What's the difference between a Soft Starter and a VFD?

What's the difference between a Soft Starter and a VFD?

Soft starters as well as variable frequency drive (VFD) are different types of motor starters. For smooth operations, both starters use semiconductor components. This is the reason many people think that soft starters and VFDs are the same devices as you can safely stop and start an induction motor with these starters.  However, the fact is that the features of a VFD are quite different from that of a soft starter. Before we start a comparison between VFD vs soft starters, let us first understand what VFD and starters are in the first place. 

So, what is a motor starter and how do motor starters work?

The term motor starter is self-explanatory. It is a device which safely stops and starts an induction motor. As an induction motor has low winding resistance, it consumes a huge volume of electrical current during starting. This huge current during the initial stage is called the inrush current. This current can damage the motor’s internal windings and minimise its life span. Even burnouts can happen in the worst cases, which can also lead to the complete damage of the machine or the motor.

The motor starters reduce the starting current so that the motor can start safely without any damage to the mechanical parts or windings due to the sudden jerks. Once the power is switched off, the motor starter stops the induction motor. Motor starters provide overcurrent protection and protection against low voltage, which protects the machine parts from electrical damage and burnouts. 

When to Use a Soft Starter?

A soft starter is a kind of motor starter which reduces the high inrush current by minimising the incoming voltage to the motor. Wondering How do soft starters work? These starters use semiconductor thyristors to control the supplied voltage. A pair of thyristors is used in a back-to-back formation to control current flow in both directions. 

Here is a detailed explanation of how soft starters work - there are six thyristors in a three-phase soft starter offering voltage reduction in all three phases at the same time. There are three terminals in the thyristor – cathode, anode, and gate. The current starts flowing when the gate signal is applied and the thyristor is triggered. You can vary the angle of the gate signal (firing angle) to decide the amount of voltage which is allowed by the thyristor. This reduces the inrush current reaching the motor. 

When to Use a Variable Frequency Drive?

A variable frequency drive or a VFD is a motor starter based on a semiconductor. Along with starting and stopping electric motors safely, this motor starter controls the motor speed completely during its operation. It not only controls supply voltage, but its frequency at the same time. An induction motor’s speed depends on supply frequency. A VFD varies the speed of the motor during its operations. 

There are three circuits in a VFD – Rectifier, DC Filter, and Inverter. The rectifier circuit converts the AC line voltage to DC. It is then passed through the DC filter. By using the inverter circuit, the DC voltage is converted to AC. As VFD can alter the speed of the motor during operations, it is mainly used in applications where motor speed needs to be varied from time to time. 

Read Also: Variable Frequency Drives - Everything You Need To Know About VFD

Understanding the Difference Between A VFD And A Soft Starter 

This table helps in understanding the difference between VFD vs soft start clearly. 

Soft Starter


  1. It uses thyristors for regulating the AC voltage. 

VFD converts AC to DC and back into the AC of the required value.

  1. A semi-conductor-based motor starter, which starts and stops an induction motor well.

VFDs are semi-conductor-based motor starters too, which start and stop the induction motor and vary the motor’s speed. 

  1. At full speed, the soft starter is bypassed by a contactor. 

It operates continuously throughout the whole operation of the motor. 

  1. It can change the supply voltage only.

It can alter the frequency and the voltage. 

  1. When compared to VFD, it is smaller in size. 

VFD is large. 

  1. Soft starters don’t need additional filters. 

VFDs require fillers because of harmonics injection in the line. 

  1. It is cheaper than VFD.

VFDs are pretty expensive. 

  1. Used for constant motor speed application. 

Used for variable motor speed application. 

Purchase the high quality and affordable VFD and soft starters from the Schneider Electric eShop

Now that you know the difference between VFD and soft starters, you can purchase motor starters as per your requirements. If you are looking for a reliable place to purchase these electrical items, look no further than the Schneider Electric eShop. Schneider Electric is a trusted brand in India that offers high-quality electrical appliances and devices. The eShop has a plethora of choices of electrical solutions, ranging from electrical protective appliances and electric supplying devices to home automation systems and other electrical components. You can browse through the different motor starters as well as the other electrical items available on the website and find the electrical components you are looking for. 

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