What to choose – VFD or Soft Starter?Girish KR
VFD is a variable speed drive and speed control is achieved by adjusting the frequency. Soft start is one among the many features available in the drive.
A soft starter is a reduced voltage starter, which limits the inrush current during the motor start. Speed control is not possible in a soft starter
While determining which device among them is required for our application, the parameters an application requires and other relevant factors are to be considered
Below are the major factors that are to be considered while deciding about going with a VFD or a Soft Starter.
1. Speed Control
If the application does not require speed control but requires only inrush current limitation during a motor start a soft starter is to be chosen.
VFDs are used in an application where speed control is essential.
VFDs are generally priced higher when compared to the Soft Starters. This is because VFD has several features apart from soft start whereas soft starters have very limited control features.
Lastly, the device size is also a determining factor. VFDs are bigger than soft starters.
Real-world applications are a great way to identify the best to choose between a VFD and a Soft Starter.
Below are a few examples of where we can use VFDs and Soft Starters.
1. Water Treatment Facility
While choosing between a soft starter and VFD with the application being a wastewater pump and given the water exiting the plant is constant, Soft Starter is a better pick.
This is because when a water pump is started within an application; there can be a large flow of current on our motor, which can be easily controlled, and ramp up the pump gradually.
The speed of the pump does not vary as the demand and supply of water exiting the plant and supply into the plant are respectively equal.
In case, the demand for the water exiting the plant increases or decreases, we then need to adjust our speed for which VFD is a preferable choice for the application.
2. Cooling Fan System
For our next application, let us consider a cooling fan which will be turned on once the plants air temperature reaches the highest temperature set point.
After reaching the set point, the cooling fan shall slow down to maintain the desired air temperature. The cooling fan will shut off once the temperature drops below a low-temperature set point.
However, if there is a surge in the temperature and surpasses the high-temperature set point, the fan needs to rotate faster.
If you are wondering which device would better suit the situation, then VFD is your answer. It is because VFD supports to vary the speed of the fan.
In case, if you only wanted to switch the fan on and off depending on the temperature and not its varying speed, then considering for a soft starter is a suitable choice.
Both VFD and a Soft Starter carry out almost equal functions as long as the ramping up or down a motor during start is considered.
When the need for application arises, price and size benefit a soft starter from choosing it. But if you are looking for controlling speed, then VFD is the choice you need to make.
Both VFD and Soft Starters can be replaced within the application depending upon the needs of the speed control requirement. if the VFD needs to be replaced with a soft starter or vice-versa within an application, it is possible.