With so many different applications in industry and numerous variances of an application types, such as water/wastewater, paper mills, nonwovens manufacturers, where do you begin selecting the correct drive type for your application? First off, you need to understand what the differences between a soft starter and a variable frequency drive (VFD).
The Differences between a Soft Starter and VFD
A soft starter slowly ramps up the torque producing capability of a motor over a defined period up to the motor rated speed. The VFD on the other hand is a means of providing a continuously variable frequency, which can deliver increased efficiency and reduced wear on motors, bearings, belts etc. With a fundamental understanding of the differences between the soft starter and the VFD, you can now look further into application specific variables to further guide you.
The number one question you need to ask yourself here is – Do I need to continuously vary the speed or not? If you are only going to run at a constant speed, then by all means go with a contactor or a soft starter. However, if you ever want to continuously vary the speed of your motor, you will need a VFD. Typically, with a VFD, your motor rated speed will be your top end, with an incredible amount of flexibility through all speeds below. We’re talking about fans, pumps, winders, extruders; applications that demand variable speed control for successful operation.
The overall size, footprint, and cooling needs of the soft starter vs. a vfd is important to understand as there are significant differences between the two. In a lower horsepower range, 15hp and below, the soft starter and the vfd will be somewhat similar in overall size. As you go up horsepower, the vfd begins to be much larger than the soft starter. With the increased size of the VFD comes increased price as well as increased operational temperatures, which can open more questions about circuit protection and loading.
Now, to the defense of the VFD, with this increased price and size, the VFD offers more diagnostics, programming and operational methods. While the soft starter does have some of these capabilities, the VFD’s are normally more robust and in-depth in these areas. Make a note here that the capacitors in a VFD do have a shelf life. If the drive is shelved dormant for a year or two, the capacitors will have to be methodically built back up in order to avoid damage to the VFD.
While a VFD can successfully replace a soft starter in just about every application you throw at it, the soft starter does have its advantages in the correct application. Either way, having a sound understanding of your application’s criteria and constraints will guide you in selecting between a soft starter and a VFD.