to minimize inrush current. over a set time, reducing inrush current.
• Once the motor reaches full speed, a bypass contactor engages to maintain efficiency.

This results in reduced electrical stress and enhanced control over acceleration.

Core Components of a Soft Starter

1. Thyristors/SCRs

These are semiconductor switches that regulate how much voltage is applied to the motor. They enable gradual voltage increases, controlling the acceleration of the motor.

2. Control Circuit

This circuit determines when and how the SCRs are triggered. It governs the ramp-up and ramp-down times, and may also house features such as current monitoring and overload protection.

3. Bypass Contactor

Once the motor reaches full speed, the SCRs are bypassed using an electromechanical contactor. This reduces power loss and heat within the starter.

4. Monitoring Display

Many soft starters, especially when using a soft start feature, include an LCD or LED interface that displays status information, error codes, and allows configuration of parameters such as voltage ramp time and start torque.

Key Benefits of Soft Starters

Soft starters reduce both electrical and mechanical stress during motor startup, which helps extend the life of motors and associated mechanical systems. They are often more compact and economical than VFDs, particularly in fixed-speed applications. Additionally, they produce less heat and fewer harmonics, enhancing energy conservation and overall power quality.

Key Factors to Consider When Selecting a Soft Starter

Motor Specifications

Selecting the right soft starter begins with knowing your motor. This includes its horsepower (HP) or kilowatts (kW), voltage and frequency (such as 460V, 60Hz), and the motor’s Full Load Amps (FLA), which can be found on the nameplate. You’ll also need to consider the motor type—squirrel-cage or wound-rotor—as this affects compatibility.

Load & Application Type

Different applications require different torque characteristics. Constant torque loads like conveyors and crushers need a different approach compared to variable torque loads like pumps and fans. Additionally, whether your application has high or low inertia—centrifuges versus elevators—impacts soft starter selection.

Starting and Stopping Requirements

Ramp-up and ramp-down time affect how smoothly a motor starts and stops. Torque limits in soft starters, especially reduced voltage starters, prevent over-torquing sensitive loads like mixers and blowers. For pump systems, soft stop functionality helps prevent water hammer, a damaging hydraulic shock.

Duty Cycle & Starts Per Hour

The frequency of starts per hour can lead to thermal buildup. Manufacturers specify thermal ratings such as Class 10, 20, or 30. In high-frequency applications, you may need to oversize your soft starter.

Environmental Conditions

Ambient temperature, altitude, dust, and humidity all impact soft starter performance. Use appropriate enclosure ratings such as NEMA or IEC (e.g., IP65) to ensure reliable operation in harsh environments, especially when dealing with high amounts of current.

Power System and Generator Sizing

When using a generator, it's essential to size it properly—typically three times the motor’s FLA. Soft starters help reduce the voltage dip during startup and allow for extended ramp times, which are gentler on gensets.

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Main Types of Soft Starters and Their Use Cases

Primary Resistor Soft Starters

These use resistors in series with the motor to limit starting current. While cost-effective, they offer low torque and are mostly suited for legacy systems.

Autotransformer Soft Starters

These starters use multi-tap voltage settings to provide better torque and lower inrush currents. They are commonly used in HVAC systems and large motor-driven compressors.

Part-Winding Soft Starters

This method energizes the motor windings in sequence to reduce inrush current. It is typically used in motors designed with two windings.

Wye-Delta Starters

Open Transition

This method introduces a brief power dip during the transition between star and delta connections. It is simpler and more affordable.

Closed Transition

This approach avoids power interruption during switching, offering smoother operation but with added complexity and cost.

Solid-State (SCR-Based) Soft Starters

Modern SCR-based soft starters provide stepless, precise control and often come with integrated protection and diagnostics. They are the preferred choice in critical and continuous-process environments.

Step-by-Step Soft Starter Sizing Guide

1. Gather motor data: HP, FLA, voltage, frequency.

2. Classify the application: light, medium, or heavy-duty.

3. Determine starts per hour: important for thermal stress considerations.

4. Check environmental conditions: temperature, altitude, and enclosure rating.

5. Apply derating if needed: based on manufacturer guidelines.

6. Example — 500 kW wood chipper:

• FLA = ~620A, starts/hour = 4

• Recommend Class 30 soft starter with thermal derating
  
  

Soft Starter vs. Other Motor Starting Methods

Selecting a Manufacturer & Model

Top Soft Starter Brands

Notable brands include Eaton (DS7 Series), Schneider Electric (ATS22, ATS48), Siemens (Sirius 3RW series), ABB (PSTX series), and WEG (SSW series) for various motor starters. Each offers a range of models suited for different industrial environments, making it easier to select the soft starter based on the type of motor.

Features to Compare

Compare built-in bypass contactors, diagnostic interfaces with LCDs, and protection features like phase loss and overload safeguards. Also consider support for communication protocols such as Modbus, EtherNet/IP, and Profibus. Look for options with inside-delta compatibility and flexible mounting configurations.

Installation & Best Practices

Ensure correct wiring—either traditional three-phase or inside-delta—as per your system design. Proper grounding helps prevent transients and surges. Good ventilation and thermal spacing extend device life, especially under varying operating conditions. Stay current with firmware updates to maintain peak performance.

Maintenance & Troubleshooting

Routine maintenance involves checking LED indicators and reviewing error codes. Periodically tighten electrical connections and conduct infrared thermographic inspections. Track starts per hour to monitor thermal stress on the motor soft starter. With proper care, most motor soft starters have a service life of five to ten years.

Frequently Asked Questions

Q: How do I select a soft starter for a motor?
A: Start by gathering the motor nameplate data for the motor soft starter: horsepower, full load amps (FLA), voltage, and frequency. Then, identify the application type—constant or variable torque—and the number of starts per hour. From there, determine the required soft starter class (10, 20, or 30), environmental protection needs (IP/NEMA rating), and desired features such as communication protocols, overload protection, or built-in bypass contactor.

Q: How do I size a soft starter for AC motors?
A: Begin by identifying the motor’s FLA and voltage. Adjust for altitude and ambient temperature using manufacturer derating tables. Consider the motor’s service factor and the start frequency when selecting a soft starter for your application. For high-inertia loads or frequent starts, oversizing may be necessary. Always consult the soft starter's datasheet for matching capabilities.

Q: What’s the difference between a soft starter and VFD in the context of induction motors?
A: A soft starter limits voltage during startup to reduce inrush current and mechanical stress, making it ideal for fixed-speed loads. A VFD (Variable Frequency Drive), on the other hand, can control the motor’s speed continuously by varying both voltage and frequency. VFDs are better for applications requiring dynamic speed adjustments or precise process control, while soft starters are simpler and more cost-effective for basic acceleration/deceleration.

Q: What size generator is needed for a soft starter?
A: A good rule of thumb is a generator with a capacity of at least three times the motor’s full load amps (FLA). Soft starters are beneficial in generator-powered setups as they reduce the motor’s inrush current, minimizing voltage dips and helping maintain generator stability. Extended ramp times can also ease the load on smaller gensets.

Q: When should I choose a soft starter over VFD?
A: Choose a soft starter when your motor operates at a constant speed and only requires controlled acceleration or deceleration. They're ideal for conveyors, pumps, compressors, and HVAC systems. If speed variation, feedback, or process control is essential, a VFD is the better fit.

Q: Are there any disadvantages to using soft starters for your application?
A: Yes. Soft starters do not offer speed control beyond startup and shutdown. They may not be effective for very high starting torque applications. Additionally, while soft starters improve power quality during startup, they do not mitigate harmonics like some VFDs do. They're also generally less effective in regenerative or braking operations.

Conclusion & Next Steps

Choosing the right soft starter begins with understanding your motor, your load, and your environment. From selecting the proper starter class to verifying enclosure ratings and ensuring compatibility with your power system, every decision impacts reliability and energy efficiency.

Use this quick checklist for selecting the right soft starter for your application:

• Motor Data → HP, FLA, voltage

• Load Profile → Constant vs. variable torque

• Application Class → Light/medium/heavy

• Environment → Temp, altitude, ingress protection

• Brand & Features → Diagnostics, comms, bypass
  
  

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