The Difference Between Soft Starters and VFDs

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Industrial motors require large quantities of energy when rapidly accelerating to full speed. To curb this issue, soft starters and variable frequency drives (VFDs) can be utilized to reduce inrush currents and lessen mechanical stress on the system - protecting your expensive equipment and expanding the overall life of your motor by minimizing heating caused by frequent stops and starts. Choosing between a soft starter and a VFD is dependent on a number of factors including the associated application, system requirements, and total cost. Below, we’ll break down the distinct differences between soft starters and VFDs, the advantages and shortcomings of both, and hopefully help you decide which is the best fit for your business.

The Basics
Perhaps the best distinction between soft starters and VFDs is outlined by the company RealPars as follows: “A soft starter is generally used in applications where there is a large inrush of current that could damage the motor while a VFD controls and can vary the speed of a motor.” Breaking down the mechanics of each device gives us a better understanding of how they function and how they might be incorporated into your business.

A soft starter consists of six thyristors or silicon controlled rectifiers (SCRs) used to start the electric motor smoothly. Each thyristor contains a logic gate, a cathode, and an anode. When an internal pulse is applied to the logic gate, it allows a current to flow from the anode to the cathode and then out to the motor. No internal pulses applied to the logic gate means the current is restricted from the motor, limiting the applied voltage and slowing down inrush current.

Alternatively, a variable frequency device consists of a rectifier, filter, and inverter. The rectifier acts as a diode, taking the incoming AC voltage and changing it to DC voltage. Then, the filter uses capacitors to clean the DC voltage. Lastly, the inverter uses insulated gate bipolar transistors (IGBTs) to convert the cleaned DC voltage and send the motor a frequency in Hertz, driving the motor to a specific RPM. In this way, we come to realize that a VFD is basically a soft starter with speed control.

The Pros and Cons
But should you choose a soft starter or VFD for your business? The decision is ultimately dependent on how much control your application needs.

The primary distinction between the two devices is that a VFD can vary the speed of a motor while a soft starter can only control the starting and stopping of that motor (no speed control). While a soft starter is smaller and more cost-effective, a VFD is a better option if speed control is necessary for your application. However, should you choose a soft starter over a VFD, or vice versa, and later decide the other would better suit your business needs, they are both easily interchangeable on existing applications.