Lathe conversion to 3-phase power

[WillieL]

I just thought I would put up a few pics of my attempt at making a silk purse from a sow's ear.   

I bought this lathe (12x36) to be a step up from my little 7x10 mini-lathe. It was nice to be able to work on larger diameter and longer items with the new lathe. But something that I missed compared to the mini-lathe was the restricted number of spindle speeds available to me on the larger lathe. This became a real problem for me when threading to a shoulder because the lowest speed on the lathe was 60 RPM (66 actual). The chuck would free wheel for quite some time after being shut off, and it is not equipped with a spindle brake. I'm afraid I am just not that coordinated to pull off that kind of timing.   

So I set about trying to resolve that issue. I purchased a new 2 HP 3-phase motor to replace the factory 1-1/2 HP single phase motor. Bigger is better, right? (LOL) One of the first problems encountered was that the motor shafts were a different size, which meant that the factory drive pulley would not fit the new motor. And of course I could not find a new double pulley in the configuration I needed. But I did find single pulleys of the proper sizes. Time to modify!   

     

After getting the motor mounted it was time to install a VFD to supply the 3-phase power to it. My lathe sits in the middle of the floor so there aren't any near by walls to mount it to. I just attached it to the rear of the factory electrical box and attached some lexan sheets to serve as a shield against getting any swarf/chips inside the VFD. I also added a large braking resistor.

 

To finish it off I mounted the VFD control unit remotely near the front of the lathe for convenience and also installed a "tachulator" from LMS, which displays spindle RPM and SFM readouts. I made an encoder wheel and mounting block to simplify the installation of the optical sensor on the lathe.

       

I also added a toggle switch on the right side of the VFD control box, that allows me to switch control of the lathe motor between the VFD panel and the factory control lever on the lathe apron. Flip the switch UP, and I can start/stop the lathe with the VFD push buttons. With the switch DOWN, the normal lathe lever is in control.



After some fine tuning I am now able to run the lathe as slow as 2 (TWO) RPM!
Threading to a shoulder is no longer a problem.   

[John Hill]

Great work and description.    

But I am a little concerned at that switching between VFD panel and lathe control lever, I hope you have it so that the control lever will always stop the lathe no matter what the position of the toggle switch.

[WillieL]

I see your point John, but unfortunately this VFD does not have a way of wiring/programming that kind of function. Actually I always use the normal lathe control lever anyway. I just added the toggle switch as an after thought, in case I ever had a switch or wiring failure on the apron control. That way I could at least finish whatever job I was working on at the time.

If I ever had a problem while using the start/stop buttons on the VFD control, I would head straight for the big red E-stop button on the front of the lathe anyway, instead of trying to find the little stop button in a panic. I have the E-stop wired to kill the single phase input power to the VFD. It stops everything.

That's the best I could come up with given the options available on the VFD I have.

[DaveH]

Hi Willie,

Nicely shown, a good job.

Because I don’t know a lot about these things I would like to ask a few questions.


The last photo - The VFD unit it is displaying 52.9 Hz – so what, does that tell me? What is it for?

Underneath (still on the last photo) you have a display showing RPM – was this purchased separately?, and did it come with the “encoder disc”.

One more,  why did you need a double pulley?

As my lathe is the same as yours so I have quite an interest in it.

 
DaveH

 

[doubleboost]

Hi Willie,

Nice job on the conversion
I have a VFD unit on my boxford it ios great for screw cutting
Be carefull you dont run the motor that slow that it overheats
John

[John Rudd]

Hi Willie,

The last photo - The VFD unit it is displaying 52.9 Hz – so what, does that tell me? What is it for?

DaveH

The rev counter is the tachulator from LMS...He made the encoder wheel himself...

a "tachulator" from LMS, which displays spindle RPM and SFM readouts. I made an encoder wheel

The frequency readout you see, is the frequency of the electrical power supply going to the motor...
Normal mains frequency is 50Hz, so for 3 phase motor its speed will be  3000rpm.  Changing the frequency alters the speed at which the motor runs...

 If you look at the formula for a typical 3 phase motor which is:
         nS=(120*f)/nP

Where: nS = nominal speed
             nP = number of pole pairs ( minimum number of two...)
              f = frequency

So (120*50)/2 = 3000rpm

So if the number of poles is constant then speed becomes a function of frequency...

[John Hill]

Mains frequency in the US is usually 60Hz, also Canada, parts of Japan and a few other countries (which I cant remember) the rest of the world uses 50Hz.

The standard motor speeds in 60Hz countries is higher therefore than in 50Hz countries and I presume 50Hz motors will tolerate lower speeds than 60Hz motors but dont quote me on that!

[John Rudd]

John,

I specifically mentioned 50Hz in my post as DaveH currently lives in South Africa and having lived there, I know the mains is at 50Hz...

But you are correct that elsewhere there is the possibility of 60Hz mains, notwithstanding that, the formula still stands and motors will runs as expected when powered by a VFD irrespective of mains frequency..

[DaveH]

John & John,

Thanks.

I haven't finished yet

So do all VFD's just show the frequency, don't suppose they show motor speed, or say % motor speed. Such as 50hz = 100%?

Just thought I would ask.
 
DaveH

[John Hill]

I dont know what the VFDs display as I do not have one however we should bear in mind that the speed of induction motors is only 'nominal' and all such motors will run a bit slower than you might expect from calculation of frequence and number of poles.  The difference is 'slip' and is essential to the operation of the motor.

[philf]

Dave,

I have a number of inverters driving a bench drill, bandsaw, lathe and CNC miller. The most recent one (an Allen Bradley Powerflex) is for my CNC miller and it has by far the most comprehensive set of display features and general parameter settings.

For a machine with only one pair of pulleys and no gearbox between the motor and spindle it can be set to display the approximate spindle rpm. Most inverters display the output frequency but mine has a parameter - Process Factor - which you can set to anything between 0.00 and 9999 which is a multiplier for the output frequency. So if your spindle rotated at 3000 rpm for a 50Hz input frequency you would input a value of 60 for the process factor. (60 x 50 =3000)

If your machine has more than one pulley or a gearbox then it's of no practical use and a seperate speed sensor would make more sense and would (hopefully) give a more precise speed reading.

My CNC has 3 pulleys so I have a magnet embedded in the spindle pulley and a hall-effect sensor connected to the Mach3 controller which displays the speed accurately - unfortunately the displayed speed is never the speed I select - it's always about 50 rpm higher! (Not a major problem but niggling.)

Cheers.

Phil.

[Country Bubba]

My vfd on my mill/drill can be configured to show Frequency or speed amongst other things. As noted above, it is not the most accurate thing in the world, but a lot closer than when I was having to guess based on the chart of various belt change speeds which was also wrong by several rpm

[John Rudd]

I dont want to divert from the original theme of this thread...
But, I own a machine that has pulleys for varying the spindle speed and it also benefits from a vfd....

So to recap..

The vfd has a readout that can display the frequency of the electricity going to the motor...
If there are no pulleys involved then frequency is a direct function of motor speed...refer back to what I said about speed and frequency....

If you have pulleys then all is not lost...if you know the ratio of the pulleys, then factor that with the frequency readout of your vfd to arrive at spindle speed.

Or else use a seperate tacho that reads direct spindle speed via an interruptor mounted on the spindle or some other means be it a Hall device or proximitor...

Suffice to say, the best combination is a vfd with pulleys, as this allows the motor to run at a speed using the lowest pulley ratio to give the lowest spindle speed desirable without overheating the motor....

[DaveH]

Phil & Art,

Thanks guys appreciate your help. I also understand it can get a bit tricky with the gear box 

I just think a % "speed" output would seem a little more sensible. Than,  what's the motor speed DaveH, " No idea but the frequency is 32.7 hz" 

I suppose if one is good at metal arithmetric then it's fine.
 
DaveH

 

[John Rudd]

I just think a % "speed" output would seem a little more sensible. Than,  what's the motor speed DaveH, " No idea but the frequency is 32.7 hz" 

I suppose if one is good at metal arithmetic then it's fine.

 If the mains frequency is 50Hz and the nominal motor speed at the that is 3000rpm, the its a simple ratio of 32.7/50*3000...which is errmmmm( reaches for calculator.... )   1962rpm.. Easy aint it...( if its direct drive....)

[DaveH]

Thanks John,

 

If they are available with a speed output reading, or a % output, I would find that a lot easier.

Even using the gear box. If the selected gear(s) gave a spindle speed of 580 rpm and the VFD says 60% that gives approx. 360 rpm.

Any spindle speed (+ or -) 100rpm  won't bother me much, I guess most of the the time anyway

I can certainly see the usefulness of a proper spindle speed measurement/display - a separate one that is.

 
DaveH

[75Plus]

On this side of the pond all our AC power is delivered at 60 hz. The calculated speed for a 4 pole motor is 1800 rpm but due to internal losses the full load rpm is listed as 1725. Two pole motors F.L. rpm is listed as 3450.

Joe

[Country Bubba]

Again on mine, I have one basic pulley set that I use most of the time. So I set the max rpm reading to what the spindle will be at the max freq. setting. In my case, that is 4000 spindle rpm. Adjusting the vfd potentiometer to say 3000 (and that shows on the display when the motor is not running) and the speed of the spindle will be close to 3000 rpm.  It just so happens that if I choose the other belt change the ratio is approximately 2:1 so if I set the speed to 3000, then the spindle will be running about 1500 rpm. Still a lot closer than the amount available with the belt changes.
 

[philf]

Dave,

The easiest way to do what you want is to make a scale for your potentiometer scaled appropriately.

On my lathe I use a 10 turn potentiometer with a 15-turn dial which starts at 2 and goes up to 12 corresponding to 20% to 120% of the speeds on the gearbox plate. (The inverter is hidden away in an enclosure and I can't see the display.)

Hope this makes sense.

Phil.

[DaveH]

Phil,

 

Dave,
On my lathe I use a 10 turn potentiometer with a 15-turn dial which starts at 2 and goes up to 12
Phil.

"Whot"
   
 
DaveH

[kvom]

It's always been my assumption that switching power other than with the VFD switch will eventually cause the VFD to fail.

I am also wondering why you would need to power off the lathe when single-point threading.  The technique I was taught and use has one hand on the cross feed and the other on the thread-dial lever.  At the end of the thread the cross slide is retracted quickly and the feed disconnected.  I leave the spindle turning, and 60 rpm is actually pretty slow.  In class, the first thread I ever did was at 80 rpm and wasn't that fast. 

[philf]

Hi Dave,

I guess you dont want me to try to give a simpler explanation!

 

Phil.

[DaveH]

Hi Phil,

It's ok, I'm with the 2 to 12 bit - 20% to 120% of the gear box speed. Thats fine Phil.

10 turn pot. with a 15 turn dial - now that's tricky - not a clue 
 
DaveH

[philf]

Kvom,

I agree that you should never switch the output of a VFD when it's output is on and should use the VFD control inputs to start/stop/reverse the motor.

You say "I am also wondering why you would need to power off the lathe when single-point threading."

If you are single point cutting say a metric thread on a lathe with an imperial leadscrew it's usual to stop the lathe, retract the tool, (or vice versa) and then reverse the lathe to get the tool back to the start of the thread. A VFD can usually stop a spindle very quickly and so cutting up to a shoulder isn't that difficult.

Tapping in the lathe is made a joy with a vfd. At low speeds switching from forward to reverse is almost instant.

I should add - with screw on chucks there is always a danger of the chuck unscrewing on quicly reversing the direction. Mine is a camlock so can be reversed quickly with no problems.

Cheers.

Phil.

[DaveH]

Sorry Willie,

Sort of messed your thread up a bit,

 
DaveH

[WillieL]

Sorry Willie,

Sort of messed your thread up a bit,

 
DaveH

Not messed up at all Dave. You stimulated some great discussion!   

As was mentioned previously, the power here in the States is supplied at 60 Hz. So the frequency display just gives me a rough idea of what speed the lathe motor is running at as compared to it's rated speed. 60 Hz = 100% / 30 Hz = 50% etc.
Also mentioned above - is the fact that you have to be careful about running the motor too slow -or- too fast as this can cause it to overheat and cause damage. Don't forget that the fan blade that cools the motor is attached to the same shaft as the output pulley! You can however buy inverter duty motors that have a separate motor built in for the cooling fan, that runs at full speed all the time. Those types are designed to be run at slower than "normal" speeds and controlled by a VFD. Of course those are much more expensive motors.   

I only run my lathe at slow (motor) speeds for short periods of time, so I'm careful to not let it overheat. And I try not to run it below 20 - 25 Hz. But having the display there is a good reminder if I slow the motor down to hit a specific SFM number. If the frequency drops too low I know to "gear down" the spindle to get the motor speed back up again. The reason for the two pulley sizes is because the lathe is set up for two speed ranges. High range (I) and low range (II). See the spindle speed chart on the front of the headstock.

A lot of guys run a single pulley and set the machine in high range and leave it there. (Usually on milling machines.) They simply control all speeds with the VFD alone. You can overdrive a motor as well. I could run mine at 90 Hz and get 150% of its rated speed. But again, that is hard on a standard motor and something I chose not to do.

The things I like about the "tachulator" display is that it gives me a very accurate display of the actual spindle speed, as well as a direct reading of the SFM for a given diameter of my work piece. If I am turning a 4.3 inch diameter piece of steel that needs to be cut at 420 SFM, I don't need to do any math to figure out what spindle speed to use to get there. All I have to do is input my 4.3" into the unit just by turning the knob, push the knob in to display the SFM reading, and then adjust the speed of the lathe spindle via the VFD control knob until it shows 420 SFM on the display. Fast and easy!   

It's also nice to have an infinite spindle speed adjustment for dialing into the "sweet spot" for a good finish, instead of being stuck between two set speed choices. Sometimes the best speed is somewhere in the middle.

Almost forgot.... the tachulator unit uses an infrared optical sensor that just needs to "see" a reflected beam. It can be as simple as a single strip of reflective tape, or a painted white line etc., on the shaft that you want to measure the speed of.
The display is adjustable for the number of inputs it sees per revolution. The more divisions you use, the more accurate the display is when measuring SLOWER speeds. I made the encoder ring to slip on the outboard end of my spindle with 10 divisions. According to the manual, 10 divisions will give a resolution of 5 RPM at slow speeds. As I noted in my OP, mine will read accurately down to 2 RPM.

I am VERY happy with the way it performs. 
Cheers mate!   

[DaveH]

Thanks Willie,

It's becoming clearer.    The encoder ring you made yourself, is it painted black, white.?
 
DaveH

[WillieL]

Dave,

The ring was cut from a piece of aluminum round stock that I had left over from another project. I cut shallow recesses in the OD on my mill using a rotary table to mark out the divisions I needed. Then I polished up the OD a bit to make it more reflective, and then painted the recesses with flat black paint to provide the contrast between the sections.



I could have simply painted black lines on the spindle shaft itself and mounted the sensor down closer to it. But it would have been a little more difficult to get the sensor mounted in that position. The small piece of aluminum square I used for the sensor mounting block was also a piece from the "scrap bucket".  Waste not - want not... 


 


 The Tachulator kit contains the display unit, a power supply (wall wart) and the sensor. You have to provide your own way of mounting the sensor to whatever equipment you are using it on. Mini-lathe, milling machine, whatever.... 
 

[WillieL]

The last photo - The VFD unit it is displaying 52.9 Hz – so what, does that tell me? What is it for?

DaveH

Dave,

Something else I forgot to mention... 

On the VFD display you will also see an indicator led for Amps. You can change the readout to show how many amps the motor is drawing if you prefer. Sometimes useful to see how much of a load you are putting on the machine if you are making HEAVY cuts. It will give you an idea of the impending doom if you are trying to push your machine too hard or if that loud screeching noise you hear is a bearing that is about to lock up.   

 

[DaveH]

Willie,

I'm with it all now

Thanks
 
DaveH

[hopefuldave]

Most (if not all) VFDs have an analogue output available - on my setup it's programmed to put out a 0-10V signal proportional to 0 - 150% motor current (to a nice WW2 Admiralty Model meter with a home-made scale -  Bakelite's contemporaneous with the lathe!) and gives a good indication of the load on the motor, but it can be set to give a 0-10V signal proportional to output frequency - if this is taken to a cheap (£10?) panel meter, a new scale can have speed in each gear/belt position marked out on it (with a little calculation).

The tacho' will need to have Nixie tubes (a' la atom bomb countdown in Goldfinger) to look right, though...

Re Emergency Stops - the spindle will brake faster if the power's still on at the VFD and braking's programmed... I've set my EStop up that way, with a separate emergency power off - YMMV

Dave H. (the other one)