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Soft Starter or VFD - Which to Choose?

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Processing plants are filled with motors that drive pumps, fans, augers, compressors, conveyors, mills, mixers, and other equipment.  Many of these motors need or could benefit from a reduced-voltage soft start or a variable-frequency drive (VFD).  Both soft starters and VFDs substantially reduce the electrical and mechanical shock caused by across-the-line starting, and both help reduce electrical demand by reducing a motor's start-up current.

Soft starters and VFDs can reduce field building or inrush currents and machine starting torque by as much as 30% to 75% when compared to an across-the-line starter.

In applications such as pumps and compressors, which can require many starts per hour to maintain critical process flows or pressures, these reduced starting currents result in significantly less motor heating and longer motor life.  A softer or less rapid increase in motor torque at motor starting can also greatly extend the life of belt-driven and mechanically geared equipment.

Benefits of Using a VFD

Although both soft starters and VFDs reduce inrush current and torque, a VFD can also "vary the output frequency from zero to above base motor frequency, allowing for set-point control to maintain the constant flows and pressures required by many critical processes.  VFDs also offer a larger number of diagnostic analog and digital signals for interface with plant control systems, allowing for much greater automation of the process.

Variable-torque applications such as centrifugal pumps and fans can provide very fast paybacks in energy savings when required process flows are maintained at reduced speeds, offsetting initial investment costs.  In fact, many pump and fan applications can pay back the entire cost of a typical VFD in a matter of months by reducing energy consumption.

Soft Starters are More Efficient

VFDs provide many advantages over soft starters, but they cost more and are less efficient.  A soft starter is generally in the neighborhood of 99.5% efficient, while a VFD is usually about 95% to 97% efficient.

Some soft starters also have another energy-saving feature—the ability to dial back the output voltage going to the motor.  In lightly loaded applications, this reduces the motor's magnetizing current, which doesn't need to be at 100% when the motor is not running at full load.  This reduced voltage results in less current flowing to the motor and increases efficiency, all without sacrificing speed.

Finally, some soft starters can be connected in a motor's delta.  Since the current flow inside the delta is 58% of the line current, a much smaller soft starter can be used, further enhancing the cost advantages of soft starters over VFDs.

VFD or Soft Starter? - Which to Choose?

For small motors, VFDs can be cheaper, as sheer volumes make up for the additional components required in a VFD.  But above the 5 hp to 10 hp level, soft starters start to become much more attractive.  One well-known manufacturer offers a 100-hp soft starter for around $3,000 while selling a VFD to be used in same application for almost $7,000.

For sizes over 10 hp, soft starters are cheaper and more efficient than VFDs, so they should be used to limit inrush current and torque in applications where precise speed control isn't required.

Large pumps, fans, mixers and centrifuges in the 200 hp to 500 hp range generally require very few starts and stops, have little or no integration into plant control networks and are required to run at full-rated speed during operation.  These characteristics make these applications a good fit for soft starters.

Contact Ocean City Electric to find out more information or help in determining when to use a soft starter or a VFD.  We are fully licensed and qualified electrical service company serving the pharmaceutical, chemical, industrial, commercial, and residential sectors.

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