VFD Buying Guide

Posted by Paul Rossiter 18/01/2014 19 Comment(s) Variable Frequency Drives,

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19 Comment(s)

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.