What is the Difference between VFD and Soft Starter?


In previous videos, we have talked about variable
frequency drives (VFD) and soft starters. If you need a refresher on the detailed
working of either a VFD or Soft Starter be sure to look back at those videos. The devices are similar in that they control
the starting and stopping of industrial motors but have different characteristics
that we will identify in this video. before we get started on today’s video if you love our videos, be sure
to click the like button below. then make sure to click subscribe and the little bell to receive
notifications of new RealPars videos. This way you never miss another one! A soft starter is generally used in applications
where there is a large inrush of current that could damage a motor. While a VFD controls and can
vary the speed of a motor. 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. From what we just covered we can see that a VFD
is basically a soft starter with speed control. 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 first application will
be a wastewater pump. A water treatment plant typically has a
constant flow of water coming into the plant. Let’s assume the demand for water exiting the plant
is constant with the supply entering the plant. Which device would be the best option? That’s right! A soft starter
would be a great choice because in this application
when starting the water pump there would be a large inrush
of current on our motor that our soft starter could handle
and gradually ramp up the pump. Since the demand for water exiting the
plant is the same as supply into the plant the speed of our pump would not vary. However, if the demand for water
exiting the plant raises or lowers, we would want to vary our speed making a
VFD a better choice for the application. The next 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 setpoint. Once the setpoint 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 that
a VFD 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. Want to learn PLC programming
in an easy to understand format and take your career
to the next level? Head on over to realpars.com

77 thoughts on “What is the Difference between VFD and Soft Starter?

  1. The best explanation and illustration in automation out there on the Planet and that's why when you sign up to www.realpars.com, where you learn to program PLC, you wouldn't regret it.

  2. Please make a video on difference on et200s and et200m
    Plus
    Use of each wire in 4 core profinet cable
    And explain each part of s7 400
    Also scalance,OSM and Optical link module and ASM 450 And SLG 75 please thank you

  3. What a great video
    Can you please update a video for parameters of both VFD and Softstarter. What are all the most important parameter setup to be known?? How to setup speed reference and speed control ? Etc

  4. Never heard about soft starter, but now I'm curious – if soft starter can ramp up/down speed, couldn't it be used as cheap VFD in applications where it's needed just to reduce speed? I imagine I could connect sensor to motor shaft and "soft starter" would regulate speed to set value.

  5. Thank you fro the explanation. But i have a question : can the VFDs used to start the motor (by decreasing inrush currents)?

  6. 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.

  7. Wow , wow , wow great effort , thank you very much realpars team for these high quality videos 🙂
    Ahmed from egypt

  8. Hey RealPars. Is it better to use soft starter or star delta starter when i am rebuilding older milling machine? Motor that drives the spindle is 11kw. Thank you for your help. PS i don't need speed control…

  9. Hi.. what happen to your realpars website? Cause everytime I want to log in.. a blank white screen appear show "Forbidden".. before this I can log in using laptop and hp.. now I can't login.. is your website doing maintenance?

  10. Hello guys I have (tia portal v. 15.1) and soft starter ES v 15.0 but it is (stand alone) and now I want integrate soft stater es in tia portal v 15.1 but I don't now how…. Please help

  11. This is great! I hope someone can help me with this question though. In the event that a VFD within a circuit board experiences very high values of instantaneous current, can a soft starter be used to control this be connecting it in series with the VFD? (note that the VFD circuit has overcurrent/abnormal voltage shields e.g. varistors, surge arresters, and choke coils)

    Thank you in advance for the response.

  12. May I know what is the name of software that used to make animations of these video?

  13. Not trying to be picky but going way back when VFDs were a rare and costly we installed soft starting to protect the machine as a starter is harsh on big machines

    Currently VFDs have reached a price point were the only traditional starters we have in use on on the boiler feed water pumps as it’s so hot it would be challenging for a VFD

    Even the standard starters our much starter now as they have and led that blinks codes to set the overload

  14. Did not mention that some VFDs has internal PLC to perform specialized functions. And VFDs has inputs/outpus aswell as analogue inputs/outputs. You can program the VFD to perform based on inputs trigger and give analogue outputs for monitoring like frequency amps voltage and much more. VFD is always the best choice.

  15. 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.

  16. My Bosch Professional Powerdrill/Boschhammer seems to have VFD judging from the description in this video as the machine can vary the RPMs, starts and stops gradually and also allows for presetting of maximum RPMs.

  17. OK, I have absolutely no Idea of what you are even talking about. Watched video still don't understand what vfd is just because VFD means, veritable frequency drive I still don't know what that means. I know that radio signals work at different frequency's but what does that have to with this??????????????????????????????????????

  18. 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.

  19. In the case of the fan you could just use a fan damper for some simple applications to modulate air flow

  20. How can i soft start the single phase asynchonous motor or 2hp single phase induction motor? that drives compressor

  21. Thanks for all of your educative videos! Please make some videos on AC ratings of motors and what is meant by SIL rating of PLCs. Thanks again!

  22. Amazing job …. this is my first time to watch your any video and now i am one of your subscriber and wanted to learn more from you ….. Thank you so much for providing us such knowledgeable videos …. Love you man

  23. My built in mag start might have been good if I already had 3 phase service, but only a VFD is convenient for converting single phase 220 to 3 phase 220 as well as programming start and stop and many other features. So a VFD and soft start are nothing alike.

  24. Built elevators for Otis all their equipment used either or depending on hydraulic or traction got to love union school learned a lot about this stuff but they used Siemens starters

  25. VFD's are most commonly used on commercial/industrial HVAC equipment with a 2-10 VDC signal to control speed. Power companies charge a fee to customers based on every 1000 watts of in rush current to start motors. That is why commercial/industrial buildings have soft starters and VFD's to keep the amount of in rush current limited as much as possible to lower the fees from the power company.

  26. Can you please also make a video on complete circuit/operation of UPS, and different types of protection how they work practically?

  27. Thank you sir good explanation……can expalain about the other types of drives……..and wiring connetions & how to interface with the plc's

  28. How to check the inverter (IGBT) whether it is functioning well or not ,could pls explain it ?

  29. how can vfd keep the goodhealth of an AC motor? when vfd only supplies dc.. i think vfd is only for dc motors only..

  30. thanks for this video, you just closed safely an argument between me and my manager on some simple application which i decided it'd need only a soft starter but he said it needs a VFD with MC 😁😁😁, i win only by your video, thank you.

  31. My lathe, two milling machines, shaper and drilling machine are controlled by VFDs, only the lathe having a dedicated unit, the other 4 machines sharing two VFDs with each having a three position 3-phase selector changeover switch to direct power to a single machine of choice, power switched off of course.
    What a pleasure being able to select the correct speed depending on circumstances and what type of metal used and rate of removal.
    The hassle of stepped pulleys and belt changing a thing of the past.
    The drilling machine and lathe are now an absolute pleasure to use as speed is now instantly controlled to suit depth of cut with metal being used.

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

  33. Very good info. I have a situation where the aircon units I’m running, when at nominal output are close to our power limit from the utility. These are a good quality brand of ac but not inverter technology. Often when first turned on or sometimes after running for a while during cycling the compressor on and off, one unit will trip our power as there is a much larger current draw during first seconds of startup. So I am considering installing a soft starter for this and would like your opinion on whether this will solve the issue and also can I employ a soft start on an ac unit without doing any damage to the compressor? Thanks.

  34. I actually operate a 3mgd water treatment plant and and have been an operator for 20 plus years and have always been under the impression that to have a vfd powered motor you had to have a motor designed to be able to handle it or am I wrong?

  35. These videos are really amazing, I'm a mechatronics student and a hobbyist and always try to rescale the instruments seen on the industrial area to a more home-friendly or toys applications, by watching how the industry works I can also learn to adapt my prototypes in a robust and straight-foward way. Thanks!!

  36. 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.

  37. Amazing explaining even though I'm not American you guys use an easy vocabulary, keep going this way!!! And thank you for such an amazing content!!

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