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VFD vs Softstarter – An Interview Question

What are the differences between VFD and Softstarter?

If you’re planning to join a company with equipments or products like those mentioned above. You should study the differences between the them in different aspects. It would be probably asked during the interview stage. This is one of the questions that I encountered during job interview for a sales and distribution company. Aside from how to install and program it, to study the working principle is a must for an easy explanation.

For the main difference, a Softstarter is commonly used in applications where there is a large inrush of current that could damage a motor while VFD controls and can vary the speed of a motor. This article will discuss more of their differences in terms of working principle, construction, parts, etc.

Softstarter for blower application

Nowadays, VFD’s and Softstarters are the most commonly used devices for motor and pump control applications. VFD’s and Softstarters although both built with semiconductors and almost similar in construction, they have difference in working principles. Why do we use these two devices? Simply because of high in-rush current when motor is started with DOL and motor starting torque.

Internal Parts and Workings:


A typical three-phase softstarter uses 6 thyristor or Silicon-controlled rectifiers. Thyristor is made up of three parts; Logic Gate, Cathode, Anode.

Pulses will be applied to the gate to allow the flow of current from Anode to Cathode. Which means if no pulses will be applied, SCR will be in off state.

Pulses also responsible for ramp up/down time to allow the motor to start and stop smoothly.

VFD for roller conveyor application

Variable Frequency Drive

A typical three phase VFD consist of a Rectifier, a Filter and an Inverter. Rectifier are bridges of Diodes that convert AC supply to DC. DC voltage will be cleaned by Filter by means of Capacitors. Finally, Inverter which consists of IGBTs convert the DC to a Simulated AC Wave form.

Simulated AC wave form is controlled by means of control electronics unit which is the brain of the VFD. It carries all the parameters needed for specific applications such as Ramp up/down time, RPM, Communication, Start/Stop Signals, etc.

When we should use VFD and Softstarter?

Choosing between the two will always boils down on Application, Price and Size.

Application – We should always consider where we it will be used. Do we need to control the speed? Do we need to integrate it to other control system? Or just a control for ramp up/down time. Most common misconception on choosing VFD just for the rump up/down time application which is supposed to be the main function of Softstarter.

Price – VFD will always be expensive than Softstarter because of features and softwares essential for use. VFD has more accessories and parts for the whole construction to be completed.

Size – VFD is obviously larger than Softstarter. It can eat up space to your panel which should be considered in design. It has bigger boards as it process more data than Softstarter.


Therefore, a VFD and a Softstarter can do same functions when it comes to ramp up/down a motor.

The main difference between the two is that a VFD can change the speed of a motor while a Softstarter only controls the starting/stopping of motor.

Application, price and size are in the favor of a Softstarter. However, VFD is the better choice for speed control and if more control feature will be required.

The thing is, we can also replace the Softstarter to VFD when we found out that speed control will be required. Either way, we can also replace VFD to Softstarter when ramp up/down time is only needed.

Credits to: Realpars

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