What is a Reduced Voltage Soft Starter (RVSS)?

If you’ve ever floored the gas pedal in your car, you might have an idea of why soft starters can be helpful in electric motor applications.

As a driver, you’re used to the smooth, gradual build-up of speed as your vehicle gets from 0 to 60. Imagine if the moment you touched your gas pedal, the engine exploded with power, launching to full speed in the blink of an eye. To say nothing of what that’ll do to you, what could that be doing to your motor?

Thankfully, this doesn’t really happen with cars, and truthfully, a gas-powered engine isn’t really capable of that kind of high torque start anyway. Electric motors, however, aren’t just capable; it’s precisely what they’re designed to do. With proper controls in place, you could be doing a real number on the life of your motor on each startup.

Engineers, operators, and electricians want their motors to live a long life and be as efficient as possible, and one of the more common ways they can is through the use of a Reduced Voltage Soft Starter (RVSS).

What Exactly is a Soft Starter?

A hard start on an electric motor can either be an across-the-line (ATL) start or a direct-on-line (DOL) start. In both cases, the power demand to start up a motor is relatively enormous, typically around 600-700% of its full load current and double the amount of regular torque. Imagine your car using 600% more fuel than normal to launch immediately to full speed. At best, it’s a rough start, and at worst, it’s damaging to the load.

The stress caused by the sudden change in torque will wear on an electric motor and the driven equipment as well as stressing the power supply. This can cause failures of bearings, couplings, pulleys, belts, etc. This is compounded when a motor needs to start and stop frequently.

For applications like these, enter the RVSS, often just called a soft starter for short.

A soft starter is precisely what its name implies for electric motors. As a device that provides a “soft start” for the motor by gradually ramping up the voltage, soft starters are used to control torque and reduce starting current, preserving the life of the motor by softening the demand on startup.

A soft starter works because it essentially comprises two main systems: the power circuit and the control circuit.

On the power circuit, a soft starter takes in the power supply through a bridge of receptors called Silicon Controlled Rectifiers (SCRs). These SCRs can regulate with variance the amount of power going through the system and into the motor.

On the control circuit are the circuits responsible for managing and protecting the power components. Here’s where you’ll have communication devices and other external controls.

Check out this whitepaper from WEG for more in-depth knowledge on how soft starters function.

When Would You Want a Soft Starter?

An RVSS carries a number of advantages, and they’re often compared with variable frequency drives (VFDs) in terms of their feasibility as a motor control solution. They are not, however, interchangeable, so it’s important to know when a soft starter would and wouldn’t be a smart idea.

If you don’t need speed control but have limitations on your line power or want the benefits of smooth starting and stopping, using a soft starter is a great solution to protect your motor and equipment.

As previously mentioned, an uncontrolled motor will demand over 600% of its rated current on an ATL or DOL startup. Using a soft starter doesn’t make everything perfect, but it does cut that percentage in half, roughly 300-400% of the motor’s rated current.

Soft starters will often be less expensive than VFDs as well due to having less functionality than a drive. You’ll also not have to deal with harmonics for the most part, since the soft starter’s harmonic frequencies will be bypassed once the motor is in full operation. A VFD will also cause some efficiency loss in the ballpark of about 3% where a soft starter doesn’t burden your system at all after startup.

In short, if easing the burden your load shoulders on startup is your one and only problem to solve, then a soft starter is a good way to go.

Common soft starter applications include blowers, compressors, conveyors, fans, pumps, and rock crushers.

When WOULDN’T You Want a Soft Starter?

As previously touched on, soft starters DO NOT have the speed control that is inherent with a VFD. Ultimately, in terms of what it can do, a soft starter is really a one-trick pony.

Not all motor applications need speed control, but that doesn’t mean the lack of such a need always makes a soft starter the better option. For some situations, reducing the inrush current to roughly 300% might not be good enough. In large applications, that’s still a heavy cost to the whole operation.

There is also a limitation on the number of starts per hour when using soft starters. Since soft starters still deal with a relatively huge amount of inrush current on startup, they cannot maintain consistent startup current without generating an unsustainable level of heat. The frequency of starts an RVSS can induce depends on the model/series and the sizing of the soft starter compared to the load. Generally, this is limited to 10 starts per hour whereas a VFD would not have a concern with frequently starting and stopping the motor.

A VFD has far more control over the motor and limits inrush current on startup to 100%, eliminating any excess of the rated current or amperage.

While your VFD will have resolved a major efficiency issue, it does cause another efficiency loss by continuing to run. Depending on your application, this could end up as a net positive and be easily disregarded, but it’s also possible to set up a bypass around the VFD after startup through bypass features in the drive itself, setting up bypass contactors in the system, or purchasing a drive with synchronous transfer capabilities.

If you need to have better speed control of your motor for the application you’re working with, a VFD isn’t just the better option; it’s the only correct answer.

Soft Starters? On VFDs.com?

With VFDs.com, our ultimate mission is to provide the best solution possible. If that’s a soft starter, rest assured that we know when to point you in that direction.

At VFDS.com, we primarily supply soft starters from WEG, one of the largest suppliers of soft starters in the industry. Our soft starters are readily available in both low voltage (208V, 230V, 480V, 600V) and medium voltage (2300V, 4160V, 6900V, and so forth).

VFDs.com manages the distribution of dozens of trusted brands in the electrical engineering industry. To work with a representative on your customized solution, contact us at 1-800-800-2261 or email us at sales@vfds.com.