Soft Starters Vs. Variable Frequency Drives (VFD)

soft starters vs variable frequency drives vfd car starts

One of the key pieces of advice for extracting the maximum fuel efficiency and longevity from a car is avoiding jackrabbit starts. We’ve all seen people rocket away from a traffic signal, only to brake hard and stop sharply at the next intersection. 

That’s when you come along and glide placidly to a stop — right next to them. 

While this is obviously wasteful — as well as hard on an automobile — you might ask what this has to do with determining whether to use soft starters vs. variable frequency drives? 

Read on. 

How Soft Starters Work 

Simply put, as the name implies, soft starters set electric motors into action gently, and gradually ramp them up to their operating speed. This reduces wear and tear on the motor, uses energy more efficiently and can reduce the “shock” the system within which the motor is installed experiences. 

“A soft starter helps protect the motor and connected equipment from damage by controlling the voltage supplied to the motor,” says Daniel Weiss, senior product manager at Newark element14, in an interview with Control Design magazine. “This limits the initial inrush of current, extending the life of the motor, and reduces mechanical shock sustained during starting by providing a more gradual ramp-up to full speed.” 

In the article, Weiss recommends using soft starters in situations in which speed ramping and torque control are desired for starting or stopping a motor. He says they are also useful where supply network issues or penalty charges may occur while starting a large motor, or you want to avoid a high inrush of current. 

Weiss says soft starters are also beneficial in instances in which gradual controlled starting is required to avoid shock and tension in conveyors and belt-driven systems. They also ease strain on gears and couplings in drivetrains. 

How Variable Frequency Drives Work 

To continue our automotive analogy, variable frequency drives (VFDs) work similarly to the new generation of cruise control systems that are capable of varying the speed of a car in sympathy with traffic flow. 

While VFDs are also capable of starting and stopping an electric motor, they are best specified in applications in which you’ll also need the speed of the motor to vary during operation. In other words, the soft starter’s sole purpose is to introduce current to a motor gradually and let it ramp up to a constant speed. Meanwhile, a VFD will allow you to do that, as well as change the speed of a motor once it’s running. 

Chip McDaniel, a technical marketing engineer at Automation Direct told Control Design, “VFDs provide power-factor correction and low power consumption, along with full- speed control as well as start/stop control. But the additional cost of the VFD must be justified.” 

According to McDaniel, VFDs also provide dynamic braking, PID control, Safe Torque Off (STO), built-in I/O and logic, fire mode, circulatory control mode, multi-motor control, communication interfaces and network control. 

So, Which Is Better? 

Again, soft starters improve the reliability of mechanical systems by controlling the voltage supplied to a motor. They’re ideal in simple situations. Meanwhile, VFDs enable you to derive more flexibility in terms of the speed at which a drivetrain operates. 

However, VFDs also introduce harmonic distortion and a small amount of additional heat. Further, while their cost has come down of late, VFDs are more expensive than soft starters. Specifying one could add needless cost to a system if the motor in question is tasked only with the need to start and stop at prescribed intervals. 

The Bottom Line 

So, when it comes to soft starters vs. variable frequency drives, a VFD is the solution you need if your system must operate at varying speeds. Going with a soft starter will be less expensive and preserve your equipment if your configuration operates at a steady pace.

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