What is the Difference between VFD and Soft Starter?

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In this video, we will look at the internal workings of the devices, how to determine which device you might want, and look at some applications to better understand these differences.

Let’s first talk about the internal working of the two devices. A typical three-phase soft starter uses six Thyristors or Silicon Controlled Rectifiers, oriented in an anti-parallel configuration to start the electric motors smoothly.

A Thyristor is made up of 3 parts; a logic gate, a Cathode, and Anode. When an internal pulse is applied to the gate it allows current to flow from Anode to Cathode which then sends current out to our motor.

When the internal pulses do not apply to the gate, the SCRs are in the OFF state and therefore they restrict the current to the motor.

These internal pulses limit the applied voltage to the motor slowing down inrush current. The pulses are sent based on ramp time so the current will be slowly applied to the motor.

The motor, attached to the soft starter, will start up at a nice smooth current and top out at the preset maximum speed.

The motor will stay at that speed until we stop the motor where the soft starter will ramp down the motor in a very similar way as the ramp up.

VFD’s have three main components, a Rectifier, a Filter and an Inverter.

The Rectifier acts like Diodes, takes the incoming AC voltage and changes it to DC voltage.

Next, the Filter uses Capacitors to clean the DC voltage making it a smoother incoming power.

Finally, the “Inverter” uses transistors to convert the DC voltage and sends the motor a frequency in Hertz.

This frequency drives the motor to a specific RPM. We can set the ramp up and down times just like in a soft starter.

So how do we know which device is needed for our application? The decision on which device you choose comes down to how much control your application requires.

If your application requires a large inrush of current but does not require speed control, then a soft starter is the best option.
If speed control is required, then a VFD is a must.

Also, the price can be a determining factor in a lot of real-world applications. Since a soft starter has fewer control features, the price is lower than a VFD.

And finally, if the size of our device is a determining factor, soft starters are generally smaller than most of the VFDs.
Let’s look at some real-world applications to help us see the difference between a VFD and a Soft Starter.

The application we will look at is a cooling fan. In this scenario, a cooling fan will turn on when the plant air temperature hits a high-temperature set-point.

Once the set-point is reached the cooling fan will slow down to maintain the air temperature unless it drops below a low-temperature setpoint, where it will shut off.

However, if the temperature continues to rise and pass the high-temperature setpoint, the fan will need to move faster.

Which device would work best in this situation? That’s right! A VFD would be better than a soft starter because the speed of the fan will need to vary.

If our scenario was only turning the fan on or off based on temperature and not changing speed, then a soft starter would be a better option.

In closing, a VFD and a Soft Starter can do similar functions when it comes to ramp up or down a motor.

The main difference between the two is a VFD that can vary the speed of a motor while a soft starter only controls the starting and stopping of that motor.

When faced with an application, price and size are in the favor of a soft starter.

A VFD is the better choice if speed control is required. The good news is that if an existing application has a soft starter in place and speed control is later determined to be a requirement, a VFD can easily replace a soft starter. The opposite is true as well, a soft starter can replace a VFD.

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[juanzingarello]

One important thing to note here. When adding a VFD, in many times the motor will need to be replaced as well. The motor needs to be Inverter Duty rated to operate with a VFD. If you use a non-inverter duty motor it won't cause immediate damage but it will in the long run lessen the life of the motor quicker due to the harmonics which can damage the motor windings. I see it all the time in HVAC applications where they replace Vortex dampers by adding a VFD to control fan speed to duct static pressure and motors burning out after only a year or two. Ideally, if you replace an HOA or Soft Starter with VFD, make sure the motor is Inverter Duty and if not, the motor should be replaced as well.

[deadspeedv]

VFD's are also great for power savings if you have long piping systems with low head pressure required, thus saving on friction losses with higher fluid velocities. One of my primary jobs for awhile was analyzing pumping systems to determine the feasibility of retrofitting VFD's using power savings to build a business case. Saved millions at a water utility company for both water and waste water systems

[MyKarur]

Excellent presentation. In addition, use of a VFD for a conveyor drive helps because we need max torque and low speed

[ngtrki]

Your videos are totally helpful. I'm a last year student majoring in Control Engineering & Automation. Your channel helped me understand a alot about industrial devices, programing, communication protocols, ... Moreover, your channel is one of the best channels for me to learn English. Huge thanks for this channel and the creative team! Hopefully, there will be more better videos in the future

[assioshungwe]

still an electrical trainee these are videos for my training keep it up

[oznol]

Awesome video! I will just comment on one point to clarify. @1:03 - "A soft starter is generally used in applications where there is a large inrush of current that could damage a motor" The arrow appears to come from the electrical grid into the motor. It is the motor that creates this inrush (drawing on the grid) and as a result can have adverse affect on the electrical grid and other equipment (like a voltage drop dimming the lights). Soft starters are used to protect the integrity / strength of the grid voltage, avoiding dips during motor starting. Soft starters are horrible for motors if starting under any loaded conditions, this is because the torque generated by the motor when operated on a soft start is a squared function of the voltage drop. 50% drop in voltage to start the motor means 25% available rated torque! A 'no load' or 'reduced load' condition would be okay but needs to be reviewed.
Both VFD and Soft Starts can be used to protect the grid strength, just a difference in how they do it. Thanks for the video!

[DelbertQuentin]

Back in the day, a soft start was used to reduce the current inrush to the motor for about a few seconds. They are still out there, especially for large direct drive compressors in the refrigeration and HVAC applications. Great video!, but I'm an old coot that rembers those days.

[Bramon]

Great simple explanation. I use VFDs with a pressure sensor on vacuum systems at tunnel car washes. As more people use the hoses the system ramps up. Saves money for the owners and wear and tear on the producer and motor.

[erickamarianaescobedobarro]

I'm recent graduate from the control and automation engineering. However I need to learn more and more tasks and your videos are helping me lots of. Thanks for your collaboration. I'm from México City so, at the same time that I reforce my knowledges, I practice the English language.

[jsnyara]

Keep in mind, depending on brand, some soft starters can only start a motor 2 times in an hour. Also, they reduce motor voltage on startup which causes them to draw VERY high amperage through the soft starter.

[prabhathkanishka]

As an engineer who has worked on industrial drives for various applications, this is the only channel which explains these concepts very effectively.
Keep sharing these knowledge and also would love if you could share the logic in which parameters are set in the control board.

[robbymashegorobby]

we are getting skilled with the knowledge you giving us. thank you.

[WireWeHere]

Your fan animation will be subject to induced vibrations at higher speeds due to blade geometry. Suggest a VFD and through the versatile settings limit maximum RPM to a value below the tested unstable zone or find a fan supply source with a design department consuming far less Red Bull.

[wiseowldisplays]

VFD also have the added advantage of being able to converting single phase to 3 phase.

[cordaxg]

Im now further in to my Automation education.
These movies i have been watching for some weeks. Let me tell you !
Very formal and very easy to follow. English isnt my main language, but listening to the very calm and easy to hear voice, and looking at the pictures and examples REALLY makes this a whole lot easier to grasp and even learn than any of my teachers ^^.
This is very good if you are just starting up with Automation.
Kudos !
Greeting from Sweden :)

[nikhilhegde]

As an engineer who has worked on industrial drives for various applications, this is the only channel which explains these concepts very effectively.
Keep sharing these knowledge and also would love if you could share the logic in which parameters are set in the control board.

[WXUZT]

Thank You For such a nice Explanation ! Never imagined so much Technology involved in Startup. We take it for granted ! Just a flick of the switch I used to think. Changed my perspective.

[MehediHasan-hjio]

This video is very effective and most useful for an engineer.. Thank you very much sir.

[djblackarrow]

I like the IGBT's in the VFDs. When i found defective VFDs that are thrown away i like to remove these IGBTs and use them for my own projects.

[anuraagannu]

Thanks for getting it with so much of clarity