Soft Starters vs. Variable Frequency Drives

Soft Starters vs. Variable Frequency Drives

Industrial motors may require quick acceleration at times to deal with a peak load requirement. This requires the inrush of large amounts of energy, and thus, current which can damage the motor as well as the adjacent electrical equipment. Variable Frequency Drives or Soft Starters are commonly used to mitigate these issues, with the choice depending on the type of application.

Soft Starters

These are solid-state devices that shield AC electric motors from the damage poised by inrush currents when a motor starts. Soft Starters provide a ramp output that raises the speed steadily and can be used only when the motor is starting. Common applications include:

  • When speed and torque control is required only during startup

  • Limit the startup inrush current when a large motor is turned on

  • The mechanical system doesn’t have the capacity to bear with torque spikes

Soft starters decrease the voltage or current for a short period of time by reducing the torque, usually employed for control of both single and three phase devices. The construction is simple, relying on solid state devices, i.e. thyristors, and contactors.

The apparent benefit of choosing a soft starter is the economics associated with it. The hardware is more affordable than a VFD when the application requires protection from inrush current during motor startup. Furthermore, they have a compact form factor which makes them a natural choice for applications where space is a concern.


Variable Frequency Drive

In retrospect, a VFD is a device that can be used to control the speed of an AC induction motor, with the features of protection being integrated. VFDs are usually used when:

Variable Frequency Drives convert input power to adjustable frequency through which the speed of an AC induction motor is controlled. Note that the mechanical speed of the motor is directly dependent on the electrical frequency and the number of poles.

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A VFD consists of the following four major parts:

  • AC Supply

  • Rectifier – converts AC into DC

  • Filter and DC bus – to smooth the rectifier output and convert it into a low ripple one

  • Inverter – converts DC into AC power through pulse width modulation

The benefits of using a VFD can be summed up as follows:

  • Helps save energy

  • Reduces peak energy demand

  • Allows the speed of pumps, conveyors and fans to be controlled over a wide range

  • Dynamic torque control

  • Maintains the speed of a motor to a set level for a specific application

  • Holds programming and communication capabilities similar to that of a PLC

One of the biggest advantages a VFD has to offer is energy savings for pumps and fans, whose speed can be adjusted according to the load requirements. Affinity laws states that if the motor speed drops then the power consumed decreased by a cubic factor. This shows the magnitude of energy that can be saved through a VFD implementation.


Making the Choice

The end-application determines whether a Soft Starter or a Variable Frequency Drive would be a better option. Soft Starters are compact, less expensive and have simpler implementations compared to VFDs controlling motors of the same horsepower rating. In contrast, VFDs provide greater energy savings of up to 50 percent and allow speed control throughout the operation of the motor, making the case for a higher ROI for more complex control requirements. It must be known that Soft Starters don’t offer speed control and are simply protective devices. Furthermore, they usually don’t sport features such as communication, digital input/output, etc. which is one of the reasons as to why they are space- and cost-savvy.  

If constant acceleration and torque control is not a requirement, and only initial protection against inrush current is required, then Soft Starter should be the choice, otherwise, a VFD would suit the needs of an application where energy savings and continuous control is essential.


Interested in learning about the benefits of soft starters? Contact our Solcon professionals




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