What is a Variable Frequency Drive?

Posted by Craig Hartman 20/03/2014 72 Comment(s) Variable Frequency Drives,

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72 Comment(s)

25/11/2015, 07:24:01 PM

i am a bit lost on VFD/rotary phase converters. would they supply constant on demand power while it is on? i've got a sewing machine with a servo motor. its MAYBE 1/4hp (the motor itself is about the size of a 12oz soda can around, slightly shorter height)

the only thing i am not sure about is if VFDs would be okay for low load frequent start/stops

this machine also has electronics/small LCD display. would a VFD be okay to use with the electronics? i really want to avoid using a rotary converter if i can because of the size/noise

25/11/2015, 07:41:47 AM

VFD work in zero speed control

Presley Chilumbu:
24/11/2015, 12:40:54 AM

Good topic on VFDs, just made my understanding. Thanx

Lisa Chen:
22/11/2015, 11:24:31 PM

Nice post. If using VFD motors, better install an insulated bearing at the non-drive end.

23/11/2015, 09:28:11 AM

Insulated bearings are a great option. You can also use shaft ground rings to protect the bearings already in place: http://www.vfds.com/shaft-grounding-rings

Often, for large power motors, we recommend a combination of both insulated bearings and shaft ground rings.

20/11/2015, 03:19:40 AM

nice yr its esey to understand thanx tu u yr thnxx

Abdul Basit:
10/11/2015, 09:05:04 PM

Please share that what will happen to the motor power and torque after the installation of VFD when it is operated at the lower frequency and rpm then its designed one. Will the torque reduce after reducing the rpm?

Abdul Basit:
10/11/2015, 09:05:03 PM

Please share that what will happen to the motor power and torque after the installation of VFD when it is operated at the lower frequency and rpm then its designed one. Will the torque reduce after reducing the rpm?

Abdul Basit:
10/11/2015, 09:05:01 PM

Please share that what will happen to the motor power and torque after the installation of VFD when it is operated at the lower frequency and rpm then its designed one. Will the torque reduce after reducing the rpm?

27/10/2015, 07:42:51 AM

Great article!!! Thanks for sharing

16/10/2015, 08:21:34 PM, none

Why is the voltage to frequency (V/Hertz) important for inverters?

Is the switching time programmeable??
And why?

Thanks for any input

Appreciate your Time

Ken Salvail:
15/10/2015, 07:57:22 AM

Is it possible to utilize a VFD on a single phase system?

19/10/2015, 08:30:09 AM

Hi Ken,

If you have a single phase motor, no.

If you have a three phase motor and single phase power, yes. You can find single phase input VFDs from many manufactures. Typically they go up to 3HP. Anything above 3HP, you will have to derate the drive by getting a VFD that is twice the size (in Amps) of your motor.

If you go the route of derating the VFD, it is important to know that the VFD has to go directly to the motor and nothing else should be powered downstream by the VFD. Any lights or switches should be powered from a separate source.

Hope that makes sense. If you have other questions, feel free to send us an e-mail. Thanks.

Joe helms:
26/09/2015, 04:20:40 PM

I've noticed some inverters can be fed with single phase but output three phase. Is this true of al inverters. How does it produce the phase shift required for the third leg. Are the three legs 120 degrees apart. Unlike single phase.

28/09/2015, 04:02:35 PM

Hi Joe,

All AC Variable Frequency Drives are designed to control 3-phase motors, but not all of them will accept single phase power input. The output of the drive is not exactly a true 3-phase AC power, it is a DC generated square-wave, simulated 3-phase power.

All of the input power is inverted through diodes to create DC power so the drive is able to monitor and control the power, then the DC power is ran through IGBTs that “pulse” the DC + and – currents to create the simulated 3-phase power.

The output legs are 120 degrees apart.

AC drives are specifically designed to control 3-phase AC motors, they are not designed for any other function. For more information about how a VFD works, here is a video of our VP of engineering explaining what a VFD is: https://www.youtube.com/watch?v=wDZANW2HeJ8

Hope that helps. Let us know if you have other questions.

Tom oconnell:
25/09/2015, 11:52:30 AM

When do you know when a vfd has past it operating life . Do you have a standard to follow . A motor has a L 10 life .

01/10/2015, 09:19:22 AM

Hi Tom,

Every drive is different, but most modern drives have a 10 year expected operating life if they are kept in a cool, clean, and dry environment. Hope that helps.

mosoti D:
17/09/2015, 03:21:05 AM

thanks it is a crucial information.

12/09/2015, 11:48:21 AM

Thanks for your explanation. And I understood total theory of vfd.and we need more information about vfd

Sanket Naresh Bavkar:
31/08/2015, 10:48:16 AM

Dear Admin,

Many many thanks for this very useful information.
Your Diagrams and simple explanations has made this complicated topic very easy.
I feel like I understand all about VFDs now only :)
and I am excited to read and reply to all your blogs .

26/08/2015, 07:59:13 AM

Dear Admin,

Could you explain how a VFD really saves electrical power with the help of equations?



si bolah:
24/08/2015, 12:30:09 AM

The first time I visit your kesitus this unique and interesting I really like it

24/08/2015, 12:29:03 AM

I am very happy to read this article because of you greatly helped and thank you very much

shad md:
11/10/2015, 08:07:53 PM

I am very happy to information

Mas bram:
24/08/2015, 12:27:43 AM

thanks for sharing dude, I was greatly assisted by the article from you this

24/08/2015, 12:25:34 AM

wow this is information very important

15/08/2015, 10:57:04 AM

Hi, thanks for nice site. Does the output of the inverter 200 V to 12 V, 12 V, 50 Hz with reduced and motor launches. Thank you. - See more at: http://www.vfds.com/blog/switching-it-up-igbt#sthash.3koTVK4B.dpuf

11/08/2015, 03:19:11 AM

How the VFDs handle the power factor correction and the reavtive power?

09/08/2015, 08:16:55 AM

That was good information regarding VFD, VFD itself the answer of speed control.My question is if my Process Reactor's RPM is set with 68 max.so is it possible with the help of VFD i can make my RPM more than 68? if no that mean VFD means always decrease the RPM not increase...am i right?

abhas singh:
07/08/2015, 07:27:03 AM

sir, by which mechanism the voltage is reduced in vfds

07/08/2015, 04:00:44 AM

sir plz tell me how much % of total current take v.f.d. in starting.

06/08/2015, 05:30:39 PM

What is the typical efficiency range of a VFD controller? How many watts of power are drawn for the energy consumption compared to the watts of electricity passed through to the motor?

I presume this is impacted by the size of the load (motor), but am interested in a range of efficiency at the range of controlled output speeds.

Any suggestions on where I might find some information on the controllers?



03/08/2015, 08:50:22 PM

In india some wind mill generator available at 3ph 80 volt, 120 volt series. what kind of load used for this voltage? Normally in india ac is 3ph 440volt.

31/07/2015, 02:47:40 AM

What is minimum frequency of vfd that my motor will start to rotat.

bharat jadav:
31/07/2015, 02:20:12 AM

Thanks for your guidance....

bharat jadav:
31/07/2015, 02:19:04 AM

Thans for your guidance.....

sivaraj palanichamy:
28/06/2015, 09:30:35 AM

Sir actually i want to run 415v 5hp motor with this drive, so what the capacitance value i want to use for dc bus?

Prabhakar v patil:
06/06/2015, 02:49:26 AM

Good diagram & spcificatio

20/05/2015, 07:52:09 PM

Please let me know with so many electronic components in series.What is the overall reliability of VFD
RS Yadav

13/07/2015, 10:03:00 AM

If you size the VFD properly, keep it cool, dry, and clean; they should run for years. In our experience, we have heard them last between 5-25 years.

sivaraj palanichamy:
04/05/2015, 11:39:11 PM

Dear admin, actually in my area only 3hours will be 3 phase current available, the remaining timetime will be 2phase current only, in this situation can i use VFD for my submersible water pumb on 2phase current?

Azeez Monsur:
14/07/2015, 11:22:35 AM

I doubt if this can work, as some VFDs experience undervoltage when required phase is not complete. You need a complete 3-phase input to avoid the devise from being tripping off.

Nishant Shah:
01/05/2015, 12:38:42 AM

If we have VFD does there is need of Stabilizers or Voltage Controller equipment to protect our motors, VFD's if Yes why, if no why ?/ please explain.

13/07/2015, 10:06:00 AM

No Nishant, you do not need them to protect your motor. However, you might need line reactors to reduce the harmonic distortion.

Azeez Monsur:
14/07/2015, 11:24:23 AM

You don't really need any protection appliances once you are using a VDF, it comes with many protection funtionalities.

29/04/2015, 06:00:44 PM

I'm running 5.5 kw motor coupled to 7.5kw inverter. The motor draws 10.7 amp when 400v and 50hz when running with inverter its frequency will drop to 28hz and how much the vo lol tage and current would be at this time will burn my motor.

13/07/2015, 10:11:00 AM


We need a little bit more information. Did you set the parameter for speed to 50 Hz maximum? Does it ramp up to 28 Hz and stop?

Raj kumar singh:
24/04/2015, 10:33:04 PM

Does vfd reduce starting current.If yes than how much in percentage.

26/04/2015, 10:12:03 PM

Yes it does. A VFD virtually eliminates "inrush current".

22/04/2015, 07:58:09 PM, VFDs.com

Thanks For the Information.

Tim G:
18/04/2015, 09:33:47 AM

I really over spec'd a ventilation fan/motor. 30 HP 460V. Should have been 20 HP. Does the VFD have to be rated for the existing motor or the speed/draw I want to operate at?
My guess is the VFD has to be 30 HP.


13/07/2015, 09:06:00 AM

Tim, you are correct. You have to size the VFD based on the motor's spec. More specifically, you have to find a VFD that has a higher or equal amp rating than the motor.

sheetal :
16/04/2015, 07:15:52 AM

which power vfd can display on panel ? induction motor input power or shaft power?

Navneet Gill:
06/04/2015, 01:56:03 AM, www.yogijidigi.in

Thanks For the Information, It is very useful for us specially schematic diagrams helped a lot.

06/04/2015, 09:22:00 AM

You are welcome Navneet, we are glad that you found the article useful.


Pardeep :
04/04/2015, 07:26:21 AM

I have Schneider altivar avt312 ac drive
My motor running load is 15 amp
I want to get load under 12 by ac drive
Is it possible
With changing frequency

07/04/2015, 09:48:00 AM

Hi Pardeep,

What application/machine are you with your motor?
What do you mean exactly when you say "running load"?
What exactly are you trying to do?


Roberto Perez:
06/03/2015, 12:58:00 PM

How much can we load a VFD for ESP's (electro submersible pumps) applications? Its true if we under load a VFD do we have more chance to get harmonics than if we load it properly ( let's say 80-90% )

11/03/2015, 02:47:00 PM

Hello Roberto,

Sorry for the delayed response. We consulted our field engineer about your questions. The lighter you load a VFD, the higher the harmonic distortion will be. If you stay within about 15% of the full current of the VFD, it should be fine.

We are unsure what you mean exactly when you "load the VFD". If you are asking how close can you get for sizing, then it is typically within 5%; possibly closer if the pump is only moving water. Technically, you can use the full range up to the rated current rating. However, we generally like to be conservative and not be right at the top of the range of the VFD rating.

If you are asking about overload conditions, the drive will run to the preset overload values that are in the drive as it relates to the parameter set points (i.e. 110% for 60 seconds at SLD rating of hte FLA in parameter #9 [Mitsubishi], or 150% for 60 seconds at LD rating of the FLA)

Let us know if you have more questions! Thanks Roberto.


rana noman:
04/03/2015, 06:53:54 AM

can v use single phase phase motor with single phase vfd? in other words i m going to make my course project i m going to make a prototype of my idea and in this idea i m going to design a vfd motor and initially i m going to use very low power motor with vfd

Yaseen Subahani:
05/03/2015, 09:49:07 PM

Yes u can use single phase motor with single phase vfd. And I already done this project.

11/03/2015, 02:50:00 PM

Theoretically, yes you should use a single phase vfd for your single phase motor. However, single phase VFDs are not very common. We haven't heard of that many in the market.

26/02/2015, 10:32:09 AM

Does it make a difference between 480Y or 480-Delta, corner ground, services with regard to input wiring?

02/03/2015, 11:16:00 AM

Hi Mike,

No, it makes no difference for a VFD whether you have 480Y or 480-Delta input. Most incoming services are 480Y. Hope this helps!


22/02/2015, 11:16:46 PM

Is it possible to use 3 phase variable frequency drive for 1 phase motor to differentiate the speed of the motor?
Please give me your advice

23/02/2015, 10:02:00 AM

Thanks for the question Raj. No, it is not possible to use a 3 phase VFD to power a single phase motor.

Bakare Kehinde:
21/01/2015, 05:49:32 AM

How does the VFD work in varying the frequency and voltage supplied to the electric motor.

21/01/2015, 10:45:00 AM


The VFD is able to vary the frequency and voltage through the use of IGBTs. It is through the fast switching of IGBTs, which creates PWM wave, that allows the VFD to alter the frequency and voltage supplied to the motor. You can read more about IGBTs in another post on our blog here at:http://www.vfds.com/blog/switching-it-up-igbt.


03/01/2015, 05:35:59 AM

what type of motors can be used with vfd?

09/01/2015, 04:25:00 PM

For the VFDs available on our site you would need to use a three phase motor. Please let us know if you have any more questions about finding the right VFD for your motor.


silviu dima:
27/12/2014, 10:01:19 AM

How should we choose an appropriate VFD ? Let's say we have a X KW motor, what kind of VFD shall we consider ? I mean, the maximum and minimum limits in KW.

29/12/2014, 07:42:00 AM


When sizing a VFD to a motor you want to match the FLA listed on the motor nameplate to the amp rating on the VFD. You will need to consider what application you have because this will determine what overload rating you will need on the VFD. Take a look at our VFD buying guide here:


This article will help you better understand everything that needs to be considered when sizing a VFD.

24/12/2014, 11:47:19 AM

pls. advise if any motor operate with fixed speed for example (1500rpm, 50HZ) CAN OPERATE WITH VSD ?

22/12/2014, 12:31:00 AM

This is really informative material which you shared with us. Thank you very much... Can you help me out with this, I were working on micromaster 440, it gives fault indicates overcurrent error. I unplugged the motor termination, on the drive but still found the same error. I know the drive is faulty but I am interesting to know what were the reason.. Is it the transistor malfunction or capacitor or any other? I will be very thankful if you upload some troubleshooting case studies related with VFD repairing.

Service engineer

22/12/2014, 12:45:35 AM

I would be very thankful if you clarify me in below contents
I found at motor input from drive, we occasionally placed chock transformer. I heard the purpose of it is motor protection, but how it protects if AC square wave is appropriate for motor operation as you told in article..

Thanks in advance

22/12/2014, 12:38:00 PM

It is really hard to say the exact reason for your overcurrent error without being able to directly take a look at the drive. There could be several reasons why you are having problems. Our recommendation would be to take it somewhere for diagnostics. If your location is close enough to us, you could send the drive in to us fpr diagnostics to address the issue. If you would like to do that, we can send you a repair quote.

As for your other question about DC chokes. A choke reactor is usually used on a dc bus of a drive to help with harmonics that goes back on the line.A Dv/dt filter or sine filter will help with the square wave form, which you mentioned. If the motor is not too far away from the drive it isn’t too much of a concern if you don't have one, but if it is 100 ft from the drive you should take a look at using one.

Bryan Charlton:
17/12/2014, 07:48:53 AM

Why would the switchgear that is feeding the VFD show a different current reading to that showing on the VFD?

17/12/2014, 08:43:00 AM

Without knowing more about your application it is hard to give a definitive answer. But, typically they will be different because the VFD and switchgear are not calibrated the same. The amperage that you will see from your switchgear will include everything that is on the switchgear while the amperage from the VFD is just from the motor.

Bryan Charlton:
17/12/2014, 08:49:06 AM

Sorry that was a bit misleading by me the current reading on the MV switchgear is of the CB feeding the VFD so it is dedicated to this application only.

Bryan Charlton:
17/12/2014, 09:08:16 AM

I should add also that the VFD is reading less than the switchgear up until the rated frequency which then they are identical.

15/12/2014, 09:29:10 AM

What would cause a DC overvoltage message on an AB 300HP VFD?

16/12/2014, 08:27:00 AM

There are several reasons that might cause a DC over voltage. It is hard to identify the reason for this with out knowing more about the application. If you would like to call in and talk with one of our engineers at 800-800-2261 we can help you figure out what is causing your over voltage.

05/12/2014, 08:48:35 AM

Can I use your definition in a publication?

Asmat ullah khan:
28/11/2014, 04:08:29 AM

what would be the minimum DC voltage from DC voltage source on DC bus of VFD to run the induction motor?

03/12/2014, 09:27:00 AM

The DC bus voltage is directly related to the supply voltage. There is a DC bus voltage at which the VFD will trip on under-voltage and will not run at all. This will vary depending on the drive manufacturer. For one brand of drive that we offer, the DC bus under-voltage trip is set at 430V for a 480V drive. The typical DC bus voltage for a 480V drive is about 650V. Technically speaking, there is no absolute minimum voltage that the DC bus will function at, but rather a minimum source voltage that the VFD will consider acceptable voltage to operate safely.

25/10/2014, 02:50:39 AM

Does vsd help for heavy load start

11/11/2014, 04:42:00 PM

Yes, it does. Base on the overload rating of the VFD, it can give you full load torque and more at all speeds, including dead stop without the typical high current associated with across the line motor start.

Ralf Meister:
16/10/2014, 07:03:02 AM

I have seen VFDs offered with 120V single phase input and 230V output. How can you get 230V out of 120V without a transformer?

Jon Novak:
22/10/2014, 11:55:35 PM

240V output is no more than 2x 120V lines phased @ 180degrees. Likewise 208V 3phase is 3x 120V lines phased at 0, 120, and 240 degrees. If you are handy with how you capture the AC power and convert it to DC, you can get a + and - DC rail at the cost of needing more current. Which will allow you to use the VFD portion to phase the output and get you the phasing you need to get "2 phase" 240, or 3 phase 208.

FYI in your house, this is exactly how its done. You generally get a neutral and 2 hot legs legs of power each having 120V (to neutral). But hooking Hot/Hot will give you 240, since they will be 180 out of phase.

Jun Pascasio:
29/01/2015, 10:38:41 PM

A device called Joule thief can do that. It is a configuration of a capacitor and an inductor. The configuration causes a build-up of charge at the output terminals. The oscillations on the R-C circuit makes it possible.

08/10/2014, 02:48:28 AM

What is the relation between speed and power consumed by VFD

08/10/2014, 04:40:00 PM

HP = (Torque(ft-lb) * Speed(RPM)) / 5252
KW = HP * 0.7457

The above equation shows us that there is a third element outside of speed and power that must be taken into account. Torque is determined by the load. Essentially while there are small losses in power in the VFD and the motor (both are extremely efficient) you will not be able to determine the consumption of power without knowing the torque.

12/10/2014, 02:14:11 AM

what must be the rpm value, full or sync rpm?

28/08/2014, 08:59:51 AM

What controls are out there to determine the output frequency in order to match the load? In an heat pump application is this controlled by temp sensors and programmed logic similar to a traditional technology?

25/08/2014, 02:22:10 PM

I've been told to purchase an inverter for a pump application. Is there a difference between an inverter and a VFD?

12/08/2014, 01:23:56 PM

Is it possible to use 2-wire hard contact to remotely vary the speed of the motor between 50% and full speed when using your VSD device.


Engineer Samson rotimi Oladipo

12/08/2014, 06:17:00 PM

Yes, the drives include preset speed inputs. The two wire contact would be wired to the preset digital inputs.

01/09/2014, 04:51:13 PM

Thanks for the answer, the actual thing we want to achieve is, if the two-wire hard contact are bridged we want to have 100% full speed and if the wires are disconnected, the motor should be running at 50% speed. Is it possible ? and how can we achieve this with the preset digital inputs. Could you help us with a typical sketch or diagram or drawings.

Thanks for using your time to help out.

Engineer Samson Oladipo

04/08/2014, 06:39:22 PM

I feel am already expect in VFD's. This presentation was clearly explained from what is to why we use it.


31/07/2014, 08:37:14 AM

Thank you for the great information on VFDs.
When you reduce the voltage to maintain the magnetic flux ratio (V/Hz), what happens to the current?

31/07/2014, 11:32:00 AM

If the motor is running, the current will be roughly proportional to the torque required by the load.

31/07/2014, 12:56:13 PM

Forgive me if I am being obtuse; I'm new to this. If you reduce the frequency from 60 to 30 Hz and maintain the same load, what will the current do?

08/08/2014, 03:43:00 PM

If the V/Hz is constant, the Amps will be roughly proportional to torque, regardless of speed.

Ikhtekharul Islam:
13/07/2014, 12:59:11 AM


Myo Kyaw:
11/07/2014, 06:11:48 AM

Thank you very much for very useful information. I wish I could also read how switching with transistors is done to achieve required frequency and about frequency modulation.

15/08/2014, 02:57:00 PM

Thank you for your comment and the idea for additional content. We have written an article about the switching of the transistors. Here is a link to it:


15/08/2014, 02:57:00 PM

Thank you for your comment and the idea for additional content. We have written an article about the switching of the transistors. Here is a link to it:


09/07/2014, 01:11:40 AM

Very very good information

01/07/2014, 09:17:18 AM

In the V/Hz discussion I can't find the reference "(see the VFD Motor Theory presentation for more on this)" . Where is it located?

This site is a great, informative discussion of VFDs.

01/07/2014, 04:56:00 PM

Thanks for the kind words and for your question! We are currently in the process of editing the motor theory post but I think I can help answer your question about the V/Hz ratio.

Motors are driven by magnetic poles. The strength of the magnet is proportional to the ratio of the voltage divided by the frequency (V/Hz). If the ratio is too low, the motor will be too weak to provide full torque on the motor shaft. On the other hand, if the ratio is too high, the iron in the motor will saturate because it is being pushed too hard. It will overheat and start to smoke, and eventually fail. Therefore the ratio must remain constant.

To illustrate this further, at 460V and 60Hz, the magnetic flux ratio (V/Hz) is 460V/60Hz = 7.66 V/Hz.

Now it’s important to remember that this V/Hz ratio must be maintained in the VFD. When you reduce the frequency you must also reduce the voltage. If you were to reduce the frequency, say to 30Hz, the new required voltage to maintain the ratio would be approximately 230V.

V/Hz Equation:
Magnetic Flux = V/Hz
Example: New Voltage (230V)/30Hz = 7.66 V/Hz

I hope that answers your question. If you have other specific questions, let us know and we would be happy to answer them for you John.

01/07/2014, 04:03:09 AM

thank u .... super tutorial guys

29/06/2014, 10:33:04 PM


29/06/2014, 08:09:43 PM

it is a interesting subject.attach more of the same

26/06/2014, 02:07:59 AM

here good and knowledgeable information about vfd.

amancio tapasao jr:
09/08/2015, 09:55:20 AM

Even big motor its ok using vfd?