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Vfd and motor efficiency

A bit of advice on choosing a motor.

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Martin Hocking12/10/2021 19:04:08
6 forum posts

Hi there chaps and chapettes,

I don’t post very often but this place is my go to when I’m looking for advice from a grown up and I usually find what I’m after has already been asked so thanks everyone!

I’ve finally got my elliott milling machine up and running, I’m using the original motors which are wired for 415v. The vfd I have bungs out 220v. The horizontal motor seems to work well enough like this but the vertical motor won’t pull anything higher than the lower speeds (about 500rpm).

So once my kids stop needing all of my money (I can dream ha!) I’m going to get a new motor for the vertical head.

My question isn’t really specific to the machine though. I understand that motor efficiency is measured at full load. Does this efficiency at full load relationship continue as you go down in hz? So a motor that is 80% efficient at full load of 1hp at say 50hz would that still be 80% efficient at full load at 0.5hp at 25hz?

The reason I ask is that I am contemplating using a 1hp motor in place of a 0.75hp throttling it back with the vfd and adjusting for lower speed with the pulleys most of the time. Essentially with the view to save electricity. It seems motor efficiency per £ seems to jump up quite a bit at the 1hp mark.

Sorry if this is a bit garbled and equally sorry if I’m being daft.

many thanks


John Haine12/10/2021 19:48:32
4259 forum posts
251 photos

Martin, I think the first thing to do would be to re-wire the spindle motor from star to delta, as it sounds like at the too-low voltage the motor isn't generating its rated power. Then worry about changing the motor if you have to.

Are you really that concerned about motor efficiency? At 0.75 HP the motor will be consuming at most 1 kW when running which will cost you about 25p an hour. Any energy not going into making chips will help heat the workshop too.

Clive Foster12/10/2021 20:03:33
2876 forum posts
103 photos

Actual operating efficiency of a motor running at part load is a complex subject and there are no useful rules of thumb covering all motors. For well designed motors it holds up pretty well over most of the useful load range.

Modern name brand vector drive VFD boxes are generally very good at driving the motor with the minimum power needed for the load. Almost invariably doing better than the motor alone running on utility power.

Sounds like your existing motor is hard wired Y configuration for 415 volts. Best solution is to find someone able to dig out the star point where all three windings join and separate the connections so it can be re-wired for 220 V Delta connection. Then you existing inverter will be able to run it at full speed and full power.


Nigel McBurney 112/10/2021 20:07:58
944 forum posts
3 photos

on my Elliott 00 omnimill I changed the vertical head motor from 0.75 hp to 1.5 hp with inverter drive,big improvement I would say that for a milling head with a 3mt spindle and a high top speedof over 3000 rpm 1.5 hp is a minimum. I considered a 2 hp motor but being larger it would foul the round overarm.

Martin Hocking12/10/2021 22:06:24
6 forum posts

Thanks for the reply’s everyone, very much appreciated.

John: Am I that concerned with efficiency?

Erm I thought I was…having a bit of a look about at motors it seemed that getting the most efficient motor without spending silly amounts of money was the best bet. And having seen how motors seem to be listed with a fairly broad hz range I started to wonder if that could be exploited. But in a slightly perverse twist I thought that if the old motor had enough oomph I could use it to convert my myford/Drummond m type lathe to variable speed. Pretty much disregarding the emphasis on motor efficiency! I think it’s a slightly toxic mix of waste not want not and frugality that I blame on exposure to MEW at a relatively young age!

Clive/John: Your both absolutely right it would be really good to rewire the motor to delta. Clive is right, it is hard wired so I will have a bit more of a look into that, to see if it’s something I can do myself or perhaps a local motor rewinding company can do for me at something that doesn’t make my wallet cry!

Out of curiosity I did purchase a cheap energy monitoring socket, to see how many watts (roughly) the motors on the machine draw at different hz and I was very impressed by how the inverter pulled a fair bit less than I was expecting. It’s totally unscientific but the inverter was pulling more or less half the juice at 25hz than at 50hz. Which would certainly which go to back up what you were saying about how good vfd’s are at managing how much power motors use.

Nigel: I’m glad you’re here, I think your certainly partly to blame for this thought experiment rabbit hole I’ve fallen down haha! I read the similar remarks you made about your machine on another topic about omnimils. Having played around with the vertical head and an indicator, it definitely needs one of the stiffening piece you and others have made. Just to clarify your remarks, when you say you think that 1.5hp is a minimum, do you mean to fully make use of what the machine can do or to pull the skin off a rice pudding?



Gary Wooding13/10/2021 08:02:37
903 forum posts
232 photos

Martin: Clive is right, your best bet is to reconfigure your motor as Delta format so that your 220v VFD can drive it at full power. Some motors can be reconfigured by simply re-arranging the cables in the connection box, but some, like those in my Centec, have to be surgically altered by splitting the star-point.

The basic characteristics of a 3ph motor driven by a VFD are: constant power when run above 50Hz (or 60Hz in USA), and constant torque when run below. So, above 50Hz, the torque reduces as the speed increases, and below 50Hz the power reduces as the speed decreases.

Martin Connelly13/10/2021 08:21:38
1930 forum posts
207 photos

Martin, on a larger capacity machine the need for a higher power motor with a VFD is to offset the reduction in power that comes from using the motor at lower Hz. The reason for the lower Hz is usually due to larger tooling requiring a lower RPM. Couple that with the higher back torque on the spindle due to the larger diameter/radius of the tool then using a VFD and large radius tool at lower rpm can result in the tool stalling, not a good result.

If you look at specifications for machines they often state maximum drill sizes that they can cope with. This is based on either changing gears or belts on a pulley to reduce spindle speed. When you use a VFD to reduce the Hz and therefore the available power for the tool that maximum will go down. Putting in a larger power rated motor will keep that maximum diameter even when a VFD is used to reduce spindle speed.

The power of a motor is maximum rated power and will only be used if you really push the motor into doing the maximum it is capable of. When it is just spinning a spindle and the tool is cutting air the power consumed will be very low. There is a relationship between power and material removal rates. Use a low power motor and you may end up running it twice as long to do a job that would take less time with more available power. So the idea that someone is saving the planet with a lower power motor on a machine tool is not 100% correct.

Martin C

not done it yet13/10/2021 08:44:25
6430 forum posts
20 photos

What Gary says is spot on, to all intents and (practical) purposes.

Rewiring in delta will increase the motor power by ~75%, compared to running 230V three phase in star configuration.

We don’t know specifically if Nigel’s nominal 1.5kW motor is @415V and is running star at 230 volts. If it is the output would only be about 0.9kW.

Of my five VFDs, three are either dual voltage (or converteted by splitting the star point). One original motor is running (with adequate power) in star configuration for the duty it performs and the other star configured motor was chosen to have adequate power (while running from 230V) to drive the machine it is operating.

I actually use the variable speed option for my lathe, small mill, a power feed and surface grinder (only to run that machine at the optimal speed for any different diameter wheels which might be fitted).

My larger mill is generally run at constant Hertz (using the gearbox for speed selection) as the VFD is less accessible and has no remote speed control - at present. Both the lathe and small mill also have continuous variable mechanical speed control (not used very often these days) over the operating range.

My larger mill was fitted with a single phase 1HP motor and would not start in higher speeds until the oil had warmed through, during the colder winter periods. No problem now it is running at 1.2HP on 3 phase.

john fletcher 113/10/2021 09:38:53
736 forum posts

Martin, a couple of years or so ago some on this site posted a series of picture of how he located the Star point on his motor and then reconnected it in Delta worth locating. Not a difficult thing to do, but a rewinder will be able to help out for a fee, think £50 plus an hour. John

SillyOldDuffer13/10/2021 10:54:21
7675 forum posts
1693 photos

Sticking to the theory and practice of electric motors, I wouldn't worry in this case about efficiency. Unless the motor spends a lot of time running, in practice the loss of efficiency won't add up to much. In my workshop the lights use far more electricity than the motors because the lights are always on, while the motors only run in short bursts. Efficiency becomes important when motors run continuously or for significant amounts of time. In a home workshop, a motor running at 60% rather than 80% efficiency might not show on the electricity bill at all.

My WM280 lathe is fitted with an 1100W out motor, 1½HP in old money. Not a massive machine - it's somewhat bigger than a Myford 7. 1100W is a measure of how much power the motor can deliver without overheating. The motor will deliver considerably more than 1100W if overloaded, but it won't be long before magic smoke and ruined insulation. In practice, putting a wattmeter on the lathe's input shows the motor isn't stressed at all. With the change-gears engaged, the lathe consumes just over 200W when idling. Normal cutting consumes markedly less than 1000W in. I've not managed to load the motor heavily enough to draw the full 1500W input. To load the motor to pull more than 1000W in, requires heavy cutting with carbide. Swarf comes off as a spray of smoking hot blue-steel chips; production rate cutting, not at all what I enjoy as an amateur.

Power is the rate at which the motor might do work if required. No harm done replacing a ¾HP motor 1HP provided the operator doesn't abuse the extra power and overstress the machine, perhaps by accident. Always possible to drop a big souped-up engine into an ordinary car, but the brakes, transmission, cooling system, and road-wheels also need attention. Might pay to fit a roll-bar as well! But 1HP instead of ¾HP isn't over the top. The uprated motor will do the same job as the small one 25% faster - if pushed.

I wouldn't uprate the motor from ¾HP to 1HP in hope of improving efficiency. Power ratings are just a hint because so much depends on the design and build of particular motors, and they vary a lot. Perfectly possible for a small motor to be more efficient than a big one. Safer to say a recent electric motor is probably more efficient than older ones because insulation is better and the design more optimised, but even that's a lottery.

In other words, I'd be happy with any motor than worked, and wouldn't fuss about efficiency unless the machine was doing production work. Instead, replace fluorescent tubes with LED lighting - that's well worth doing.


Steve Pavey13/10/2021 22:22:58
354 forum posts
40 photos
Posted by john fletcher 1 on 13/10/2021 09:38:53:

Martin, a couple of years or so ago some on this site posted a series of picture of how he located the Star point on his motor and then reconnected it in Delta worth locating. Not a difficult thing to do, but a rewinder will be able to help out for a fee, think £50 plus an hour. John

I might be the guilty one - see this thread

There are loads of folk who can give you advice (as I obviously found when I tackled my motor and vfd setup) and when I got stuck in it was really very easy, and I wouldn’t hesitate to do another one. The motor and vfd have run perfectly ever since.

not done it yet14/10/2021 09:01:20
6430 forum posts
20 photos

…. when I got stuck in it was really very easy, and I wouldn’t hesitate to do another one.

Some (probably most) are but not all. Some can be tricky if the ends have been varnished tight to the windings. Easier for more powerful motors, too, l suspect. The windings on low power motors can be quite thin and easily broken.

Also wires should not be soft-soldered. Motor windings can become hot enough to melt some ‘softer’ solders.

Andrew Tinsley14/10/2021 13:52:30
1499 forum posts

It is very unlikely that a modern motor does not have the choice of star or delta connection. These motors are designed to run very hot!

It is older motors that are usually wound in delta only and these are the ones that need the star point digging out! I have found that, without exception, these motors have soldered connections and judging by my temperature controlled soldering iron, these joints are made with standard 60/40 solder. The reason being that the older motors are designed to run much cooler than modern ones.

I always use 60/40 solder to remake the joints on these motors and have never had a problem in many years of operation.

For insulation I use heat shrink tubing followed by glass fibre sleeving over the heat shrink. Retie the windings and then use air drying varnish to finish off.


William Chitham14/10/2021 14:46:06
125 forum posts
52 photos

I've just rewired an old Newman motor as described by Andrew. First one I've done and a bit nerve racking but it went ok. I found the star point was tied in on the same side the existing 3 tails came out, not surprising I suppose but for some reason I'd convinced myself it would be the other side so I ended up undoing all the binding unnecessarily. The windings and insulation are rather brittle once untied but I read somewhere that the existing varnish can be softened with a hot air gun and that worked really well making it possible to squash the revised wiring back into place ready to tie. I failed to find any of the special binding string within my patience span so I used cotton string but I found a supplier called Brocott who have the special varnish in small quantities and various other winding related supplies including the glass fibre sleeving.


Martin Hocking14/10/2021 22:19:27
6 forum posts

Wow! Thanks everybody, once again this forum has proven to be a mine of useful knowledge!

So much more of a comprehensive response then I could have hoped for. Starting with a fairly theoretical query about how motors work best, I seem to have most of the other things I have been wondering dealt with too.

I think as has been suggested I’ll have a go at digging out the star point on the original motor then, I’m always game for a bit of a tinker, and as Steve pointed out there is certainly no shortage of people here who know more then I ever will!

Incidentally thank you very much to Steve for chiming in because I think I got to page 26 of a forum search for ‘motor’ before I gave up looking for his thread after John fletcher mentioned it!

Thanks to William for the link to the brocott website that looks like its got everything I’m going to need. The motors on my mill are both Newman motors so hopefully my motor will have some similarity too.

Hopefully I’ll have some success and then I can see how I get on with the motor as nature intended and go from there.

Thank you so much to everyone, and watch this space (or one nearby!)


William Chitham15/10/2021 10:31:31
125 forum posts
52 photos

Happy to help Martin, here are a some photos of my motor's innards showing the star connection and the finished job before varnishing. I bought the red varnish expecting something translucent but it was actually opaque, more like paint. If I did it again I'd go for the clear. As Andrew said the connection was soft soldered, it did take quite a bit of scrubbing with fine emery to clean the insulation off the winding ends to get a good joint on to the new tails. There are a couple more photos in my Meddings album.


newman motor windings -02.jpeg

newman motor windings -06.jpeg

newman motor windings -09.jpeg

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