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What VFD should I get

Single Phase input Single and 3 phase output

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malcolm hollins23/02/2022 16:19:37
34 forum posts
20 photos

Several ME members have suggested I get a VFD for my Mill.

I would like a single phase input with single phase and 3 phase output (I could use 3 Phase if I needed to replace my Single Phase motor)

The motor on my mill is a Amtecs AMAS SR80G4 1 hp 1410 rpm.

I would like the VFD to display the Mill spindle speed , have a Filter and be able to controller my Single phase motor.

What VFD should I get?

Andrew Tinsley23/02/2022 16:49:47
1630 forum posts

You cannot get a VFD that outputs single phase as a general rule. So you will need to change your single phase motor for a 3 phase one.

As to which one to get, there simply isn't a realistic answer. They range from cheap and cheerful Chinese ones to very expensive industrial ones with all the bells and whistles. At least the latter have a comprehensive instruction manual.

To reverse the question, how much do you want to spend and what power 3 phase motor are you going to get? Answer that and people will give you a less wide range of answers!!

I tend to buy well known units second hand or even new.


Andrew Tinsley23/02/2022 16:53:21
1630 forum posts

As your mill will be geared, the VFD cannot measure the spindle speed. All it will do is to tell you what speed the motor is running at. You will need to work out the spindle speed from that. Much better to get an electronic tachometer fitted to the spindle for speed readings.


malcolm hollins23/02/2022 17:15:31
34 forum posts
20 photos

Having read up on VFD on the internet I know that I would need to remove the capacitators from the single phase motor in order for the 3 phase output from the VFD to run the motor. I was thinking if the motor was then not able to run the mill O.K. I could replace it with a 3 phase motor.

Some of the VFD I have seen on the internet reckon they will give the spindle speed.

Andrew Tinsley23/02/2022 18:10:12
1630 forum posts

I say again, that you will NOT be able to run a single phase motor from a VFD.

All that a VFD will give you is the motor speed. Your spindle does not run at motor speed and the VFD doesn't have a clue as to the gearing ratio between spindle and motor!

If you are convinced that a VFD will run a single phase motor and give a reading of spindle speed, then I suggest you demonstrate this. I for one will be most interested in the results.


Pete Rimmer23/02/2022 18:13:24
1233 forum posts
65 photos
Posted by malcolm hollins on 23/02/2022 17:15:31:

Having read up on VFD on the internet I know that I would need to remove the capacitators from the single phase motor in order for the 3 phase output from the VFD to run the motor. I was thinking if the motor was then not able to run the mill O.K. I could replace it with a 3 phase motor.

Some of the VFD I have seen on the internet reckon they will give the spindle speed.

It will give you the motor spindle speed.

Tony Ray23/02/2022 18:21:58
188 forum posts
46 photos


I don’t know what you have been reading but it’s wrong. As has already been said you need a single phase input 3 phase oiutput VFD and a 3 phase motor. The VFD should be rated to the motor in your case a 750W VFD it won’t harm anything if you go to a 1kW unit but you don’t need to. With respect it doesn’t appear that you have a great deal of experience in electrical matters so I would advise you get someone to help you with the conversion. One of the biggest challenges is understanding the terminology in the VFD manuals and some pretty poor translations.

WEG, Schneider and ABB are brands I have experience with. I personally would not trust my safety on a non branded unit.

Ian Parkin23/02/2022 19:30:30
1021 forum posts
239 photos

I cant now remember which brand VFD they are but some have a multiplication factor that could display your spindle speed…but not if theres a gearbox with more than 1 ratio

Ian P23/02/2022 19:44:23
2590 forum posts
114 photos

As others have said the VFD needs a three phase motor.

Some inverters have features that allow the display to show, output frequency, load current, input voltage, supply current, motor RPM and umpteen other parameters (only one at a time though). Some also allow the user to 'scale' the readout on the display to account for the belt or gearing ratio but again that will only works at one fixed ratio.

I have my VFD's set to display output frequency so I could estimate the spindle speed relative to normal 50Hz. On my Fobco drill an old 'squarish' Hoover motor is quite happy on anything between 4Hz and 200Hz

Ian P

peak423/02/2022 20:50:28
1714 forum posts
183 photos

It's certainly possible to get a single to single phase VFD, and as I understand it some models can have a scaling factor added to the display, though I'm not sure which ones.
A friend of mine has a 2"x72" belt grinder running off one as an experiment, though I don't know if that model has a programmable display.

It's also possible to run a single phase motor off some single to three phase VFDs, though not all will allow the correct programming. I don't know personally if any of those allow a display to be scaled.

This video is worth a look on how to run single phase motor off a specific ATO three phase output so it might be worth contacting ATO directly.
I guess you could try a single phase motor on it, and swap later to a three phase one, but it looks like they advise an inverter of double the normal rated capacity, so a higher initial expense.
You would have to check the prices yourself, as it may be more cost effective to get a smaller inverter and a new/second hand three phase motor.


Personally I'd use a separate tacho running off the main spindle, as the cheap LED ones off ebay work well enough. If you were to make a separate housing for the VFD, then adding one to the box cover would be easy enough.

They do a similar video for their single to single phase converter.


noel shelley23/02/2022 23:45:04
1353 forum posts
21 photos

Since you enjoy reading, read the article I wrote in MEW about a year ago on fitting a cheap tacho on to machines ! If you have any questions I will answer them ! As to VFDs 1ph in 3ph out is the norm ! Noel

Neil Lickfold24/02/2022 03:55:54
863 forum posts
195 photos

After looking at the comments of the ATO video, the motor can not be switched into reverse from the VFD. All it can do is control the motor rpm above the speed of the sprung switch for disconnecting the capacitors, even though they are not connected. To run in reverse the wiring of the motor will need to be altered.

So it looks like a single phase to single phase inverter can be used on a motor that has a cap start still enabled or having the cap start disabled. If the cap start is disabled, then instead of the 0.75kw unit, you will need their 1.5 kw unit. As they say that a 2 times the capacity of the plate rated power is required for the start of a singlephase no capacitor start motor.

On my mill drill recently, I did find a use to be inverting my saw blade and to run my motor in reverse. So for me, I will need a replacemant 3phase motor and single to 3 phase VFD unit .I am thinking of getting a 1.5kw motor that can run at the higher 2880 rpm. That way I can double the rpm and still have the same torque as the slower 0.75 kw motor at 1/2 motor RPM. (25hz the motor would be 1440 RPM) . The gear box lowest speed is still slow enough for everything I need, but the top end is really on the slow side, so that is why I am considering the bigger faster motor. They used to may the Mill I have with the 2880 2 speed motor but stopped doing that line of Mill drills. Maybe that could be in another complete thread. With the 3phase motor and controlled braking, I will be able to more easily do tapping of holes as well.

As for buying the VFD, I have bought all of mine off the local motor sellers. They used to sell complete VFD with a motor package, but no do this service. None of the vfd have ever given any trouble. They also sell the 3phase mototrs to suite the application etc.

ATO do make a single phase to 3 phase motor invertor and on their youtube video, do answer a lot of the comments questions about the setting of their drive etc.


malcolm hollins24/02/2022 08:07:21
34 forum posts
20 photos

What I had in mind was to remove the Gearbox and revert the Mill back to using Belt and Pulleys (I still have the original 4 pulley assembly to do this) Still would like to know the Belt sizes required.

The Single phase motor is only 2 years old so I was wondering if I could still use it. (Motor hp and rpm)

If that is not an option I would buy a 3 phase motor. (What hp, number of poles {speed}would be required)

I didn't know that some Mills have a motor reverse mine has not .

I would need a Single phase to 3 phase VFD if I want to stop changing the stepped pulleys to change the Spindle rpm.

I would like to point out that I was in the RAF for 13.5 years, Then as a Main Frame Computers engineer until I retired at 57.5 years old, .I am 77 now.

I have not kept up with the latest developments in electronics since I retired eg . VFD's etc but I have a fair idea.

John Haine24/02/2022 09:16:21
4679 forum posts
273 photos

Contact Newton Tesla for advice on which of their packages would be best. Reverse on a mill is very useful for tapping, very slow in/reverse out.

not done it yet24/02/2022 10:00:29
6812 forum posts
20 photos

As you are clearly not up to speed with this, Newton Tesla would likely be your safest bet.

My mills and lathe all have variable speed facility (two continuously variable and one with a six speed gearbox).

The three phase motors with VFDs allow me even more latitude on speeds and other features.

If you retain the belt pulley speed change, you would likely not need to change the belt position very often - but it would be an option available for the extremes of the speed range available (within the abilities of the machine). I alter the VFDs much more, on my lathe and mill with infinitely variable selection within their designed speed range.

Pulleys come in matched pairs, such that changing the belt between the pairs of grooves requires no real change in the tension adjustment range (although the tension needs to be relieved while actually moving the belt).

Little point in having reverse on your mill if it does not have a quill. I cannot recall ever reversing my smaller mill, which does not have a quill. There may be obscure uses (such as back-to front fly cutters) but not worth worrying about.

Your choice of input, to the VFD, depends entirely on the supply to which you are grid connected. Even those with three phase supply may well fit VFDs to enhance the operation of their machines.

ChrisH24/02/2022 10:29:38
1018 forum posts
30 photos

Malcolm - contact Inverter Supermarket and pick their brains, they were very helpful to me when I went down this route, supplied both motor and inverter.


malcolm hollins24/02/2022 11:23:24
34 forum posts
20 photos

I have just talked to George at Newton Tesla . He explained that a VFD was the same as an Inverter and a 3 Phase motor was to best option (single Phase motor would have problems)

I will get my Mill going using the Belt/Pulleys (remove gearbox and refit the pulley assemble that is between the motor and Spindle)

Sell the Gearbox (use the money to get VFD option)

Then buy a VFD/Inverter out of the box package (easiest option)

I still don't what the correct belt sizes are - I will put the 2 pulleys in the middle of there adjustment and measure the distance between the motor , pulley assembly and Spindle. Knowing the motor speed and the pulleys diameter I will use an internet Pulley Calculator to get the Belt Lengths.

Dave Halford24/02/2022 12:21:38
2054 forum posts
23 photos

Your Mill looks like either the old Warco Economy or Major Mill (depending on size, the Major is pretty big.)

Warco should be able to help with the belt type and size if your existing ones aren't marked.

malcolm hollins25/02/2022 08:23:39
34 forum posts
20 photos

Howard (from a previous Post) yesterday emailed me Photos of a Chester manual which I printed out. Very useful manual.

The Mill is a Chester rf-20 or a rf-25 (I need to check the specifications against the Mill to find out which one is mine)

I now have the Belts sizes .A31 (A790MC 13 x 800) & A38 (A990MC 13 x 965)

I do not have the correct Motor pulley (4 steps ) My motor pulley has 2 steps)

Will check with Chesters see if they still have one.

The original Mill had 12 speeds (a lot more than the 4 the Gearbox had

George at Newton Tesla said I would still need the correct pulleys and belts when I used a VFD

malcolm hollins05/03/2022 11:12:41
34 forum posts
20 photos

dsc04895.jpgChesters and Warco doesn't have any Motor Pulleys Couldn't find any on internet decided to make my own.

Was going to make a single step pulley out of aluminium but Macc Models was closed.

Made a pulley out of Hard Wood .

Mill now works.

Plan to fit a 3 Phase Motor , correct size pulley and a VFD in thefuture.dsc04893.jpg

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