VFD vs. Soft Starter: What is the Key Difference?

In industrial motor control, VFDs and soft starters play important roles in controlling how a motor starts, stops, and runs. These devices are intended to optimize motor utilization and enhance motor durability by regulating electrical variables such as voltage, current strength, and frequency. Although these aims may coincide, they operate dissimilarly and are best used for particular purposes. It is important to know how a soft starter and VFD differ to determine which of the two you should use in your motor control.

In this blog, we will explore what is soft starter, what is VFD, the soft starter vs VFD difference, and how to compare VFD and soft starter for various applications.

What is Soft Starter? 

A soft starter is a motor control device that controls the rate at which voltage is applied to the motor. It is desirable to minimize the inrush current and limit the mechanical stress of the motor at the moment of starting. Soft starters are typically necessary when an accurate and gradual motor start is necessary, but speed regulation during operation is unnecessary.

How Does a Soft Starter Work?

A soft starter functions by regulating the voltage supplied to the motor at start-up, gradually increasing till the maximum voltage is achieved. In the normal mode, it is similar to a manual switch, in that once the motor is at full operating speed, the soft starter simply removes itself and leaves the motor running at full voltage.

Applications of Soft Starters 

Soft starters are widely employed in inflows, including pump, fan, and conveyor applications, where the load for the motor is continuous once the motor achieves full speed. They are also applied to protect the pumping systems against water hammers and minimize stress on the belts and gears in conveyor systems.

Key Features of a Soft Starter: 

• Reduces inrush current

• Minimises mechanical stress during startup

• Does not offer speed control after the startup

• More cost-effective than VFDs for basic motor startup needs

Benefits of Using a Soft Starter

• Prolongs motor life by reducing wear and tear during startup

• Reduces power surges that can affect the electrical grid

• Cost-effective solution for applications that don't require variable speed control

However, despite their advantages, soft starters are not very versatile. After the motor attains its full speed, a soft starter affords no control again, making it incompetent for use in applications that demand a change in speed or loads while in use.

What is a VFD? 

A Variable Frequency Drive (VFD) is a much more complex unit than a switch to fully control the motor’s speed and torque. A VFD controls the voltage and frequency of the power supplied to the motor, allowing precise motor speed control during run-up, running and coast-down.

How Does a VFD Work? 

VFDs operate as AC-to-DC converters followed by DC-to-AC converters but with adjustable voltage and frequency. This makes it possible for the VFD to vary the rotor speed while the motor runs at nearly any rate. In contrast to soft starters, the VFDs are not only concerned with the initial running of the motors but offer constant control of speed and torque, which makes them suitable for application to machines that require speed control.

Applications of VFDs 

VFDs are applied across all industries in processes such as HVAC systems, gears, and other industrial equipment,requiring precise speed control. They are frequently used in pumps, fans, compressors, and conveyors to control speed and attain optimum performance and energy utilization.

Key Features of a VFD:  

• Full control over motor speed and torque

• Reduces energy consumption by adjusting motor speed to match the load

• Protects the motor from overloads and short circuits

• More expensive than soft starters

Benefits of Using a VFD 

• Energy savings: VFDs allow motors to run at optimal speeds, reducing energy consumption.

• Enhanced control: VFDs provide precise control over motor speed, allowing for better process optimisation.

• Reduced wear and tear: VFDs minimise mechanical stress on the motor by controlling both voltage and frequency, extending its life.

However, VFDs are more expensive than soft starters and might be overkill for applications that don't require variable speed control.

Difference Between Soft Starter and VFD

To make an informed decision, it’s important to compare VFD and soft starter based on the specific needs of your application.

VFD and Soft Starter Difference  

Control Capabilities

When looking at the level of control, one will realise that there is a vast difference between a soft starter and a VFD. A soft starter only regulates the motor during starting; it gradually increases the voltage fed to the motor. In particular, when the motor attains maximum revolutions, the soft starter offers no control over the motor. In contrast, a VFD consistently provides full control of the motor spas speed and torque from start to stop by varying the voltage and frequency.

Cost

In most cases, soft starters are cheaper than VFDs, which makes them ideal for use where precise speed control isn’t necessary. Typically, VFDs are more costly than other types of controllers and drives, but in industries where a variable speed control motor is required, energy saving will eventually compensate for the higher initial cost in the long run.

Energy Efficiency 

A soft starter does not result in energy saving other than the time used to start the motor. They can save significant energy due to their ability to run the motor at a lower speed, therefore consuming less power when full speed is not required.

Applications

Craning loads or applications where the motor remains in operation at a steady speed after it starts are some areas where soft starters are normally utilised, most often in pumps, fans, and conveyors. VFDs are best suited for use in processes that require speed control during operation, for instance, in AC systems, variable load conveyors and any process that may require motor speed to be changed based on the load required.

Motor Protection

Soft starters and VFDs protect the motor; however, the protection provided by VFDs is more extensive than that of soft starters. VFDs can prevent conditions such as overload, short circuits and other electrical misfortunes through protective devices. The main application or function of soft starters is to control the inrush current and mechanical stress which occurs at the time of starting.

Complexity and Maintenance

VFDs are relatively larger control devices that comprise more components than soft starters and require more maintenance. Suppose your application is not complex and you need special control that VFD offers. In that case, you should consider installing a soft starter, as it is a simpler product and eliminates many problems.

When to Use a Soft Starter? 

• When smooth startups are needed, variable speed control isn’t.

• In applications where the motor runs at full speed once started.

• This is for cost-sensitive projects where basic motor protection is sufficient.

• In systems prone to mechanical stress during startup, such as pump systems.

When to Use a VFD? 

• When variable speed control is required to optimise system performance.

• In applications where energy savings are critical, such as HVAC systems.

• This is for processes where motor speed needs to adjust based on demand or load.

• In systems requiring advanced motor protection and control.

Also Read - A Quick Guide to Soft Starters!

Conclusion: Choosing Between VFD and Soft Starter 

Soft starter vs. VFD comparison finally reduces to your application's preferences and what kind of facility you are operating. If the application requires an easy and inexpensive motor starting without continuous speed control, then a soft starter may serve the purpose best. On the other hand, if your application needs speed variation, energy conservation and complex motor protection, then the VFD is the ultimate solution.

Comparing a soft starter and VFD is easy once one understands the key difference between the two products. Soft start and VFD are two inductions in motor control. Knowing when to use either of them will enhance the system's overall performance, reduce energy expenses, and enhance the motor's lifespan.