Soft Starters versus VFDs

May 5, 2010
VFDs Provide Many Advantages Over Soft Starters, but They Cost More and Are Less Efficient
About Author
Dan Hebert is senior technical editor for Control, Control Design and Industrial Networking. Email him at [email protected]. See his Google+ profile.Process plants abound with motors that drive pumps, compressors, mixers and other equipment. Many of these motors either need or could benefit from some type of control, and two among possible choices are reduced-voltage soft starters and variable-frequency drives (VFDs). "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," notes Joe Kimbrell, the drives, motors and motion control product manager at AutomationDirect.

"Soft starters and VFDs alike can reduce field building or inrush currents, and thus machine starting torque, by as much as 30% to 75% when compared to an across-the-line starter," observes Scott Richardson, an application engineer with Yaskawa Electric America.

"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," adds Richardson.

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 setpoint control to maintain the constant flows and pressures required by many 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," explains Richardson.

"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," Richardson continues.

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," says Kimbrell.

"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," explains Kimbrell.

"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," adds Kimbrell.

"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 $3000, while selling a VFD for close to $7000," he concludes.

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," sums up Richardson.