Energy & Operating Cost Savings

Soft Starts and VFD's


September 15, 2014

A typical process plant will have many motors that drive pumps or compressors and other equipment. Many of these motors would benefit from or require some type of starter control. Two options include soft starters or variable frequency drives (VFD). Each has different benefits depending upon the type of process you are controlling. We will outline a few of the differences between soft start and VFDs.

Soft Starts: A soft start is a solid-state device that protects AC electric motors from damage caused by sudden influxes of power by limiting the large initial inrush of current associated with motor startup. They provide a gentle ramp up to full speed and are used only at startup. Ramping up the initial voltage to the motor produces this gradual start. Soft starts are used in applications where:

  • Speed and torque control are required only during startup
  • Reducing large startup inrush currents associated with a large motor is required
  • The mechanical system requires a gentle start to relieve torque spikes and tension associated with normal startup (for example, conveyors, belt-driven systems and gears)

Electrical soft starts temporarily reduce voltage or current input by reducing torque. Some soft starts may use solid state devices to help control the flow of the current. Soft starts are often the more economical choice for applications that require speed and torque control only during motor startup.

Variable Frequency Drives (VFD's) protect and control the speed of an AC induction motor. A VFD can control the motor speed during the start and stop cycle, as well as throughout the run cycle. VFDs are used in applications where:

  • Complete speed control is required
  • Energy savings is a goal
  • Custom control is needed

VFDs convert input power by adjusting the frequency and voltage source to control the speed of AC induction motors. The frequency of the power applied to an AC motor determines the motor speed. Benefits of using a VFD include:

  • Energy savings
  • Reduces peak energy demand
  • Reduces power when not required
  • Fully adjustable speed (pumps, conveyors, and fans)
  • Controlled starting, stopping, and acceleration
  • Dynamic torque control

Selecting the correct equipment for your needs: Choosing a soft starter or a VFD often depends on your application. Soft starts are smaller and less expensive when compared with VFDs in larger horsepower applications. Larger VFDs take up more space and are usually more expensive than soft starters.

That being said, while a VFD is often more expensive up front, it can provide energy savings of up to 50 percent, thereby producing more cost savings over the life of the equipment. Speed control is another advantage of a VFD, because it offers consistent acceleration time throughout the entire operating cycle of the motor, not just during startup.

It is important to note that a VFD can initially cost two to three times more than a soft starter. Therefore, if constant acceleration and torque control is not necessary, and your application requires current limiting only during startup, a soft starter may be a better solution from a cost standpoint.