Soft Start vs VFD – What’s the Difference and Which is Better?

From improving your energy efficiency to avoiding mechanical damage, the difference between a soft starter and a variable frequency drive (VFD) matters for your motor applications. If you’re looking to save money and keep your mechanical parts safe, understanding which is best for your application is important. Soft starter vs VFD: what’s the difference and which is better? That’s the question we’re here to answer. In this blog post, we’re going to compare and contrast these two motor-control options while providing insight on which might be the best fit for your particular motor.

Quick Explanation of Key Points

A Soft Start uses current limiters, resistors and/or reactors to reduce the starting current during acceleration. On the other hand, a Variable Frequency Drive (VFD) uses adjustable frequency to modulate motor speed according to demand.

Soft Start vs VFD Overview

Soft starts and variable frequency drives (VFD) are both power electronics used to control the acceleration profile of an electric motor. When motor applications require gentle ramping at start up, either a soft start or VFD can be employed to protect sensitive equipment, reduce wear and tear on the system, and generally improve performance.

Soft starts employ power resistors in series with the incoming voltage while variable frequency drives use Pulse Width Modulation (PWM) to vary the speed of the motor. Soft starts provide an effective starting solution when only the acceleration needs to be refined. However, when precise speed regulation is needed, a VFD is preferred as these are more energy efficient and provide more accurate system control over a wider range of conditions.

On the other hand, soft starts are easier to install and configure than VFDs and cost less. Depending on the application, one type may have advantages over the other in terms of scalability and flexibility as well as technical issues like torque control or overload protection during starting.

The debate between soft start vs VFD continues as both types of power electronics have pros and cons that must be weighed according to each particular application in order to determine which is better for any given situation. The next section will describe different types of power electronics used for motor control and their respective benefits for various uses and conditions.

Power Electronics in Motor Control

Power electronics in motor control provides greater flexibility and accuracy when compared to traditional methods. This type of motor control is based on the electrical current passing through it to direct the motion of the machine. By applying power to the motor, designers can realize higher efficiency, performance and lifetime for their applications.

Power electronics can be used for a wide range of applications and are commonly employed in servo motors, position control systems and even pumps. They allow motor speed and torque to be regulated accurately with response times as quick as 10ms. High-performance servo drives provide accurate and repeatable stopping points for precise positioning tasks.

Speed regulation can also be accomplished through power electronic devices such as Variable Frequency Drives (VFD). VFDs provide precise control over voltage, frequency and current when controlling induction motors. This allows the speed of the motor to be adjusted quickly while maintaining high efficiency.

On the other hand, Soft Start controllers are designed to reduce the linear ramp up time of an ac induction motor. When they are enabled, they reduce starting current by slowly increasing voltage as the motor ramps up to its maximum speed. This improves machine reliability by reducing mechanical stress on components due to sudden starts and increases lifespan with its gentle acceleration properties.

Soft starters provide smoother transitions between stops and starts which eliminates mechanical shock but falls short when compared to modern power electronics like VFDs for a variety of reasons. Soft starters can only slow down an induction motor and lack programmable functions that are available on modern power devices such as VFDs, further limiting their capabilities. The advantages found with VFDs make them suitable for applications requiring more precise control than what soft starters can achieve.

In conclusion, there are clear differences between Soft Start Controllers and Variable Frequency Drives with regard to power electronics in motor control. While both offer specific benefits, selection should be based on the particular form factor or application requirements. We will discuss each option in more detail in the following section exploring potential benefits of “Soft Starter vs Variable Frequency Drive”.

  • Soft starters have a much lower initial cost than VFDs.
  • VFDs offer greater flexibility and control than soft starters.
  • According to an IndustryWeek survey, 61% of manufacturers reported improved energy efficiency after replacing traditional motors with VFDs, while 22% reported no improvement.

Soft Starter vs Variable Frequency Drive

Soft starters and variable frequency drives, commonly referred to as VFDs, are both electronic controllers used to regulate electric motor speed. They each provide an effective means of reducing mechanical stress on the motor while also improving their efficiency. While the two components can be used together in a single system, they offer distinct advantages when used independently to ensure a better understanding of how to best manage the performance of motors for more productive operations.

Soft starters are designed with three primary stages. The first involves ramping up the voltage gradually and gently allowing the motor to spin up to its full operating speed, which is known as current limiting. This reduces inrush current, as well as thermal and mechanical stress on the motor. In contrast, VFDs feature electronic control elements that allow them to alter the power supply frequency leading to direct variation of motor speed while being capable of ramp start-up or other adjustable acceleration or deceleration profiles. This is especially advantageous since it can regulate not only the speed but also torque, meaning that on start-up there can be a much softer loading during initial operation.

Though each offers specific advantages, neither technology is necessarily better than the other as this will depend greatly on the intended application. Soft starters offer great cost savings in instances where applications do not require precise regulation of speed or torque fluctuation. VFDs however can provide finer control over a greater range of operating conditions by providing reliable frequency precision when variable speeds are required for increased reliability factor over time.

In conclusion, soft starters and VFDs each provide different types of control that should be carefully considered based on specific needs dictated by the intended application and desired outcomes. Which one works better depends on your particular application requirements and budget constraints – what may be suitable for one might not work well for another situation. In the next section we’ll explore soft starter operation in order to gain a better understanding of how they work and how they may fit into your operations plan.

Soft Starter Operation

Soft Starter Operation involves a three-phase current source compatible with most 3-phase AC induction motors. This technology utilizes a semiconductor device to reduce the amount of current used when the motor is initially started, which lessens the inrush current. As the motor’s speed increases, the voltage waveform ramps up in order to apply full voltage to the motor at or near full speed for optimal operation.

The advantages of Soft Starter Operation include reduced inrush current typically providing smoother starts than direct-on line starters and provides better accuracy when controlling large motor systems. Additionally, soft starters are often more efficient than other starter options, requiring less energy and protecting against damage to motors caused by excessive starting torque.

However, Soft Start Operation does have some downsides. Withdrawal of standard overload protection devices is necessary as soft starter performance relies mostly on electronic detection of fault currents. Soft starters also require regular maintenance since complex components may fail and lead to downtime of equipment. Also, if multiple motors are operating in parallel, there is an increased cost with soft starter operation and can be more expensive than other starter solutions.

All these factors must be taken into account when examining whether Soft Start Operation is the best option for a particular application. Ultimately it all depends on the specifics of the system and desired result of successful starting and running of an AC induction motor system. With that being said, let us now move onto discuss Starting Torque; one of the key components for achieving successful motor performance during start-up.

Must-Know Summary Points

Soft Starter Operation is a technology which uses a semiconductor device to reduce the amount of current used when the motor is initially started, reducing inrush current and providing smoother starts. It has advantages such as efficiency, better accuracy for larger systems, and protection from excessive starting torque, but it does have some downsides such as needing to withdraw standard overload protection devices and regular maintenance. Whether it is the best option depends on the specifics of the system, taking into account factors such as cost and desired result.

Starting Torque

Starting torque is the force necessary for a motor to overcome inertia and begin turning. High starting torque allows a load to be rapidly accelerated, which has valuable practical applications across several industries. In comparison, low starting torque limits acceleration, reducing stress on the motor, but can be inadequate for cycling duties. When choosing between soft start and VFD techniques, both offer advantages and disadvantages in terms of starting torque:

Soft start is ideal for providing smooth acceleration with low starting torque, making appropriate for lightly loaded motors where rapid acceleration may cause excessive wear or overload of the motor. It is also preferable when the motor needs frequent stops or starts. Soft start eliminates high current draw during motor startup, preventing early component failure while providing effective protection against thermal overloads; however, it is less efficient than VFDs due to the additional internal losses associated with the resistance used in these systems.

VFDs can build starting torque more quickly than a soft starter, as they do not need to increase voltage slowly over time. To alleviate high current demand during start up they use a technique called “V/f ratio control” that reduces the applied voltage by increasing its frequency proportionally as load increases. This allows for precise control over starting torque speed and acceleration rate if needed; however, this method does require an inverter drive which adds significant cost to a system compared to other starters.

Overall, each type of starter has advantages and drawbacks when comparing its ability to generate different levels of starting torque. Whether soft start or VFD offer better performance will depend on specific application requirements and objectives.

Moving forward, it is important to understand how variable frequency drives (VFDs) actually operate in order to fully appreciate their feature set. In the following section we will examine how VFD operation helps with achieving precise control over motor speed and improving efficiency.

Variable Frequency Drive Operation

Variable Frequency Drives (VFDs) are used to control the speed of an AC motor by altering the frequency of the electrical supply. By changing the frequency and voltage, the motor speed can be regulated without impacting the motor torque or power. When the frequency of a VFD is reduced, the motor’s speed will also decrease. The higher the frequency, the faster the motor rotates. In addition to controlling speed, VFDs are also used to optimize production speed, reduce energy consumption, and lower motor noise levels.

VFDs are an efficient way to reduce energy consumption as they enable users to adjust a motor’s speed according to production speed requirements. The lower frequency results in fewer current draw, meaning that less electricity is required and wasted when running a motor at a slower speed or low load compared to running at full speed all of the time. They are also safer than traditional starters due to their ability to limit starting current and can even ramp up start times for smoother startups without requiring large starting loads or banging noises from motors. VFDs can save time and money by reducing maintenance checks and repairs on machines that use AC induction motors.

Although Variable Frequency Drives provide many advantages, there are still some drawbacks associated with using them as well. Some consumer industries have had difficulty using VFDs because of their need for complex wiring which is costly and often beyond what consumer level systems can accommodate. Moreover, VFDs require regular maintenance because of their usage in dusty conditions which can lead to dirt getting inside and causing wear or even failure on parts of the system such as capacitors or switches. Additionally, an external fan is typically required so it needs adequate space and air flow around it which can be difficult in tight quarters or limited floor space applications.

Overall, Variable Frequency Drives offer improved performance over traditional starter solutions through reduction of energy consumption, smooth frequency changes which shorten startup times, and better power factor control; but careful planning must go into installation as they require special wiring consideration which can become expensive unless proper planning is put in place ahead of time and additional steps must be taken such as keeping an external fan clean in order for them to perform optimally over time.

For these reasons alone, it becomes clear why there is a debate about whether Soft Starts or VFD’s are better for certain applications depending on the needs of specific operations and companies. The next section will discuss why Current Control has become an important factor in choosing between these two types of controllers for AC induction motors.

Current Control

Current control is one of the primary differences between a soft start and a VFD. While both types of technology provide adjustable speed control and protection for motors, current control sets them apart. A soft start device limits the starting current, but does not limit the running current. By contrast, a VFD controls both the starting and the running current by creating an alternating voltage with adjustable frequency and voltage applied to the motor.

Both soft start and VFD devices offer unique advantages when it comes to current control. Soft Starts are able to reduce starting currents to as low as 10% of rated motor full load amperage. This allows for reduced in-rush currents that place less stress on motors and other electrical components by minimizing heat build up during startup. In addition, this allows for longer motor life due to reduced loads during startup and prevents overloading of power supply systems.

VFDs, on the other hand, enable adjustable frequency which alters the output voltage supplied to the motor in order to provide precise speed regulation throughout its entire operating range – from zero to full speed. In addition, they are able to regulate torque on asynchronous motors, enabling more control over starting conditions. This reduces mechanical stresses on the motors and drives and prevents overloads on power supplies.

Ultimately, both solutions can be used depending on the application needs; however, it is important to note that while soft starts limit in-rush currents that help reduce stress on motors during startup, VFDs provide more accurate speed regulation throughout the entire range of operation. With this in mind, comparing their respective applications and benefits is essential for choosing the right solution for any given project or application. As we move forward into the next section, let us now take a closer look at some comparisons between soft starts and VFDs as well as their ideal applications.

Comparisons and Applications

When it comes to Soft Start vs VFD – understanding the differences is key for determining which solution works best for you and your specific application. In order to make an informed decision, studying the comparisons and applications of VFDs and Soft Starts can help to provide answers.

Soft starts and VFDs are both control systems used to control pumps, fans, conveyors, pressers and other machinery. They generally act as a way to save energy and provide more consistent control performance by taking a gradual approach instead of abrupt power surges or stops.

One major difference between Soft starts and VFDs is cost. Typically, Soft Starts are cheaper than Variable Frequency Drives (VFDs) since they have fewer components. The setup also tends to be simpler with Soft Starts since all that’s typically required is installing the device itself and making some basic wiring connections – whereas a VFD is made up of several components that may require extra setup time or personnel for installation.

In terms of functionality, Soft Starts offer limited speed control compared to VFDs because their output is predetermined, while VFDs offer variable speed control and support multiple operating modes such as vector mode and sensorless vector mode. This makes them a better choice when needing precise speed control or when your application requires flexibility and stability in motor performance.

VFDs also work better in controlling start-up torque with accuracy, where as soft starts rely on internal DC injection braking for deceleration only which may limit torque accuracy. On the other hand, soft starters are known for providing smooth transitions at low voltage drops with higher g-ratings and thus decrease maintenance costs of motors since they experience less stress under such circumstances.

These differences should be taken into account when choosing between Soft Starts and VFDs since the needs and requirements of each application vary from each other greatly. With this information in mind, let’s delve into the pros and cons of each in our next section so that you can choose the best option for your needs.

Pros and Cons

Soft start and Variable Frequency Drive (VFD) are two viable speed control methods that provide built-in protection capabilities while ensuring the desired accuracy of an application’s speed control. Each of these technologies has its own specific strengths and weaknesses, making it important to consider the pros and cons of each before deciding which one is best for a given purpose.

Soft Start: Soft starts typically operate on a continuous current supply with 3-phase power being applied directly to the motor phases. This allows for more precise speed adjustments than VFD capability can provide. However, soft start systems are expensive, require large enclosures, have limited run time settings, often draw high levels of inrush current, and don’t provide overload protection.

VFD: VFD systems offer superior motor protection compared to soft starts and can provide smooth acceleration rates and torque overload protection during startup. They also have adjustable settings which allow for easy system tuning. The primary drawbacks of VFDs are increased cost, higher energy loss, and emissions at higher switching frequency ranges.

Overall, when comparing soft starts vs VFDs, it really comes down to what type of performance you need from your system and whether or not the added cost associated with one is worth it when compared to the other. In conclusion, each technology offers advantages and complications that must be weighed before settling on one as the better option for your application’s needs.

In this section we discussed the pros and cons of both Soft Start and Variable Frequency Drive (VFD) technology. The next section will delve into a conclusion about which technology is better suited for different applications depending on their needs.


Soft Start and VFD are both methods of motor control that effectively provide precise speed control with minimal stress to the motor. Both systems also reduce electrical bills significantly, as they allow the user to regulate the amount of power consumed by the motor based on its needs.

For most applications, Soft Start is a good choice. It is less expensive and offers just enough control for most applications. It can be used in a wide variety of scenarios depending on the operational requirements of any given application, such as conveyor belts or pumps. Additionally, Soft Starts typically provide smooth acceleration and deceleration that helps extend the longevity of the motor.

VFDs, however, are generally better for more complex applications that require higher accuracy levels or more features, including frequency slicing and torque control. Furthermore, VFDs can help adjust to different speeds quickly, making them perfect for applications that need frequent changes in speed or direction, such as edge banding machines or mixers.

Choosing between Soft Start vs VFD comes down to personal choice and budget availability. Soft Star will suffice for most needs while VFDs offer greater precision and performance capabilities that befit modern applications. Depending on the needs of a particular application, each system should be considered before making a decision that fits within an operation’s budget constraints.

Common Questions and Answers

How do you determine if a soft start or VFD is best for a particular application?

When determining if a soft start or VFD is best for your application, it is important to consider both the cost and technical aspects of each device. Soft starts are typically less expensive but do not provide as many features as a VFD, including adjustable speed and torque control. VFDs offer more flexibility and are more suitable for applications where dynamic torque control/speed adjustment is needed. Additionally, VFDs can help increase energy efficiency due to their ability to adjust motor speed based on the load demand of the application, resulting in improved energy savings. Finally, it is essential to assess how the specific application may benefit from either a soft start or VFD prior to making a decision. For instance, if there is a long startup time associated with the motor, then the addition of a soft start would be beneficial whereas the use of a VFD may be more suited if high-precision speed control or dynamic torque control is needed.

What are the advantages and disadvantages of using a soft start vs a VFD?

The primary advantage of using a soft start versus a Variable Frequency Drive (VFD) is that by using a soft start, you can better control the motor’s speed and torque. This allows for smoother and more gradual starts which reduces starting currents, stress on the motor and drivetrain, and potential shock or vibration. Additionally, by controlling the start-up process with a soft start, you are better able to protect both the motor and drivetrain from potential damage due to thermal stress caused by a sudden surge of current during startup.

On the other hand, one disadvantage of soft starters is their limited range of adjustment when controlling and ramping up speed and torque. Compared to VFDs, Soft starters have less functionality in terms of control. They are also only able to handle relatively low power motors whereas VFDs can be used for higher powered ones. As such, if a system requires greater control than what a Soft starter can provide, a VFD may be the better option.

Ultimately, whether Soft starters or VFDs should be used depends on the specific needs of the application in question. However, with careful consideration of factors such as power requirements, torque control level needed, flexibility needed in terms of speed/torque regulation, and cost effectiveness it’s easy to determine which is better suited for any given situation.

What are some common applications for using a soft start or VFD?

Soft starts and VFDs are both highly versatile tools that can be used for a wide range of applications.

When it comes to soft starts, they are most commonly used in large motors or pumps. They are often used to reduce the inrush current when powering these larger machines, which helps to minimize stress on the electrical grid and prevent potential damage to the machinery. Soft starts can also be used to reduce stress on mechanical components in industrial machinery, such as conveyor belts and other motion-based systems, while preventing any wastewater from entering sensitive underground pipes during excavation projects.

VFDs, or variable frequency drives, are most commonly used in HVAC, water treatment systems, pumps, blowers, mixers, and other industrial equipment that require precise speed control. They offer a much more complex suite of controls than soft starts as they can precisely modulate motor speeds—much more than just starting and stopping them. This allows processes such as temperature control, filtering items like solids from liquids, and reducing noise in a work environment to be handled with ease and increased efficiency.

In short, soft starts are great for controlling high-powered systems that may cause surges or stress on the grid upon startup. VFDs are incredibly powerful tools for precisely controlling motor speeds beyond simply starting and stopping them.