VFD Buying Guide

Posted by Paul Rossiter 18/01/2014 34 Comment(s) Variable Frequency Drives,

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34 Comment(s)

Rony George:
11/07/2016, 08:50:11 AM

Good day
I wonder if we can ran a 50HP motor with 40 HP VFD!!
What you recomend
Looking forward to your recomendations
Rony George

Yin (Admin):
11/07/2016, 08:54:11 AM

Hi Rony,

It is possible. What is the amps on your motor and VFD?
What is the application?

11/07/2016, 09:04:35 AM

The amps on motor is 45.5Amps
And on VFD is 41Amps

11/07/2016, 09:06:30 AM

The application is wastewater sludge pump

Yin (Admin):
11/07/2016, 11:45:32 AM

Rony, we sent you an email with some info. Let us know if you have more questions!

22/06/2016, 10:39:45 AM

I have single phase power to my building and I need to run 2 machines that run on 3 phase. The main machine is this popsicle freezing machine: http://www.finamac.com/en/produtos/maquinas-de-picoles/robopop-ultra
It uses 220v, 3ph, nom 27a, peak 33a.
The 2nd machine is a cooling tower for it, it uses 220v, 3ph, 2.8a.

What VFD do I need to run these machines?

Thank you!

Yin (Admin):
22/06/2016, 01:16:09 PM

Hey Scott,

I gave you a call and left you a voicemail. Give us a shout, we can help you find the right drives. Thanks Scott.

Toll-free 800-800-2261
Direct line 385-259-6897

mark gudiry:
22/06/2016, 09:57:49 AM

i need a 3hp vfd 120vac 1ph input, and 240vac 3ph output. any suggestions?

mark guidry:
22/06/2016, 10:03:23 AM

0-2000 rpm

Yin (Admin):
22/06/2016, 10:05:21 AM

Hi Mark,

120V VFDs typically go from 1/8HP to 1HP. There are no 3HP 120V 1PH input VFD that I am aware of. However, if you can supply 240V 1PH power, the Mitsubishi D720s series is a great option. Hope that helps.

18/04/2016, 08:15:03 AM

Hi Sir,

I was planning to use 1hp 3phase motor for moving the turret of the capstan lathe and control the motor using VFD. Please advise me on selecting the type of VFD to choose. like variable torque or constant torque( Machining involves vvariable torque )
And i Think 1.5 hp or 2 hp Vfd might be good for my application.
Please do get back on this doubt.

Thank You, :)

21/04/2016, 03:48:24 PM


I hope we were able to answer your questions over email. Let us know if you need more help!

Bhavesh mishra:
05/04/2016, 08:59:50 AM

I have a motor range of 0.37kw to 37kw. 1 VFD will be sufficient for this? Orelse i have to take separate VFD for different range of motors?

05/04/2016, 09:17:32 AM

Hello Bhavesh, we recommend using a VFD for each of your motor. But if you have a VFD for your 37 kW motor, you can use that with your 0.37 kW motor. The VFD needs to be big enough (in amps) for the motor. To find out what size of VFD you need, you have to size the VFD with the full load amps of your motor, as our engineer Craig Hartman has mentioned in the video. We hope this helps!

07/02/2016, 05:12:14 PM

I m building new machine with 2 motor 1 hp and 50 Hp my application is wire saw I am selecting 750 rpm motor and I am running motor only 20 percent less rpm for few hours and go maximum rpm my application need to start with low rpm around 550 then go Max rpm I need constant torque during cutting or say my load on motor depend upon contact of wire saw with stone as wire rope length reduce current drawn by motor reduce but to maintain cutting speed need full power what kind of drive I should go?

Steve Fierst:
14/01/2016, 01:13:58 PM

Is to Toshiba VFD better than an Allen Bradley VFD

Ganesh Shete Patil:
09/01/2016, 09:52:17 AM

Dear sir,
I have all the parameters of motor into my ABB350 series,but the problem is that my motor is of .5HP having 3 phase 415 supply requirment & the maximum voltage I can set into this VFD is only 350 Volt....So I am experiencing a lower speed than stated...so what will the the solution for this..

Ganesh Shete Patil:
11/01/2016, 10:39:33 PM

Sorry Its not 350 volt but its 250 volt

Ganesh Patil:
05/01/2016, 04:23:13 AM

1) I have replaced a old 3 HP two stage water pump with .5 HP motor...my question is can use the old 3 HP VFD to my new 0.5 HP motor...

2) Secondaly how can I calculated FLA for this motor..?

3) The 3 HP VFD that I have is of ABB350 series so is there any setting i have to do to drive this 0.5 HP Motor..?

05/01/2016, 11:02:49 AM

1) Please see Menaria’s comment below regarding an oversized VFD, and let us know if that does not answer your question. As long as the new motor is also rated for the same voltage, then the VFD should work.

2) A motor’s FLA can only be estimated using a number of parameters, such as voltage, horsepower, number of poles, etc. To obtain the actual FLA of the motor, the manufacturer performs a dyno load test.

3) You will have to program the VFD to the new motor. Please give us a call at 1-800-800-2261, and we can connect you with our technical support staff.

27/12/2015, 12:23:39 PM

I have a very old motor and a hoist system. Motor is 3 ph, 208v, 3HP, not inverter-duty rated, and it is linked directly to the hoist. Therefore the motor has constant torque load constantly. I used a Powerflex 525 3HP VFD to drive the motor. The VFD allows heavy-duty operation. The problem I have is that I sometimes have difficulties to start motor run using VFD. However if I just use two fingers to slightly twist motor axial, the motor start run normally. Also a VFD was burned without any error code at this situation. What would be the solution on this problem?
If I am using by-pass mode to drive the motor directly, motor never had problem to start and run normally.
Thanks for your advices.

29/12/2015, 08:36:31 AM

Hi George,

Can you provide us with some more information?
(1) How long have you run the VFD before it was burned?
(2) Is the Powerflex 525 the VFD that is out of commission?
(3) Can you elaborate the problems you are having when you start the motor with the VFD? Does it only start occasionally, or start with strange noise, or something else?

Please e-mail us at bdt@emcsolutions.com and we can continue looking into this issue. Thanks George.

29/12/2015, 08:47:09 PM

1. The burned VFD Unit was new, only used for less than 30 hours when it was burned. We do not have constant use of motor/hoist or VFD. When the VFD was burned, it operated the motor for a few minutes, then started struggling with the load. After a few seconds, without any overloading message, heavy black smoke came from the VFD's power unit.
2. We changed a new VFD, and it is in commission in a stand-alone circuit. We still have weak operation issue with the new VFD. We can feel that the VFD runs the motor weakly in comparison with the bypass mode using regular 3ph power. We observed that the VFD struggles with load, while by-pass runs the motor strongly and regularly.
3. We can use VFD to start motor run with normal load in most of time. But occasionally, and non-reproducibly motor could not start run normally. Whether the VFD runs normally, line current is about 9.6amp. If motor does not start, the current would start to increase and eventually VFD is overloaded. When motor does not start, we can use hand to slightly twist the motor axial, the motor would run normally. Without this slight twist, motor would not run.
4. When motor does not run, I think I could hear very light noise, possibly generated by current through the motor.
5. The VFD is about 7 feet distance from the motor. VFD frequency varies from 20HZ to 60Hz in 3 second. VFD is set to maximum overload values.

Hope the above information would help to understand the problem.
Thank you.

30/12/2015, 11:29:50 AM

Thank you for the detail explanation George, that certainly helped narrowing it down. We have an idea what is happening but it might take a little bit of time to explain the problem and potential solutions. Will you give us a call at 800-800-2261?

Naveen Menaria:
22/12/2015, 01:10:09 AM

Dear sir,
If we apply High rated VFD eg. 60HP VFD on 40Hp Motor then What effect will be on system. please reply

22/12/2015, 04:17:16 PM

A 60HP VFD will operate a 40HP motor without a problem. However, an oversized VFD will generate slightly higher harmonics back onto the line. VFDs are sized by the motor's FLA (full load amps). A 40HP motor may demand a FLA that is higher than the rated amps of a 40HP VFD, thus, a higher horsepower VFD with a higher amps rating is recommended. An oversized VFD will run at a lower temperature and have a longer lifetime. It is important that you program the VFD parameters to the motor nameplate specifications. More importantly, is there a reason you are considering an oversized VFD? There are many reasons a VFD would be oversized (load, environment, temperature, altitude, braking, single-phase input, etc.), and without knowing what those reasons are, we cannot be certain that the VFD is properly sized.

adam gibson:
20/12/2015, 01:18:43 PM

I have machinery, all three phase, 1 to 7.5 hp. I have 240 v single phase and 480 single phase available. Several machines are 480 v only and all machines are currently wired for 480. I will only operate one at a time. I also have a 240/480 three phase transformer. Would I be better off putting output of 240 v VFD through the transformer (I will not change frequency) or looking for a large enough VFD that will accept single phase 440. I have not found one yet.

22/12/2015, 04:41:45 PM

Just to clarify your question, you would like to know if it is more economical to purchase a 240V three-phase VFD and connect the 3-phase output to a 240/480V transformer, or purchase a 480V three-phase input VFD and connect the 3-phase output directly to the motor? The prices of 240V VFDs are only slightly lower, and you may run into some problems when installing a transformer in between the drive and the motor. We recommend that if all of your motors are rated for 480 V, you use a 480V VFD and connect it to your 480V single-phase power supply. You are correct in that your best option is to find a VFD that is large enough (de-rated) to the higher amperage from single phase input. The rule of thumb for single-phase input on a three-phase input VFD is to multiply the motor’s FLA by 2 and find a VFD with a higher amperage rating than that value. For example, if your 7.5HP, 480V motor has an FLA of around 12A, you’ll need to find a drive with an amperage rating greater than or equal to 24A. Please give us a call at 1-800-800-2261 if you would like help finding a VFD for your applications. Also, if you will use one VFD to power all of your motors, read the comment and response above regarding oversized VFDs on smaller motors.

16/12/2015, 12:26:29 PM

I have a 3HP PowerFlex 40 200-240V Single-Phase Input VFD, may I use it for a 1HP , 115V rated sigle phase AC motor?

16/12/2015, 12:31:12 PM

the reason is we only have a 115V power source, however, we have a 3HP PowerFlex 40 200-240V Single-Phase Input VFD at present, do not want to buy an extro one.

17/12/2015, 07:59:24 AM

You’ll need to power the VFD with the voltage and phase it is rated for. If you have single phase power and a single phase motor at 115 V, you’ll need a VFD rated for 115 V single phase input & output. We do not sell single phase input & out drives, but we have heard they exist.

17/12/2015, 01:42:02 PM

Aaron, I wanted to clarify that there are few technologies in single-phase motor speed control, however, they are not popular or cost effective. The most common and reliable solution people go to is converting their single-phase motor to a three-phase motor, and using a VFD to phase make from the single phase input to the three phase motor.

04/12/2015, 06:22:32 AM

I have an older belt-driven lathe with a 1 horsepower, 220 3 phase motor w/2.4 amp rating. I also have an RPC using a 15 horse motor with a knife switch mounted near the lathe. Would like to install a VFD to allow for more spindle speed variability. The motor is older and is not fan-cooled, but I can move the sheave belt to overcome any overheating problem with the motor. Would there be any problem arising from the over-voltage of the third leg that is being generated by the RPC? Would the knife-switch eliminate the possibility of overpowering the VFD upon start-up of the RPC? What size VFD would you recommend? I would say that the torque value would be variable.

Hector M:
27/11/2015, 10:28:12 PM

I have a 10hp 208-230v,3ph,60hz blower motor in a 25ton hvac application that I would like to consider using a vfd, to overcome the noise when it starts and to slow it down on the first stage of cooling and later to ramp it up when it goes into high speed cooling. What would I need to provide you to know how this can be done and about how much?

01/12/2015, 08:27:48 AM


Here is what we would need to know to give you the most competitive price:

What is the FLA of your motor?
What are the properties of your power supply? (also 230V, 3-phase, 60Hz?)
What is the environment like where the vfd will be installed? (dirty/clean, dry/humid, indoor/outdoor, etc.)
What is the elevation of the residing application?
What is the maximum ambient temperature at the site where the VFD will be placed?
What is the lead length (cable length between VFD and motor)?
How will you be controlling the VFD? (i.e. keypad, control room, any communications used)

Paul Lancaster:
19/11/2015, 06:14:10 AM

Can I use a 15 HP VFD on a 10HP motor?

19/11/2015, 08:44:03 AM

Absolutely. The motor’s output power (HP or kW) at a specific voltage (V) dictates how much current (Amps) the motor will need to draw in order to produce that power. If the motor’s Full Load Amps (FLA) - the current it needs to run at full load - does not exceed what the VFD can handle (maximum Amps rating), the VFD will be able to handle the motor’s needs.

Nagendra Yadav:
28/10/2015, 11:57:56 PM

So as I want to start a productive machine of its motor & control supply details are as,
Motor connection details - 30 kw, 3 phase 220 volt in delt / 3 phase 380 volt in star 60 Hz operated with 3 phase 220 volt VFD.
Now the power supply available at our side is 3 phase 380 - 415 volt 50 Hz.So guide me that I could start that machine with our side present power supply or need to change according to machine specification.

Also suggest what is the best option to start the machine.

R.Prabhakar Rao:
22/10/2015, 04:58:22 AM

Please give suitable VFD for RO high pressure pump motor
Motor Kw 37.0, Motor RPM 2950, voltage 450, 3 phase, Full load amps 60.0

Please give suitable VFD specifications.

with regards,
R.Prabhakar Rao

22/10/2015, 02:07:24 PM

A Reverse Osmosis high pressure pump pumping water usually requires a 110% overload – variable torque (VT) – for 1 minute. We have multiple options for a 37kW (50HP), 450V @ 60Hz motor; however, to give you the most competitive price we would need to know a little more on your application.

-What is the environment like? (dirty/clean, dry/humid, indoor/outdoor, etc.)
-What is the elevation of the residing application?
-What is the maximum ambient temperature at the site where the VFD will be placed?
-What is the lead length (cable length between VFD and motor)?
-How will you be controlling the VFD? (i.e. keypad, control room, any communications used)
-Are there any regulations set by the utility company on harmonic pollution?

Feel free to send us an e-mail at bdt@emcsolutions.com

dale race:
12/10/2015, 12:51:28 PM

I have an air compressor that is lightly used. It is presently driven by a 6 wire, 3HP, 3Phase, 1740rpm, 10.6FSA motor. I want to run the motor with a 230V VFD to be switched with the existing pressure switch. The compressor pump has a mechanical unloader.

Could you kindly advise which of your products would best suit the need described?

Thank you for your time and attention,

Dale Race

22/10/2015, 02:29:46 PM

Hi Dale,

Is the motor a WYE-Delta starter?

17/09/2015, 03:22:33 PM


I have a three phase dust collector. 4hp, 3360 rpm, 220 volts, 10.6 amps, 3kw, 60 hz. Can you please advise which vfd I need?


17/09/2015, 04:21:36 PM

Hello Kyle,

Do you have single phase or three phase power?

17/09/2015, 05:28:31 PM

Single phase

18/09/2015, 09:34:30 AM


You should get a 3 phase input, 3 phase output VFD that is twice the size of your 10.6 amps motor. It is called derating when doubling the size of the VFD in order to convert the single phase input power to three phase output. In your case, the closest VFD size would be 7.5HP (typically around 24 amps), such as the Mitsubishi E720 series 7.5HP ($910) new with 5 year warranty. You can also consider the Saftronics C10 series 7.5HP ($561) new surplus with no warranty. We have both in stock. Feel free to give us a call 800-800-2261 or email if you would like more information about them.

The E720 is IP20 rated (there is an opening at the bottom where the wires are connected, IP20 is meant to be mounted inside an enclosure/panel), whereas the C10 series is NEMA 1 rated (it is fully enclosed and can be mounted on the wall by itself).

Hope that helps.

15/09/2015, 08:28:17 AM


I would like to put VFD for two of the motors in the plant, the consumption is 18A and 30A respectively. Its a 3 phase motor which is very old and I don't think its designed for VFD service. In this case which model you recommend?

15/09/2015, 10:04:09 AM

Hi cRK,

We would recommend getting an inverter rated motor if possible. Choosing your VFD will depend on your voltage, application, and how you plan to mount the VFD. If you provide those information, we can give you a couple of suggestions. Thank you.

30/08/2015, 02:25:13 PM

Can I use the VFD for a 2.2kw,2500rpm,6.7amp single phase centrifugal fan? Can I connect it to one of the phases on the output side

01/09/2015, 09:32:36 AM

Hi Chux,

1. We always recommend that you use a 3-phase motor on a Variable Frequency Drive. You cannot use a 3-phase drive to run a 1-phase motor.
2. Single-phase drives do exist, but only work with certain types of single phase motors. We recommend you use a 3-phase motor if possible. If it is not possible, we would need all of the information on the motor nameplate to see if a single-phase VFD would work.

Let us know if you have other questions. Thanks!

24/08/2015, 02:22:45 AM

Can i use a 2 hp drive for my 0.75hp 3 phase motor to get 40rpm motor speed

14/08/2015, 08:32:40 AM

I have a Sid Harvey 1/2 HP single phase, 1725 rpm motor for a furnace distribution blower. I'm looking to reduce the fan speed to quiet the blower and increase the run-time to eliminate short-cycling. I had a Danfoss VLT2800 given to me from a job site, can I use the two together?

Frederick Cox, P.E.:
09/08/2015, 01:39:04 PM

I do not have a hardware-specific comment or request at this time. Rather, I want to congratulate you on your thoughtful and authoritative responses to all inquiries. I was seeking confirmation that an oversize (40HP-rated) VFD drive would work with a 30 HP inverter-rated AC motor; Google led me to your website and I got the answer I needed by browsing your Q and A section. Thank you. You are now on my go-to list for future inquiries and specifications.

24/05/2015, 12:09:33 AM

Is A pump motor rated: 25 Hp, 3-Phase, 230V, 60Hz can be serviced & run by a Single Phase, 230V 60 Hz power supply connected to Variable Torque VFD for motor rated 60Hp? Please advise if this possible prior purchase of VFD unit. Thanks.


13/07/2015, 09:56:00 AM

Hi Miles,

Yes it is possible but with limitations. You need to get a drive that is around 50 HP in order to accept single phase power. More accurately, you should double the amps of the motor instead of the horse power.

You have to connect the VFD to the motor, with nothing in between the two and nothing after the motor. Any lights or switches will have to go before the VFD.

Hope that makes sense. Thanks Miles.

20/04/2015, 03:09:22 PM

Miller Dimension 452 welder, Volts. 230/460/575
3 phase
Amperes. 90/45/36
kW 22.3
60 hertz

Would like to use it at home with available 120/240v Single Phase 60hz.

13/07/2015, 09:45:00 AM


You cannot run a VFD on your welder. Welders typically do not have a motor in them. They are more like a transformer. Also, this one already has a speedometer to control its speed.

VFDs are decided to run motors. One of our engineers said to using a VFD to control a welder is like using a VFD to control lights. It will not work. I hope this helps. Sorry for the slow response.

D Clark:
19/11/2015, 12:58:25 PM

Thank you for your introduction into VFD use here online. I understand that a VFD is not appropriate to run the Miller Dimension 452 Cam was asking about from a single phase supply. However, could the thinking of engineering saying all welders are transformers be slightly limiting?

For example, inverter welders are not transformer based (but also would not work with a VFD)...

But perhaps the older motor-generator welders like the Lincoln-Electric SA-800 that have a 3 phase motor could be controlled by a VFD? perhaps these machines are not easy to purchase anymore in the US but there are still a fair number around and still in use.

Jinn Liong Chin:
30/03/2015, 11:39:47 PM

I have a 803KW motor with voltage 6.6KV and 86A. I have a ABB VSD ACS2000-066-A01K-J4-101, the max output continuous current is 86A. Is it ok to use?

07/04/2015, 10:19:00 AM

Hi Jinn,

Sorry we missed your comment. Theoretically it will work as long as your VFD amp rating is higher than the motor FLA. However, depending on your application, you will likely need a little more headroom. You should consider going at least one size bigger on the VFD.

For example, for applications that are constant torque, you will need 150% to 200% overload. Typically, manufactures will call it the heavy duty rating. For applications that are variable torque, you will need 100% to 120% overload. Typically, manufactures will call it light duty or normal duty rating.

A constant torque rated VFD will have both heavy duty and light duty ratings. You will see something similar to "95/80A" or "95(CT)/80(VT)" in the specifications. You will need to determine if your application is constant torque (heavy duty) or variable torque (light duty), then match your amp rating accordingly. I hope that makes sense.

21/03/2015, 12:46:49 PM

we have an VFD written U1: 3-380V I1-385 Amps U2-3-0-u10-320 Hz.
I2 : 820 amps.

Dooes this mean that the VFD is suitable for withstanidng a current of 385 amps or 820 amps Please advice

23/03/2015, 08:54:00 AM

Hello Balaji,

Can you tell us what brand and model is your VFD?

- Yin

24/02/2015, 04:35:02 AM

can u suggest for us three phase inverter
220V .
+/- 10% voltage
36kw 220 motor
1200 rpm
we need 25~100Hz.
main shaft is pushing biomass briquette
we need to protect inverter from being burned from overload.

25/02/2015, 10:30:00 AM

Hi Naif,

We will send you some suggestions to your e-mail. Thanks!

27/01/2015, 10:06:20 PM

Please suggest me VFD drive or any other for the following two Vacuum PUMP:
1) Model & Spec: ADS-602 By ALCATEL, 3KW, 3-PHASE, 400V, 5.5A.

2)Model & Spec:DUO 65 Double stage, 3-Phase, 400V, 2.2KW, 4A.

Application: Vacuum generation
Please email me the suggested drive model.

Many Thanks and Regard

28/01/2015, 08:06:00 AM


We would be happy to point you in the right direction on your search for the right VFD for your application. I will send you an email shortly with a few additional questions I have for you.



Willy Tom:
14/01/2015, 08:01:43 AM

I have a mixer that serves 2 types of mixes. For example the thin mix will only require low power torque around half or less. The thick mix will require full power torque of the motor. Both mixes will use the same RPM.

Some of the questions I have.

1) Will a VFD work on this application?

2) Will it save and use half of the energy for the thin mix?

3) Can the VFD automatic sense the require preprogrammed require torque automatic adjust to the require torque. Let said if we mixing thin mix, the VFD will automatic use low torque.

4) Can the VDF be preprogrammed to auto sense the require torque for example during initial mix the require torque is very low but the mix will get thicker as more ingredient is added for the thick mix.


21/01/2015, 10:27:00 AM

Yes, a VFD will work for your application and will allow you some control in how fast you would like the mixer to operate at. VFDs often result in energy savings by creating more efficient operations. With applications that require high torque (like mixers) it is hard to see significant energy savings because the demand is still going to be pretty high. You may still be able to see energy savings by using a VFD on your mixer, though, most likely, it will not be half.

The VFD will only use the amount of current needed to mix based on the torque at the motor shaft. The motor develops torque as it starts, as the torque builds the drive sends more current and voltage as needed to maintain any increases in speed. Through parameters that you set in your VFD, you can adjust the speed that the mixer will mix at. We have many different drives available, some of which you can input the torque directly into the drive. If you are looking to maintain a specific level of torque you may want to look into using a drive capable of that, such as our A700 Mitsubishi drive.



19/12/2014, 04:22:04 PM

Have a 7.5hp 3 phase lathe. Trying to run on 240v single phase. What are my options? I don't need full 7.5hp of the lathe (this lathe is oversized for my needs, but got a good deal).

22/12/2014, 10:32:00 AM

Hi Ben,

We still need to use a drive that covers 7.5HP even if your lathe requires less than the maximum horsepower. Let me ask you a questions before providing a suggestion.

(1) Is it a spindle lathe?
(2) Do you have an encoder?
(3) What is the full load amps (FLA) of your motor? You can find that on your motor nameplate.

If you prefer to call us, we are more than happy to help you out! 800-800-2261

23/12/2014, 06:45:45 AM

(1) - Yeah. A 18"x59" metal lathe.
(2) - It's built in 1984. Not sure, but probably not?
(3) - I made a mistake. Looks like the motor is only a 5hp. 14amps.

23/12/2014, 12:29:00 PM


We have just sent you an e-mail. Let us if we can help you further. Thanks!

Romeo Bungay:
29/11/2014, 01:49:00 AM

We have a 10 HP 3phase motors for winching machine, is it OK to use VSD with 3phase,15 HP capacity. Appreciating your favorable feedback. Thank you very much.

01/12/2014, 12:50:00 PM

Yes you can use an over-sized VFD for your application.