What is a Soft Starter and Why Do I Need One?
Posted by Morgan Spano on Sep 26th 2022
If you’re here, you may be wondering what a soft starter is and why it’s a good idea to consider installing one. The first thing you need to know is the relationship between soft starters and inductive motors.
A good analogy for using a soft starter with an inductive motor is to think of it like how you get a car up a big hill. The motor is the car and the soft starter is your foot on the pedal. Sure, we can just pump the gas right away and do a sick burnout while trying to get thousands of pounds of metal up that hill…but what does that do to the machine?
In applications where the motor requires a large amount of torque to start, the initial voltage inrush can be quite jarring and potentially damaging to your equipment.
Image courtesy of wikiHow ‘How to Drive Uphill“
So How Does a Soft Starter Actually Work?
While technology may vary depending on the manufacturer, most soft starters today implement a series of solid-state semiconductors known as thyristors. These thyristors, also referred to as silicon-controlled rectifiers (SCRs), regulate the voltage during motor startup.
These SCRs can be configured for either open-loop or closed-loop control depending on the motor requirements.
Did You Know?
“The total annual energy consumption due to motor-driven equipment in the U.S. industrial, commercial, residential, and transportation sectors was approximately 1,431 billion kilowatt-hours (kWh) in 2006…This industrial sector motor use equates to about 17% of the total U.S. electrical energy use. Within the industrial sector, about 62.5% of the total electrical energy use is for motor-driven equipment.”
Read the full U.S. Department of Energy report here!
Open-Loop Configuration
At startup, voltage is gradually applied over time using a pre-determined voltage ramp system; regardless of current draw or motor speed.
There is no current feedback loop and provides no additional protection to the motor.
This works best for applications where a specific start torque is not required.
Closed-Loop Configuration
Voltage ramp systems continuously monitor the motors current draw and speed in order to adjust voltage input accordingly.
This feedback allows for constant modification of the starting voltage in order to gradually accelerate the motor while reducing overheating and equipment strain.
Think of closed-loop configurations as a system with feedback and open-loop as non-feedback. Let’s use a clothes dryer as an example. With older dryers, we had an open-loop configuration where the user would apply a set of conditions; such as temperature and drying time. Nowadays, we have dryers which monitor the clothes throughout the cycle and automatically adjust temperature or drying time.
Learn How a Soft Starter Can Control Voltage through “Phase-fired control/Phase Cutting” in this video:
Are Soft Starters Right for My Application?
Building on what we know about how soft starters control incoming current for gradual motor startup, let’s review the other key features to consider. We know there are options for open or closed-loop configurations. We also know they are designed to protect against phase failure, overcurrent, and undercurrent encountered during motor startup. This all translates to added equipment protection. So how do these features relate to your application?
Do you have a projet where the motor needs a lot of torque to start but is good-to-go once it meets optimal speed?
Are you dealing with water pumps where flow consistency is key?
Maybe you have conveyor belts or other sensitive equipment that need to avoid sudden jerk.
Common Applications for Soft Starters:
- Industrial Fans/Blowers: Largescale fans take a lot of torque to get spinning and are often operated using drive belts. These belts have a tendency to slip due to high motor torque during startup and be very time consuming to reinstall.
- Conveyor Belts: If you have moving parts directly affected by the position of other parts, the potential for misalignment is always high. A sudden jerk during startup or shutdown could mean hours of downtime just trying to get everything back on track. This concept also applies to ski lifts! As one can imagine, a gentle start and stop on one is greatly appreciated!
- Mills: Bandsaws, circular saws, chippers, edgers or de-barkers, all have a high starting current draw which can impact the components and even cause misalignment.
- Compressors: High current inrush required by HVAC compressors can cause havoc to your equipment or work site. Nobody wants to be the guy that powers up their compressor and causes the lights to flicker or even worse. Many manufacturers now specialize in Compressor Soft Starters.
- Mixers: Depending on the viscosity of what you’re mixing, getting the equipment started may have high torque demands. Over time these high torque start-ups can cause substantial wear on components.
- Centrifugal Pumps: What happens if a pump doesn’t maintain constant internal pressure? Pressure surges ‘hammering’ in a pump system has the potential cause complete system failure. Gradually ramping up/down motor speeds helps minimize the chance of those surges.
While there are plenty more examples of where soft starters can be used, every application shares the same factor; induction motors.
Added bonus!
Lots of electrical suppliers tack on fees for heavy usage during peak hours. Reducing inrush current spikes can save you some added dollars.
Want to learn more about VFDs?
Why get a soft starter instead of a variable frequency drive?
Now that we have a better understanding of how soft starters function and where they’re used, let’s compare them to a popular counterpart; the variable frequency drive.
Unlike soft starters, VFDs control the motor speed by varying voltage and frequency inputs. A process of input filtering occurs, creating a stable electrical flow to the induction motor.
By design, a VFD has the capacity to adjust motor speeds during operation while a soft starter only adjust startup and stopping speeds. So how do you choose?
Determining which product is best for your application comes down to three main factors:
1) Desired level of control: Soft Starters are best when you only need to control speeds during motor startup and stop. VFDs have additional features which allow for speed control during all periods of operation.
2) Installation Space: Due to the straightforward function of a soft starter, they generally are much smaller than VFDs and can easily be installed in most locations.
3) Economy: Soft Starter prices are kept low to meet customer demands while VFDs can become quite pricey depending on the additional features.
Soft Starters are an excellent way to protect your equipment from unnecessary jolts during motor startup and stopping. To see Marshall Wolf Automation’s selection from manufacturers like WEG, and Carlo Gavazzi , click here! Otherwise, contact us here and we’d be happy to assist you in finding the right product for your next project.