Whaaaat? Could it be?

[Shopgeezer]

I spent large coin buying a three phase motor and VFD motor controller to convert my 10X22 lathe into something that will change speed without the half hour of frustration unbolting idlers and swapping belts ( King Lathe, really a terrible belt setup). All the information I can find, and all the You Tube videos I have watched ( Clough42 and others ) stress that this trick can only be performed by swapping out the existing single phase capacitor start motor and replacing it with a three phase motor. The pain and agony of doing this and tearing out the forest of relays and safety switches held me back but I decided to man up and get it done. I cracked open the little instruction sheet that came with the VFD. This is one of the super cheap Chinese models that are everywhere on Amazon and Ebay. In studying the wiring diagrams I noticed that there are three models of this inverter, the AT1 which I have ( single phase input to three phase output), the AT3 ( Three phase input and output) and the AT2, SINGLE PHASE INPUT AND OUTPUT. Whaaat? The wiring diagram shows hooking this model up to a single phase capacitor start motor. Could it be? Would this work on my existing lathe motor? Could I avoid all the hassle of replacing the motor?

My electrical knowledge is limited. I have included copies of the part of the manual that describes the single phase in and out motor setup. Does anyone have any idea if this is actually a thing? The AT2 VFD is widely available on Amazon.

 

[minh-thanh]

If you have purchased a 3 phase motor use AT1 ( single phase input to three phase output) or the AT3 ( Three phase input and output ) - if your home has 3 phase to adjust motor speed .
If using AT2, and you want to use it to adjust the 1-phase motor speed, the motor will lose torque.
Using VFD and 3 phase motor is the best choice ! You can adjust the speed at a very wide range without affecting the torque

 

[Shopgeezer]

My question on that would be how much torque is lost and at what speeds? I would guess that torque decreases with speed and running very slowly would be very low torque. The only operation done at low speeds is thread cutting which is a fairly light cutting load. I am interested in spindle speeds from 500 to 1500. I wonder what the torque map would look like for those speeds? That could be done with one belt change to keep motor speeds higher.

 

[Vixen]

I thought you said you wanted a VFD to avoid having to change belts. Now you are talking about belt swapping and a VFD for thread cutting to overcome the problems associated with keeping the single phase motor. Why not do the job properly and fit a three phase motor controlled by the VFD.?

 

[minh-thanh]

My question on that would be how much torque is lost and at what speeds? I would guess that torque decreases with speed and running very slowly would be very low torque. The only operation done at low speeds is thread cutting which is a fairly light cutting load. I am interested in spindle speeds from 500 to 1500. I wonder what the torque map would look like for those speeds? That could be done with one belt change to keep motor speeds higher.

I don't know how much torque it will lose, and I don't think there will be any research on that.
How much hp is your lathe motor ?

 

[mu38&Bg#]

VFD for single phase isn't exactly common, but it exists. https://www.regalbeloit.com/brands/LEESON/Technical-Information/Technical-Manuals The Leeson FHP manuals (at the top) imply that many 3PH VFD claim to drive 1PH motors, but do so unreliably? Phase Technologies well pump specific 1PH VFD has winding phase angle options. It's hard to find data about torque or efficiency. Only thing I saw showed a 1PH motor pulled much more current at low Hz with a fan load, than 3PH.

For machine tools, you can't go wrong with a 3PH motor. It will certainly run smoother. Don't regret your decision at this point.

 

[lemelman]

As others have said, replace the 1ph motor with a 3ph motor and a VFD. As a general rule of thumb, when running a motor with a VFD, the motor will give constant torque when run at frequencies at or below it's rated frequency (ie 50Hz in UK or 60Hz in USA), and constant power at frequencies above. Since power is the product of rpm and torque, power is proportional to rpm below its rated frequency, and torque is proportional to rpm above.

 

[Steamchick]

I can only chuck (sorry about the pun?) -in my pennorth of experience.
I have a lathe that can with a 0.75hp 1ph motor and variable speed drive. - Because it was at least 6 incheces shorter than the gear version. No regrets, the gear version wouldn't fit my space (and allow access to change wheels, etc)... The variable speed is supposed to go from 11rpm to ~1250rpm.... - then have a belt drive change for double that speed. Oh well, so after more than 10 years andf a few odd "flash-bangs" inside the electronic enclosure, it does 15rpm to ~1550rpm.... Don't explain that one - I'm awaiting the day it goes "flash-bang" for the last time! I limit myself to about 1200rpm to avoid whatever causes the "flash-bangs".... I have had the mains trip a few times, and once the surge sensitive fuse on the electronic board went pop. I don't know what the "flash-bangs" are all about, I just see the flash at the edges of the cover plate, and the bang makes me jump!
So Torque.
I can hold the chuck in 2 hands and stall the motor at 11rpm - not above 50rpm - so for thread cutting I am limited to 1/4" on steel using a regular die in a tailstock holder. But it's OK for starting and progressing by hand. Machining at 100rpm and up means fine cuts... - fine feeds - and isn't clever for de-scaling large diameter cast iron sufaces. (over 1"). But at 1000rpm I have bags of torque - although the higher speed naturally reduces the "cut" for a particular feed. Actually, the motor has a suitable torque for the stiffness of the bed/saddle/cross-slide and tool post. Occiasionally I have been too brutal when removing larger amounts of metal (at higher speeds) and stalled the motor and you can see the lathe twist as the tool digs-in at the stall. Any more torque couldn't be used without loosing accuracy. I probably need a much stiffer frame to stiffen the bed.
So all I can suggest is that you take care when you fit a "bigger" motor. Add gearing on a lathe gives you torque at low speeds that I simply can't imagine any variable speed device can achieve. But Variable speed does make using the lathe and changing speed for cuts a doddle (even during parting off a larger lump).
Having spent the ackers already, get on and fit the 3ph motor and VSD and enjoy the jobs you do! - But take care and learn how it works on your lathe - "the weakest link" and all that... you don't want to destroy the accuracy of the expensive lathe by using too much motor/cutting torque. Hobby lathes are much cheaper than Industrial lathes simply because everything is made as thin as they can get away with - which destroys the stiffness of the tool. (stiffness = ability to resist torque and maintain accuracy between mainshaft and tool = cut). 4 previous second-hand hobby lathes I owned were all much less accurate that the cheap Chinese lathe I now own. All the beds were twisted and worn and they all had more motor torque than the original...!
If you need more torque, buy a bigger lathe, if you want variable speed, accept the low speed limitations after being familiar with geared torque.
Finally: 1ph = 1/3rd torque of 3 ph (I think?).
K2

 

[Shopgeezer]

The lathe is 1 hp. From what I understand from those who have done this conversion you may want to do a belt change even with a 3 phase motor. Having speed control from, say 300- 1000 spindle rpm and making the odd belt change for a higher speed set up is a galaxy of difference from having to change a belt on this machine for every speed change you want to make. Drilling a hole, reaming it to size and running a tap through it would ideally use three different speeds. I would be all afternoon changing belts for that, compared to turning a knob between each setup.

So research on the web on using a VFD with a single phase motor and all the above comments reveal some issue with torque at lower speeds, probable increase in noise when operating at slow speeds, chance of heating, and possibility of damaging the windings in the motor if the wire insulation isn’t suitable for the square waves generated by the VFD.

So sadly I will resume work on the 3 phase motor swap. It’s going to be ugly. The new motor is rated 1hp same as the lathe motor. I’m not after more power. This lathe cuts just fine for my modest requirements. Despite my frustration with the belts I will admit that this lathe is built heavier than most 10” hobby lathes I have seen. If I can get the motor change done it should make a very good shop lathe.

 

[minh-thanh]

Shopgeezer ,
1 hp = 745.7 w
You can try Dimmer 1000 w ( 2000 w ) for fan (not for lamp).
Test it for a period of about 10, 15, 30 minutes ... at a speed of about 500, 800 or 1000 rpm
If the motor is not too hot (just 20-30 degrees hotter than usual) and not too noisy then you can use a Dimmer
Do not adjust the Dimmer to make the motor speed too slow !!

 

[Steamchick]

Hi Shopgeezer. Don't be sad. I think from your explanation you have been through a logical assessment of the options and made the right decision - long-term. So feel good about the decision, and that might ease the paim of the effort required for the immediate conversion. Look forward to more fun with an "easier-to-use tool". After all. Isn't that what tools are for? - Making things easier so we can have more fun?
Enjoy your hobby!
Incidentally, I made a change to variable speed on my little watchmakers lathe - because I was too stupid to read the instructions, and I overheated the original motor after fitting new bearings! - Now that was painful! (I couldn't get a direct replacement to the original motor so had to make brackets for a new motor - then buy and re-wire with a variable speed control to get the speed right).
But life is so easy (most of the time) with variable speed.
K2

 

[el gringo]

Each time I see this discussion I have to mention the conversion of my old treadmill motor and DC drive into service on my 9" Southbend mode A ~15 years ago. Nice speed range for me and plenty of torque, still making model engines and fixing neighbors things.
I have since converted a friends lathe with a drive found on a treadmill in a thrift shop ($25usd).

 

[Steamchick]

I have a large resistor that a Dentist had used to provide variable speed to his drill... Gets a bit warm on a 350w Brush motor, but will slow it from "finger-grip" stall to full speed. (I think he only used a 75W motor to drill teeth? WHzzzzzzz... Ouch!). Nice in winter when you need a bit of heat in the garage...
Also have a couple of thyristor controls from the 1960s to 1980s that give half-wave rectification then chop based on back-EMF from Brush motors. But they warm-up the motors a lot at low speed when the fan isn't going fast enough to cool the motor! 10 mins stop time per 2 mins running. As one thyristor has blown at odd times, I have one controller that needs an £8 thyristor - but I can't find one - so I have replaced the whole thing with a £3 Chinese Variable Speed circuit board and have much better full range control.
These new fangled things are simply superb by comparison with my old controllers.
And less than 1/3rd the cost! Not sure about if they are better at avoiding overheating.... Can any electrical boffins advise?
"Ya peys ya Money" .... etc.
K2

 

[Ken I]

Shopgeezer, You might want to look up my lathe conversion here :-

Single Phase To Three Phase Rewind - Lathe Uprate

Plenty of information there - I rewound my 550W single phase to 2.2kW 3 phase.

Curiously I ran the bejeebers out of the lathe over the weekend for 6 hours in 30°C ambient - the headstock got to about 10°C over ambient but the motor maybe only 5°C over ambient.

There is an attachment on rewinding for more power and changing from single to three phase.

Hope it help.

Regards, Ken

 

[Tomcat]

I did the three phase conversion using a VFD and it works great, but I still need to change the drive belt pulley for low speeds. The torque is affected dramatically at low speeds even with the 3ph motor and VFD. DC motors (like the threadmill example above) work fine for low speeds but they use Pulse Width Modulation, which means that the peak DC voltage always remains the same, but the wave is segmented into smaller and smaller chunks as you decrease the speed, and at full speed the waveform is a full DC supply.

 

[minh-thanh]

Steamchick !

But they warm-up the motors a lot at low speed when the fan isn't going fast enough to cool the motor! 10 mins stop time per 2 mins running.
K2

A long time ago I had the same situation, but with some 3-phase motors often running at low speed, heavy load
The environment is quite hot and the fan speed is too slow so the motor is quite hot
My solution was to remove the motor fan, and install a fan like the one you would normally find in computer cases, but bigger, and use the 220v power source directly, the motor temperature was pretty good.

 

[Jules]

Remember that a belt or gearbox will transfer (approximately) the same amount of motor power to the spindle regardless of the speed selected.
A VFD will loose power as you lower the speed (frequency)
That’s why people often fit larger motors when using a VFD to control machine speed.

 

[Ken I]

The iron core of a motor can only put out so much torque - not a Nm more.
The only way to get more power out of a motor is to spin it faster using the VFD's higher frequency.
But that won't work unless the motor impedance is changed or the voltage applied is raised to maintain torque.
But that won't help deliver more torque to the spindle either unless you gear down.
Its that simple - and that complicated.
The attached Zip file is the MS-Word *.docx version of my tome on the subject.
Regards, Ken

 

[Steamchick]

Almost "common sense", except that has never been "common"! It's what experts have experienced many times to become, well, Expert!
Ask Ken1. He is what I call an Expert.
I am just an ex-spurt... (drip under pressure).
K2

 

[awake]

Shopgeezer ,
1 hp = 745.7 w
You can try Dimmer 1000 w ( 2000 w ) for fan (not for lamp).
Test it for a period of about 10, 15, 30 minutes ... at a speed of about 500, 800 or 1000 rpm
If the motor is not too hot (just 20-30 degrees hotter than usual) and not too noisy then you can use a Dimmer
Do not adjust the Dimmer to make the motor speed too slow !!

I have a large resistor that a Dentist had used to provide variable speed to his drill... Gets a bit warm on a 350w Brush motor, but will slow it from "finger-grip" stall to full speed. (I think he only used a 75W motor to drill teeth? WHzzzzzzz... Ouch!). Nice in winter when you need a bit of heat in the garage...
Also have a couple of thyristor controls from the 1960s to 1980s that give half-wave rectification then chop based on back-EMF from Brush motors. But they warm-up the motors a lot at low speed when the fan isn't going fast enough to cool the motor! 10 mins stop time per 2 mins running. As one thyristor has blown at odd times, I have one controller that needs an £8 thyristor - but I can't find one - so I have replaced the whole thing with a £3 Chinese Variable Speed circuit board and have much better full range control.
These new fangled things are simply superb by comparison with my old controllers.
And less than 1/3rd the cost! Not sure about if they are better at avoiding overheating.... Can any electrical boffins advise?
"Ya peys ya Money" .... etc.
K2

I am out of my depth on this, so may well be wrong (that happens regularly!) - but I was under the impression that the dimmer controls, thyristor controls, etc., only work on universal type motors, not on induction type. I am assuming - perhaps incorrectly - that the existing 1hp motor is an induction motor, and so would not respond to this type of control

Again, take this with a large grain of salt! Hopefully someone else will corroborate or correct ...