Applying Electrical variable speed drives to lathes

[Don Baer]

Tod suggested that I use my background applying variable speed drive to write a tutorial. I have been doing this for over 30 years so I felt that other could benefit from my experience. This will be long and I hope not to boring. Keep in mind that I was a Physics major in college .
When applying variable speed controls one to machinery on must keep in mind the application. Turning is one of the few applications where more torque is needed a slow speed then at high speed. It requires more torque to cut material the further you are away from the center on the turning axis. A step pulley system is well suited for this since as you slow down the speed by changing pulley ratios you are actually increasing the torque. This is true for most mechanical variable speed drive systems. DC motors with permanent magnet motors and the lesser expensive inverters with 3 phase motors are constant torque devices so great care should be taken when applying them on lathes. A 1 HP motor designed to run at 1750 RPM will produce 36 in.lb. of torque throughout its speed range. A 1 HP motor designed to run at 3450 RPM will produce 18 in.lb. of torque through out it's speed range. Using my little Rikon lathe as an example the following table will tell you the speed and torque that the present drive system has. It has a 1/2 HP 1750 RPM motor. So the motor produces 18 IN.LB. of torque at the motor shaft.
Speed RPM Torque produced at spindle
430 73 in.lb.
810 39 in.lb.
1230 25 in.lb.
1810 17 in.lb.
2670 11 in.lb.
3900 8 in.lb.

If I wanted to replace the motor step pulley arrangement with an inverter or DC system I would need to figure an equivalent HP and then select a motor pulley ratio for the top speed. Lets say I wanted to use a 3450 RPM so that I can have the motor speed close to the 3900 RPM top speed. Then I take the spindle speed of 3900 and divide it by the motor top speed. This would mean I would use a 1.13:1 ratio (3900/3450). Now to figure the HP needed by taking the low speed torque multiplied by the highest speed and divide by 63025. HP=TorquexRPM/63025. In this example it tells me that I need 4.5 HP to have an equal DC or inverter based system. This would be a real waste of energy and my mini lathe would have a vary big motor hanging off of it. I would need a bigger bench to set it on also. Another way around t would be to limit the speed range of the lathe. I could for instance give up the highest two speed and use a 1750 RPM motor an belt it in so that the top spindle speed was 1810. This would mean using a 1.03: pulley ratio . The and would only require a 2.1 HP motor. 73x1810/63025. Do able but still not that practical.

[Don Baer]

There are inverters out there that can produce constant HP as well as constant torque. They do this by increasing the frequency beyond the 60 Hz that the power company produces. They way they work is from 0-base speed of the motor they produce a constant volts per cycle then once the motor reaches base speed they hold the voltage constant and increase the frequency. There are limitations to this and they lie within the motor. Things like bearing etc don't like being run too fast. A practical limit would be 3600 RPM.
If I were to use an 1150 RPM Motor it and run it up to 3600 RPM's I can use a slightly smaller motor. Doing the math on this example I take the highest torque required and multiply it by the base speed of the motor and come up with a 1.34 HP requirement and a pulley arrangement of 3.4:1.
Ok I hope I didn't confuse anyone and also hope it is a help for anyone contemplating doing a conversion.

[tod evans]

don,
thanks for the education! you`ve explained why so many folks i know who have tried to adapt dc motors to their lathes have been disappointed with the results. (myself included many moons ago)
now for the 24 dollar question....say joe owns an old delta step pully lathe, or a newer mini for that matter, is there an affordable way to incorporate variable speed (with the nifty little dial) to his lathe without sacrificing torque at low rpms? better yet can joe really improve on what`s offered by the factory regarding the torque-vs-rpm equation?
and if he can what would be the most cost effective way? tod

[Don Baer]

now for the 24 dollar question....say joe owns an old delta step pully lathe, or a newer mini for that matter, is there an affordable way to incorporate variable speed (with the nifty little dial) to his lathe without sacrificing torque at low rpms? better yet can joe really improve on what`s offered by the factory regarding the torque-vs-rpm equation?
and if he can what would be the most cost effective way? tod

Tod it realy depends on what you are trying to achieve. If you just want variable speed within a given speed range you could keep the step pulley arangement and then use a larger motor. For instance on my Rikon I could replace the 1/2 HP motor with a 1 HP and then set then pulley arangement on the 810 RPM step. That would allow me to turn the speed down to any speed and get 76 in. lbs of torque that I now have on my lowest speed for ruoughing. Then I could turn the speed back up to the 810 RPM with no loss on power. The same would be true for the higher ratios. If I used the 1230 RPM step I could turn it down in speed and get 50 in. lbs of torque at any speed. This would be fairly in expensive and cut down on the amount of times one would have to change the ratios. I generaly run my rikon between 810 if the blanks not to bad for ruffing and 1810 so if I were to use this arrangement I'd get 34 in.lbs of toqrue throughout the speed range where now I get 39 in.lb. at 810 and 17 at 1810.
It's all a compromise.

[John Timberlake]

Your explanation answers a couple of question I had in the past.

1. When using the newer Jet variable speed mini-lathe, I was able to stop the lathe while turning a pen at low speeds. You say the torque goes really low, so that would explain that.

2. The old Nova lathe with a DC motor had a step pulley with a range of speed. I guess that is why you could get away with it at very lost speed (less than 100 rpm) and still be able to rough out odd shaped blanks.

[lou sansone]

Tod suggested that I use my background applying variable speed drive to write a tutorial. I have been doing this for over 30 years so I felt that other could benefit from my experience. This will be long and I hope not to boring. Keep in mind that I was a Physics major in college .
When applying variable speed controls one to machinery on must keep in mind the application. Turning is one of the few applications where more torque is needed a slow speed then at high speed. It requires more torque to cut material the further you are away from the center on the turning axis. A step pulley system is well suited for this since as you slow down the speed by changing pulley ratios you are actually increasing the torque. This is true for most mechanical variable speed drive systems. DC motors with permanent magnet motors and the lesser expensive inverters with 3 phase motors are constant torque devices so great care should be taken when applying them on lathes. A 1 HP motor designed to run at 1750 RPM will produce 36 in.lb. of torque throughout its speed range. A 1 HP motor designed to run at 3450 RPM will produce 18 in.lb. of torque through out it's speed range. Using my little Rikon lathe as an example the following table will tell you the speed and torque that the present drive system has. It has a 1/2 HP 1750 RPM motor. So the motor produces 18 IN.LB. of torque at the motor shaft.
Speed RPM Torque produced at spindle
430 73 in.lb.
810 39 in.lb.
1230 25 in.lb.
1810 17 in.lb.
2670 11 in.lb.
3900 8 in.lb.

If I wanted to replace the motor step pulley arrangement with an inverter or DC system I would need to figure an equivalent HP and then select a motor pulley ratio for the top speed. Lets say I wanted to use a 3450 RPM so that I can have the motor speed close to the 3900 RPM top speed. Then I take the spindle speed of 3900 and divide it by the motor top speed. This would mean I would use a 1.13:1 ratio (3900/3450). Now to figure the HP needed by taking the low speed torque multiplied by the highest speed and divide by 63025. HP=TorquexRPM/63025. In this example it tells me that I need 4.5 HP to have an equal DC or inverter based system. This would be a real waste of energy and my mini lathe would have a vary big motor hanging off of it. I would need a bigger bench to set it on also. Another way around t would be to limit the speed range of the lathe. I could for instance give up the highest two speed and use a 1750 RPM motor an belt it in so that the top spindle speed was 1810. This would mean using a 1.03: pulley ratio . The and would only require a 2.1 HP motor. 73x1810/63025. Do able but still not that practical.

Hi don

please allow a little clarification on the table of speed and torque ratings. it may not be obvious that the speed reduction and the torque increase is due completely to the fact that you are running the motor at a constant speed, but reducing the speed by mechanical gearing and not electrically reducing the speed. it is because of the mechanical advantage via gear reduction , that your constant toque motor can produce greater torque at low speeds and not the fact that you have a constant hp motor running at low speeds. What I have found to be a good solution for constant torque motors is to use them in conjunction with a 4 speed gear box ( drive all ) and vary the speed to meet the torque requirements of the work at hand and also not run the snot of the motor.

I can provide photos of my set up if folks are interested
lou

[Don Baer]

Hi don

please allow a little clarification on the table of speed and torque ratings. it may not be obvious that the speed reduction and the torque increase is due completely to the fact that you are running the motor at a constant speed, but reducing the speed by mechanical gearing and not electrically reducing the speed. it is because of the mechanical advantage via gear reduction , that your constant toque motor can produce greater torque at low speeds and not the fact that you have a constant hp motor running at low speeds.

Thanks for helping. Sometimes what I know in my head doesn't get tranlated into my typing.


What I have found to be a good solution for constant torque motors is to use them in conjunction with a 4 speed gear box ( drive all ) and vary the speed to meet the torque requirements of the work at hand and also not run the snot of the motor.



Thats why when Tod asked for a an inexpensive solution I suggested leaving the step pulley in and driving it with a 1 hp motor drive combo.


I can provide photos of my set up if folks are interested
lou

That may be of some interest to some.

[Rich Souchek]

Thank you Don and Lou,
You nailed my current problem and some mis-understandings.
Been looking at all the ac & dc speed contyrollers, and am unsure of any results with them. (& don't like the $$$.)
Keep coming back to putting a 1 to 2 hp ac motor w/ 1750 rpm in a old "new style" Rockwell/Delta lathe and a set of 4 step pulleys to get the speeds low.
Any similar experience out there?
Rich S.

[Don Baer]

Rick,
If you can tell me what you have interms of a motor and drive train maybe I can recommend a solution that will work.

[Rich Souchek]

Don,
Long story.
Lathe#1 was an “old style” Rockwell/Delta 46-??? lathe that had a 3/4hp, 1750 RPM, ac motor with a 2”-to-4” od four step pulleys (approx. sizes.) It was a joy to learn turning on and must have got down to 800 rpm or so. It was almost slow enough to turn 10” and 12” rough blocks for bowls.
It is going to my brother-in-law and it was replaced by another old “new style” Rockwell/Delta 55-201 lathe that was repowered with a 1/2hp 3750 RPM motor that is just too fast and too weak. It is also on a metal stand (very noisy, bounces all over like a piece of dog doo-doo), the bed has the cut-out for larger bowls that can’t be used with the offset of a 4 jaw chuck, the drive belts must go down (not to the rear as the “old style” permitted and was VERY convenient to change speeds.
Lathe#2 is not near the lathe that the old style was, and believe that giving it more power and slower speed will help considerably.
Have looked for 1 ½ weeks at AC & DC motors and speed controllers, and am now only sure that I don’t know what to believe. My wife and life make $ an important factor right now also. Figured a ¾ hp motor and controller will run the best part of $350, mainly because I don’t know what will actually work, what to get, and that seems to be a going price.
I keep going back to bigger AC motors with slow speeds, and believe this will end up at about $150.
(Say 1.5 hp, 1100 thru 1750 RPM, single phase, either 120 or 230 VAC, reversible directions if possible.)
Intend to build another very stout, solid, wood base for lathe #2 and leave room for a big motor.
This should get me the power I want, and get the speed down to 550-700 RPM , depending on the motor. BUT with the pulleys down low and my belly going both out and older, changing speeds is a royal pain.
I got to spend about ½ a day on a big 24” Oneway lathe recently and feel in love with that speed controller. It spoiled me. So the real big question is what will work really good for lathe # 2 within a $300-$350 range?
Any ideas?
(Yeah, jackshafts…. But …….NO.)
Thanks,
Rich S.

[lou sansone]

rich ... I don't like pulleys for lots of reasons. If you can find a variable speed gear box and hook it up to the 1.5 hp motor you may like it. I use a 3 hp 4 speed gear box for my lathe and it is a pretty nice set up. Variable speed boxes are available from ebay it you look around. I built a very nice one on the delta platform that you are describing and ended up selling it to a nice gent.

best wishes
lou

[Dick Strauss]

Don,
I've got a Delta 1440 (mod #46-715...1750rpm 3/4HP Reeves drive) that I'd like to upgrade. I'd like to have a 220V motor in the 1.5-2HP range that is reversible. I might want to run it as low as 100rpm and don't mind getting rid of or upgrading the reeves drive (given that the current one probably won't handle the HP).

What would you suggest in my case? What will it cost to do it right?

Thanks in Advance,
Dick

[Don Baer]

Rich,
If I were refitting that guy I'd look for a 1 to 1 1/2 HP DC motor and controller. I saw several motors and controls on the auction site that would do. I'd say you could get a workable motor and controller for less then $200. anything over 1 hp will require 230 volts. Once you get the motor and controller we still should talk about the pulley sizes you have.

[lou sansone]

don
Is your reason for not recommending a VFD the cost associated with it? Or is there a technical reason? Having built many machines myself, I have found the modern VFD, with the proper motor, superior at low speeds to the garden variety DC motor and controller.

lou

[Don Baer]

Yes lou, I was thinking cost. That and the fact that DC motors are thermaly rated for 20:1 constant torque. Standard industrial AC motor don't like to run under heavy loads at low speed due to the reduced air flow. That said I was just over at the auction site nosing around and I did fin a 1 1/2 HP Marathon inverter duty motor rated 10:1 constant torque that someone should be able to pick up for less then $100 and I found several inverter for under $100 also so I would also recommend one of those.

[Don Baer]

I posted in this thread about speed range of motors. Thought I'd clarify the statement a little so as not to confuse folks. Motors need to be cooled especially when they are running close to full load amps. The methods of cooling are a two fold. There is usually a fan attached to the motor shaft either outside the motor for totally enclosed motors or internally for open motors. One of the basic laws of physics as it applies to fans says the airflow goes down as a cube of the reduction of speed. So if you slow a motor down the airflow over it will be reduced drastically. Motor manufacturers design around this by either adding iron to the motor so it can radiate more heat or by using an external fan that runs at a constant speed and is power by a different source of power. The when applying any variable speed drive system you should always get a motor that is designed for variable speed or your motor will soon turn to toast.
When I said that a motor was rated for 20:1 constant torque that means that you can run a 1750 RPM motor down to 87 1/2 RPM and pull full load al day without hurting the motor. I hope this clears up any confusion I may have caused.

[Don Baer]

Rick sent me a PM with several items from the auction site. I'll answer his PM here so that others can learn also. I'll be posting the Items ut not a direct link. I hope this solves any problems the moderators have.

1. 1HP-AC-Motor-Variable-Speed-Control-Package-110V_W0QQitemZ7631531941QQihZ017QQcateagoryZ11810Q QtcZphotoQQcmdZViewItem (http://gi.ebay.com/1HP-AC-Motor-Variable-Speed-Control-Package-110V_W0QQitemZ7631531941QQihZ017QQcategoryZ11810QQ tcZphotoQQcmdZViewItem)

This motor and controller are not a good choice. First of all the motor is a 3450 RPM motor (18 in.lb. of torque). In order to get down to a decent low speed say 400 RPM would require a large pulley ratio in order to get any decent torque and this would severly limit the top speed. While the inverter is capable of runn the motor to almost 14,000 RPM (:eek: yikes) I certanly wouldn't try it. The bearing in this type of motor probubly arn't good for much beyong 3600 RPM's. The motor would decintregrate.

2. 3-4-HP-C-Face-Variable-Motor-Speed-Control-Package-56C_W0QQitemZ170002049874QQihZ007QQcategoryZ11810Q QtcZphotoQQcmdZViewItem (http://m/3-4-HP-C-Face-Variable-Motor-Speed-Control-Package-56C_W0QQitemZ170002049874QQihZ007QQcategoryZ11810Q QtcZphotoQQcmdZViewItem)

Actually a better choice. At least the motor has more torque (27 in lbs) and you could run it up to 3600 RPM's safely. Certainly not the 6900RPM's they claim.

3.BALDOR-ELECTRIC-CO-2-HP-REVERSIBLE-115-230V-AC-MOTOR_W0QQitemZ330005374701QQihZ014QQcategoryZ2622 6QQrdZ1QQcmdZViewItem (http://cgi.ebay.com/BALDOR-ELECTRIC-CO-2-HP-REVERSIBLE-115-230V-AC-MOTOR_W0QQitemZ330005374701QQihZ014QQcategoryZ2622 6QQrdZ1QQcmdZViewItem)

This is a single phase motor and cannot be run off of an inverter. You'll burn it out. You need a 3 phase motor.

4.2-HP-VFD-VARIABLE-FREQUENCY-AC-MOTOR-SPEED-DRIVE_W0QQitemZ140005857507QQihZ004QQcategoryZ7139 3QQssPageNameZWDVWQQrdZ1QQcmdZViewItem (http://cgi.ebay.com/2-HP-VFD-VARIABLE-FREQUENCY-AC-MOTOR-SPEED-DRIVE_W0QQitemZ140005857507QQihZ004QQcategoryZ7139 3QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)
or
1-HP-VFD-VARIABLE-FREQUENCY-AC-MOTOR-SPEED-DRIVE_W0QQitemZ130006011184QQihZ003QQcategoryZ7139 3QQcmdZViewItem (http://cgi.ebay.com/1-HP-VFD-VARIABLE-FREQUENCY-AC-MOTOR-SPEED-DRIVE_W0QQitemZ130006011184QQihZ003QQcategoryZ7139 3QQcmdZViewItem)

Either of these would work fine with the appropriate motors. Like this one.
Marathon-inverter-duty-motor-2-HP-blue-max_W0QQitemZ7608004249QQihZ017QQcategoryZ26226QQs sPageNameZWD1VQQrdZ1QQcmdZViewItem (http://cgi.ebay.com/Marathon-inverter-duty-motor-2-HP-blue-max_W0QQitemZ7608004249QQihZ017QQcategoryZ26226QQs sPageNameZWD1VQQrdZ1QQcmdZViewItem)
or this one
1-5-HP-VARIABLE-SPEED-DC-MOTOR-MAGNETEK_W0QQitemZ260008239759QQihZ016QQcategoryZ2 6226QQrdZ1QQcmdZViewItem (http://cgi.ebay.com/1-5-HP-VARIABLE-SPEED-DC-MOTOR-MAGNETEK_W0QQitemZ260008239759QQihZ016QQcategoryZ2 6226QQrdZ1QQcmdZViewItem)
Idealy I'd like to find a 1 or 1 1/2 HP1150 RPM motor.

I hope this helps.

[lou sansone]

another option for VFDs and motor is automation direct and you can forget the fleabay game

lou

[Don Baer]

Yes Lou they are indeed a good source. I've used them in the past.

[Rich Souchek]

Don, Lou, and whomever,
Couple of more items, if you would look at them.
1.) AC drive wise, how about this AC drive from Automation Direct http://web1.automationdirect.com/adc/Shopping/Catalog/AC_Drives_-z-_Motors/GS1_(120_-z-_230_VAC_V-z-Hz_Control)/GS1-21P0 (GS1 1.0 HP AC DRIVE 230V 1 PH INPUT 3 PH OUT for $125) to go with this motor from Houston Motor and Controls http://cgi.ebay.com/WEG-Electric-Motor-1-HP-1800-RPM-143TC-Frame_W0QQitemZ7611346968QQihZ017QQcategoryZ42922Q QcmdZViewItem (new WEG Electric Motor . 1 HP, 3 phase, 60 Hertz, Frame - 143TC, RPM 1800, Volts 208-230/460 , ENCL -TEFC - Explosion Proof. with 18 Months warranty for $110)
Would they work together good for the old lathe?
2.) Is it really a simple matter of hooking up electrical wires, plugging them in and everything works as it should?
3.) Finally found the simple AC drives, any particular brands to pick or avoid?
4.)Same question as above with motors?
Would this motor be worth another $100?
http://web1.automationdirect.com/adc/Shopping/Catalog/AC_Drives_-z-_Motors/Marathon_Inverter_Duty_Motors/Marathon_Inverter_Duty_Motors_(by_HP)/Y364 (Marathon Electric MicroMAX series, 1.0 hp, 1800 RPM, inverter-duty AC motor, 230/460 VAC, 3 phase, 56C frame, $198)
Thank you Don and Lou,
Rich S.

[lou sansone]

rich

I like the "GS-2" because it has a removable key pad that you can teather to your lathe in a variety of positions. The drives work great and I have been using one for at least 3 years now with out any problems. I would try to get into the 1750 rpm and 2 hp range if you can. That will give you a little more low end torque.

For the motor, I ended up getting a very nice reliance inverter duty motor from "HGR" surplus for about $30.

lou

[Don Baer]

Rick,
What size lathe do you have, what is the max diameter it will swing. does it do outboard turning if so how big a piece can it swing.

[lou sansone]

rich and Don

here is a link that will take you the to the tsubaki web site .... once you get there open up the PDF file at the bottom of the page and you will see a very unique gear reducer that I think makes a great building block for any wood lathe

http://ustsubaki.com/ptc/disco.html

I have built machines, including wood lathes that use the DISCO infinitely variable speed gear reducer and they are really fantastic machines. if you couple them with a dc motor or a VFD you have a very versatile machine

enjoy
Lou

[Rich Souchek]

Don,
I currently have an old "new style” Rockwell/Delta 55-201 lathe that was repowered with a 1/2hp 3750 RPM before I got it. Turns 12"dia over the bed, with a short cutout by the headstock that will turn 16" dia platters or tops.
Yes, it can turn outboard, just haven't tried it and have not made a separate tool rest to do that.
I believe that a full 1 hp setup would be perfectly fine for this application.
Rich S.

[Bobby Wartman]

Rich,
I saw several motors and controls on the auction site that would do.

Sorry for this question.:confused:

Which auction site are you reffering to? Ebay?