What is the difference between VFD’s and Soft Starters?

Starting a new project or adding additional equipment to an existing process can be a challenge.  Each piece of equipment requires electrical control and researching how to control the current demand can lead to numerous possibilities.  Two of the more common approaches involve Variable Frequency Drives and Soft Starters to limit current demand and prevent equipment damage.  Do you know which is best for your new application?  If not, we’ll explain what each device does and how to determine which may be better for you.

What is a Variable Frequency Drive (VFD)?

A Variable Frequency Drive is a device used to control the speed of an A/C electric motor.  It has the ability to vary the speed of the motor, and also change the speed at which the motor increases and decreases at start up or shut off to limit electric demand.  They simply adjust the frequency of the motor to change the RPMs of the motor speed.

In order to adjust the frequency of the motor on demand, the VFDs first convert 3-phase A/C power coming into the electric motor with a set of diodes.  The A/C power is converted to D/C power, then cleaned with a capacitor, and then converted back to A/C power with transistors acting as a set of switches.  The switches adjust the frequency, which in turn adjusts the speed of the electric motor.

Variable Frequency Drives are commonly used in systems that require monitoring and adjustment of input and output processes to ensure the complete systems stay balanced.  A lumber manufacturing facility is a simple example to use as it continually has a steady input of trees entering the facility.  As the input trees increases and decreases, the output of processed lumber flow must be balanced to match the input rate.  Variable Frequency Drives can also work in teams for additional conveyor belts and secondary processes to keep the system flow balanced as the materials are moved throughout the facility.  The VFD + A/C Motor combination also has an advantage that it can be monitored and automatically controlled with a Programmable Logic Controller (PLC) to reduce manpower inputs. 

What is a Soft Starter?

A Soft Starter is a device that limits the electrical current entering an A/C motor at start-up.  Large electric motors require a high amount of electric current to start spinning, and that increases demand on the electrical system.  The Soft Starter limits the electrical demand to slowly start the electric motor.  The slower starting of “inrush current” prevents potential damage and reduces heat and stress on the electrical motor, its internal shaft, and other components.

If we go back to the lumber facility, a Soft Starter in that system would be ideal to start the process as it requires a large current demand to begin turning the large saws and conveyor systems to move processed lumber.  The Soft Starter would manage the demand to slowly begin turning the saw blade, and additional soft starts would work in tandem to slowly begin conveyor systems moving materials as the saws start turning trees into lumber.  Soft Starters are also beneficial for dust collection systems, HVAC systems, or anything that requires a heavy current load at start-up.

Soft Starters have two ways to be controlled: either by direct start / stop wiring, or over ethernet.  Control via ethernet requires an additional PLC, which can add cost but allows for finer control over the current demand entering the A/C motor.  Direct wiring is less expensive as it doesn’t require the PLC for control, but it also doesn’t offer the feedback opportunity ethernet provides.

Should I Choose a VFD or Soft Starter on My Next Project?

Many of our industrial control cabinets include Variable Frequency Drives, Soft Starters, or a combination of both.  The question is which is better for the application based on the function required.  The VFD can be used as a Soft Starter with the added benefit of speed control, but it still may not be the better choice for your application.

Other factors that should be considered:

• Do you need continuous speed control?  If you just need a slow rate at the initial start-up, a Soft Starter will be the better choice.  If you need continuous speed adjustment after start-up, the VFD will be a better choice.
• Do you have budget restrictions?  If you just need initial start-up control, a Soft Starter will be the better option as it has fewer features and a lower price.
• Does your project have size limitations?  If space is limited, a Soft Starter may exactly what you need.  They are typically smaller in size than a VFD.

A Variable Frequency Drive and Soft Starter offer comparable functions when it comes to starting and stopping an A/C electric motor.  The VFD offers additional capability for controlling speed after start-up too.  Deciding which should be used can be complex process, and Automation Electric and Controls can offer support to your business to determine the better choice for your application.  AEC is a licensed ETL 508A panel building shop, and our work is defined by the quality products we produce. 

We offer service for every industrial panel after installation, and can support your business with enclosures, breakers, motors, wiring, relays, contactors, and much more.  We are locally owned and have operated in the Pacific Northwest since 2003.  When you’re ready for a new custom designed industrial panel, a custom mobile trailer and container, or simply need support of one currently in use, give us a call at (866) 452-6947 or email us at sales@automationelectric.com.