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Forum topic by NCMau posted 04-10-2019 10:06 PM 433 views 0 times favorited 9 replies Add to Favorites Watch
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NCMau

45 posts in 115 days


04-10-2019 10:06 PM

If you use a VFD system with your lathe, please chime in and let me know the good, the bad and the ugly about your specific drive. Among other things, I’m wondering if you experience any loss of torque in slow speed.
In not so distant future I’m planning to get a VFD system for a 3-phase motor 2HP that I had in the shop for the last 1000 years, just itching to be used. My lathe right now runs at 850 RPM at the slowest speed. I guess it’s okay for most applications that I have experienced so far, but I can see that now that is too fast for some.

-- Maury, NC


9 replies so far

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TheDane

5637 posts in 4027 days


#1 posted 04-10-2019 10:43 PM

Loss of torgue at lower speeds is a legit concern. Maybe the solution is to do what Powermatic (and some others) did and use a two position pulley system. On my lathe, I can change pulleys to offset torque loss at lower speeds.

-- Gerry -- "I don't plan to ever really grow up ... I'm just going to learn how to act in public!"

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MrUnix

7361 posts in 2563 days


#2 posted 04-10-2019 11:18 PM

3 phase motor + VFD has full torque (not HP) until rated speed:

Which is why using belt selected “speed ranges” is frequently implemented. You want to keep the motor as close to rated speed as possible so you have maximum power + torque.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

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mike02719

101 posts in 4150 days


#3 posted 04-11-2019 12:59 AM

My Laguna runs on 220 with the vfd. This has opened up a whole new world of turning to me. I made a 16” sanding disc mounted in my chuck. I slow it down to 200rpms an it sands segmented rings great. speed it up to sand; terrific. Whatever I try, no speed issues at all. I have the two pulley system, and it works great. Will this work in a retrofit situation, I can’t answer.

-- Mike, Massachusetts

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AndyJ1s

36 posts in 119 days


#4 posted 04-11-2019 01:38 AM

While technically a VFD driven 3ph motor does not “loose torque” at lower speed, it does not gain torque at lower speeds like fixed speed motors with variable or multi-speed transmissions/belts, or DVR motors for that matter.

Andy

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gwilki

293 posts in 1838 days


#5 posted 04-11-2019 12:26 PM

I have a Toshiba VFD on my Vega and have no issues with it. Vega has a two speed pulley set up so that I can get very low speeds and plenty of torque for things like coring and drilling.

-- Grant Wilkinson, Ottawa ON

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GrantA

1481 posts in 1772 days


#6 posted 04-11-2019 12:49 PM

You can use a sensorless vector drive to get full power (or nearly) at lower frequencies. You’ll pay for it though, it’s another class of VFD. I have 3 ABB drives (ACS150 series) purchased from wistex (best prices I’ve found!) and am very happy.
What are you going to do with this motor that loss of low speed torque concerns you?

Here are some good explanations

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Nubsnstubs

1536 posts in 2094 days


#7 posted 04-11-2019 02:07 PM

NC, from looking at you lathe in the Chuck adapter thread, you have a step pulley on the spindle. Leave it there, and if the current motor pulley fits 3 phase motor you have use it. installing a VFD while using the same pulleys, you should have all the torque you need at the lowest speed while still getting the cooling the motor requires. when you want higher speed on the lowest speed pulley setting, just crank it up and you’ve got the 800 rpm that it will allow. More speed, change the belt to the next setting, start up and ramp up to a comfy speed. Turn your piece and change to the other belt speed if you like. Test all belt speeds by starting at zero and slowly or quickly ramp up to what you think is safe for you.

I told you in another post that I had that same lathe. After getting it in 2009, I quickly converted it to a 2 hp 3 phase motor. I made one J8 pulleys with 2” and 4” steps for the spindle. The motor delivered 3450 RPM, so I made a 2” pulley for it. The whole time I used that lathe, the belt was on the 4” spindle pulley.

That gave me a maximum of 1725 RPM. I never had a power nor heat issue with it. I believe if you have a good quality industrial motor, you won;’t have heat issues at low speed because you won’t be running it low for more than a few minutes before stopping to check what you’re doing. Then start up again, turn for a few minutes, then stop, and repeat. After you get your piece done , the motor or lathe won’t get used again until you have time to get out and turn some more. Heat build up is more than likely to occur if you turn it on at very low speed and walk away for several hours or longer. Then, there might be a problem.

I used a Hitachi 3 HP 10 amp VFD 10 amps. I’m currently using it on my Powermatic. I’m turning large very out of balanced pieces using under 100 RPM for at least an hour. As the piece is worked into balance, I ramp up to get the speed up. By the time the piece is balanced, I work it at 3000 RPM until it comes off the lathe as a completed piece.

My only complaint about that lathe was I couldn’t fully eliminate the whinny bearing noise, even after replacing them. There was no change, but is a solid lathe. .........Jerry (in Tucson) .

-- Jerry (in Tucson) www.woodturnerstools.com

View NCMau's profile

NCMau

45 posts in 115 days


#8 posted 04-11-2019 10:14 PM

Yes Jerry, I plan to leave the two existing cone pulleys in place, and like you said, it would gave me a better speed range. I was just a little concerned about the torque because few years ago I tried a DC motor with a controller for my drill press. That was a total failure and a waste of money, I got speed reduction but no torque. At the time I did not know about VFD.

Thanks everybody for the inputs

-- Maury, NC

View AndyJ1s's profile

AndyJ1s

36 posts in 119 days


#9 posted 04-14-2019 12:32 AM



You can use a sensorless vector drive to get full power (or nearly) at lower frequencies. You ll pay for it though, it s another class of VFD. I have 3 ABB drives (ACS150 series) purchased from wistex (best prices I ve found!) and am very happy.
What are you going to do with this motor that loss of low speed torque concerns you?

Here are some good explanations

- GrantA

Grant,

Thanks for the link, it is very informative.

As speed decreases, for power to be maintained, torque must increase.

The problem is, for a given motor winding, torque is determined by current. The windings on a standard induction motor are designed to handle the current for the amount of torque to deliver the rated HP at or above about half the rated speed (peak torque/current is at about half rated speed). But at lower speeds than that, it takes more torque to create the same HP, so it takes more current than the windings are designed for. So, the motor will overheat unless significant additional cooling is available. Most motors are cooled with a fan that runs at the shaft speed, so they actually get less cooling when they need it most, at low motor speeds.

Now, compare that with mechanical variable transmission (gears, belts, etc.): as the output speed is reduced, the available torque increases proportionally.

In wood-turning, low speeds are generally used for larger diameter work pieces, where a given cutter load creates a larger torque load, compared to smaller diameter work pieces. With a mechanical variable speed system, you have more torque available at the lower speeds, to match work load.

VFD lathes often have two-step, belt & pulley systems to increase available torque at the low end.

DVR motors and their drives typically deliver more torque at lower speeds than electronic variable speed drives on same HP induction motors, and generally do not need the two-step belt & pulleys for wood lathes.

But DVRs (or VFD/SVDs) are much easier to adjust the speed to your needs, and need less maintenance than Reeves drives. And Reeves drive speed cannot be adjusted unless running.

Andy

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