What are the main differences between Variable Frequency Drives and Soft Starters?


02 June 2022

Differences between Variable Frequency Drives and SoftStarters.

Product Line:
All VFD and SoftStarter products.

All models, all serial numbers.

Information needed to determine which product will best suit the application.

Soft starters are designed to ramp a motor between stopped and full speed, and can also ramp the motor from full speed to stop.  They can not run continuously at a reduced speed.  They are best suited to applications where a mechanical issue exists with a motor reaching speed too quickly.  They are commonly used on belt driven applications where the belt may be damaged if the motor comes up to full speed too quickly or in HVAC applications to prevent the ductwork from being over pressurized at startup.  They are also commonly used in pumping applications where a back flow valve is present.  They allow the pump to reduce speed slowly to prevent the check valve from slamming shut, potentially damaging the valve.    Soft Starters are able to extend the ramp up or ramp down time of a motor, but they have a much higher initial inrush current than would be present if the motor were being started across the line.  The Soft Starters are reduced voltage devices.  As the voltage is reduced, the current must increase in order for power at the motor to stay constant.   Typical inrush current can range from 350 to 500% of the FLA rating of the motor.  This is acceptable as the voltage is reduced.  Soft Starter should not be used in applications where the motor will cycle on/off more than 8 to 10 times per hour as they can overheat.  A variable Frequency Drive can perform all of the functions of a Soft Starter but does not generate heat the same way a Soft Starter does so they can be cycled more rapidly without overheating.

Variable frequency Drives are much more advanced products and process power differently than a Soft Starter does.  Variable frequency drives also give you the ability to control the speed of the motor.  Where a soft starter can only ramp a motor to full speed, a Variable Frequency Drive allows you to run the motor at any speed the motor is capable of, up to a maximum of 1000Hz*.

Variable frequency drives can also provide full torque, or greater than rated motor torque, regardless of the speed of the motor.  WIth our ATV71 series drive, with an encoder feedback from the motor, you can achieve up to 350% of rated motor torque at zero speed.  Variable frequency drives can be used in PID loops to maintain a specific pressure or level in a system and have advanced features specific to winding / unwinding applications, hoisting, and elevator applications.

For further information on our Soft Starters and Variable Frequency drives, or for answers to specific questions, please contact your local Schneider Electric Distributor.