[How-To] Conversion to VFD

[Ben17484]

I have a smart and brown SAB lathe (south bend 9 clone) and I’d like to give it variable speed. I see that a VFD is the way to do this. My question is - do I still need the pulley assembly hooked up that would normally change the gearing for speed? I would assume not, but I do see people leave it.

Picture for reference

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[Just for fun]

You will need a 3 phase motor along with the VFD for it to work. Maybe you are already aware of that.

Your pulleys will be just fine, I would think. Pick a set that covers the range you want and then with the speed control of the VFD you can go up and down from there.

 

[theresamartinez]

the pulley seems to be well placed

 

[rabler]

Motors come in certain RPMs based on their internal construction,
roughly 1200, 1800, or 3600 rpm.
Start by replacing your existing motor with a 3 phase motor of the same HP or slighly more HP than the current motor, and of the same RPM. Note that 3 phase motors maybe rated at slighly less, such as 1750 RPM, that's close enough. Then add a VFD of the size for that motor. Use the same pulleys.

VFDs can run a motor from roughly one third to twice of their rated RPM. If you get an expensive motor you can get a broader range but there are trade-offs in torque/power at the spindle. Because of that it is usually best to keep the same pulley setup as you get the speed change options of the VFD plus you can use the pulleys to select the best "range".

 

[tq60]

Vfd allows for varying speed.

Pulleys allow for torque.

You need more torque at the slow speed so leave pulley system intact.

Run it in a middle range and use vfd for wide range "general" stuff.

When you need real slow r mare power shift belts to do so.

Our 14.5 SB has 2 hp 3 phase with original everything.

Added basic AB 1300 series VFD.

can go fairly slow to fairly fast in Middle range but will go from RPM to MPR if we go to slowest pulley and back gear.

And serious torque in that mode.

Leaving in higher belted speed but slow vfd speed. Greatly reduces torque, this can be useful as safety clutch as well.

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[Ben17484]

Thanks everyone for your responses. I did know that I needed a new phase motor for a VFD. Is that the only way to get variable speed? Is there a way to achieve the same thing with the original single phase motor? Do people generally use some sort of spindle tach to figure out the actual speed of the chuck when using a VFD with the pulley gears?


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[DavidR8]

Thanks everyone for your responses. I did know that I needed a new phase motor for a VFD. Is that the only way to get variable speed? Is there a way to achieve the same thing with the original single phase motor? Do people generally use some sort of spindle tach to figure out the actual speed of the chuck when using a VFD with the pulley gears?


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I used an optical tachometer to determine the speeds of all the various pulley and back gear combinations on my South Bend 10K.

https://www.amazon.ca/gp/product/B004Q8L894/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

It has a horizontal drive with two sheaves on the motor pulley, and three pulley diameters on the spindle plus back gears so has twelve possible speeds.

 

[matthewsx]

Thanks everyone for your responses. I did know that I needed a new phase motor for a VFD. Is that the only way to get variable speed? Is there a way to achieve the same thing with the original single phase motor? Do people generally use some sort of spindle tach to figure out the actual speed of the chuck when using a VFD with the pulley gears?


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For variable speed you'll need either a DC motor with controller (lots of folks salvage these from old treadmills), or a 3 phase motor with a VFD. There are advantages for each which I won't go into here but if you're looking to learn check out @mksj on here who probably knows about as much about this stuff as anybody.

With an older machine that uses a screw on chuck some of the stuff like braking won't apply but I can definitely say that this approach is one of the best things you can do for an older lathe (I have 3ph w/VFD on both mine).

Cheers,

John

 

[Ben17484]

For variable speed you'll need either a DC motor with controller (lots of folks salvage these from old treadmills), or a 3 phase motor with a VFD. There are advantages for each which I won't go into here but if you're looking to learn check out @mksj on here who probably knows about as much about this stuff as anybody.

With an older machine that uses a screw on chuck some of the stuff like braking won't apply but I can definitely say that this approach is one of the best things you can do for an older lathe (I have 3ph w/VFD on both mine).

Cheers,

John

Thanks for this John. Why won’t braking work for a screw on chuck? Too much chance of it undoing under the pressure of suddenly stopping?


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[Ben17484]

Motors come in certain RPMs based on their internal construction,
roughly 1200, 1800, or 3600 rpm.
Start by replacing your existing motor with a 3 phase motor of the same HP or slighly more HP than the current motor, and of the same RPM. Note that 3 phase motors maybe rated at slighly less, such as 1750 RPM, that's close enough. Then add a VFD of the size for that motor. Use the same pulleys.

VFDs can run a motor from roughly one third to twice of their rated RPM. If you get an expensive motor you can get a broader range but there are trade-offs in torque/power at the spindle. Because of that it is usually best to keep the same pulley setup as you get the speed change options of the VFD plus you can use the pulleys to select the best "range".

Thanks Rabler. Is the same speed necessary or would a faster speed be ok too? I know my lathewasn’t designed to take faster speeds, so I wouldn’t run it that fast, but if a faster motor at a good price comes up for sale, there’s no reason not to buy it and run it slower, right?


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