Adding VFD to Drill Press

[Jim Peck]

As the title says, I'm adding a VFD to my drill press. For various reasons (with which I won't bore you), I need to replace the pulleys. This leads to the question of what size pulleys I should get. And that leads to the question of how much should I expect the VFD to slow down the motor.

USAGE: Hobbyist use in my garage. 90% for wood, 10% metal. No boring--just standard drilling. I don't see having this press running more than 5-10 minutes at a time, so overheating the motor doesn't seem likely.

Ideally, I'd like speeds from ~150 to ~3,000 RPM. The motor's speed is 1750 RPM, so:

--If I use the same size pulley on the motor and the quill (assuming no idler pulley), then the max speed is 1750. If the VFD can slow the motor down to 10% of its native speed, I can go down to 175 RPM, which should be fine on the low end. But the max speed of 1750 is not ideal.

--If I use a motor pulley that is twice the size of the quill pulley (example, 6" motor and 3" quill), then the max speed is 3500. If the VFD can slow the motor down to 10%, then low speed will be 350 RPM. If the VFD can slow the motor to 5%, then I can go down to 175 RPM.

The second example makes more sense to me, but I don't know how much I should expect the VFD to slow the motor down. Is it safe for the motor to run at 5%? 10%?

Should I run a couple of idler pulleys and switch belts for high and low RPM use? (Would rather not do this for obvious reasons.)

I'll appreciate any thoughts!

[Bill Dufour]

My 20" drillpress I put the belts in the middle range and run the VFD from 30-80 HZ. I have not switched the pulleys in 15 years.
Bil lD

[Jon Nuckles]

I am not an expert, but in researching a vfd for my own drill press I read two things relevant to your question. First, you lose torque when slowing down your motor with a vfd. That may not be a concern if your motor is powerful or your torque needs are low. Second, slowing your motor will slow the fan that cools it, which could lead to overheating. May not be an issue with a drill press getting hobby use.

I hope someone with more knowledge will join the conversation and give better direction, but wanted to alert you to potential issues.

[Steven Cooper2]

Do it!

I put a vfd on my drill press (1 hp delta) and it is great. I usually run between 20% and maybe 150% of 1750. It does go slower, but not with any torque. I don't run the thing in a production environment, maybe a couple dozen holes at a time so I'm not worried about cooling. We could do some math here, but you should be fine if you stop every now and then.

I usually leave it around 1:1 but have stalled it running a 1.5" step bit into a stainless beer keg, but I do feel this was a ridiculous thing to do. 1750 is pretty fast for what I end up drilling, I usually run around 30 hz, so 875 rpm.

Consider that on a 12 speed drill press you may be able to get 10:1 turndown ratio and a VFD you should be able to do about the same with no belt changes, I'd park it just below the middle of the range, maybe 1200 rpm that way with 150% overspeed and 10% underspeed you get ~15:1 and generally modest torque.

Torque above max rpm decreases linearly at constant HP, but HP below max RPM decreases linearly at reasonably constant torque. ish. This statement would indicate that you would want to run your motor near a lower base rpm if it is rated to run at a higher speed, but maybe depending on your v/hz it is a bad idea.

In general, somewhere between 1:1 and 1:2

[Curt Harms]

I have a benchtop 5 speed radial drill press. Replacing the driven step pulley is not an option, it's splined and a metric size. I bought a 1/2 h.p. 3 phase motor and VFD. The motor has a pretty long shaft so I can move the 3" motor pulley up and down to align the motor pulley with the step pulley to some extent. I've found having the belt on the slowest speed has worked out well. I have the VFD set for 90 Hz max and find about 10 - 15 Hz the minimum for adequate torque. I need low RPM much more frequently - large forstner bits, fly cutter, that sort of thing - than I need very high speed. This is one place I find a link belt pulley useful. The motor mount doesn't have any tensioning capability so It's necessary to adjust belt length to pulley sizes. I'm happy with the setup so far.

[Roger Feeley]

I did this a while back. It wasn't that expensive and works pretty well. I was disappointed in the torque at the low end. Fortunately, I don't go that low much. Controlling the speed in the ranges I use most is very convenient.

It all started because the motor on my old Craftsman DP was making this buzzing sound. I thought that was a good opportunity to get a VFD and stop changing pulleys all the time. But the motor I found had a key. So I took my pulley to a machinist to have a keyway cut and he cracked the pulley. I coudn't find an exact replacement so I found a single pulley at Graingers.

One of those "in for a penny, in for a pound things".

I have a Jet lathe with a VFD drive and it works great at all speeds. I wish someone would come out with a DP with that same motor configuration.

[Jim Becker]

If I'm not mistaken, to use the VFD, you'll also need to replace the motor with a 3-phase motor. You can likely find one used, but finding on "that small" used might be a larger challenge.

[Dan Friedrichs]

Jim, here's my thread on doing this, which has a lot of details you might find interesting:
https://sawmillcreek.org/showthread.php?250653-How-I-put-a-VFD-on-a-drill-press-for-no-more-belt-changes (https://sawmillcreek.org/showthread.php?250653-How-I-put-a-VFD-on-a-drill-press-for-no-more-belt-changes&highlight=)

A few notes:
1) Are you sure you can easily replace the pulley on the quill? I may have slots for splines that make it hard to source a cheap replacement.
2) If you get a VFD with space vector control, you mitigate some of the low-speed torque loss.
3) I left the quill pulley on mine, but have never once changed the belt to a different range now that I have the VFD. Realistically, the difference between 200 and 300 RPM (or 2k and 3k) is not impactful enough to bother dealing with it.

[Jim Peck]

Jim, here's my thread on doing this, which has a lot of details you might find interesting:
https://sawmillcreek.org/showthread.php?250653-How-I-put-a-VFD-on-a-drill-press-for-no-more-belt-changes (https://sawmillcreek.org/showthread.php?250653-How-I-put-a-VFD-on-a-drill-press-for-no-more-belt-changes&highlight=)

A few notes:
1) Are you sure you can easily replace the pulley on the quill? I may have slots for splines that make it hard to source a cheap replacement.
2) If you get a VFD with space vector control, you mitigate some of the low-speed torque loss.
3) I left the quill pulley on mine, but have never once changed the belt to a different range now that I have the VFD. Realistically, the difference between 200 and 300 RPM (or 2k and 3k) is not impactful enough to bother dealing with it.

Dan--Yes, I did see your write-up. It was very helpful and one of the things that made me think this is a viable course of action. Thankfully, no splines on the quill. I'm more concerned with the quill diameter, which appears to be 63/64ths of an inch--or right about 25mm if I measure in metric.

I've been able to source a cheap VFD ($20!, which should at least get me through proof-of-concept even if I have to upgrade it later; not sure if it has space vector control) and 3-phase motor. The real conundrum has been finding satisfactory pulleys!

Thanks,

Jim

[David Buchhauser]

If I'm not mistaken, to use the VFD, you'll also need to replace the motor with a 3-phase motor. You can likely find one used, but finding on "that small" used might be a larger challenge.

Jim,
It would appear that some VFDs will work to run a single phase motor with variable speed (if I am understanding this video correctly).
David

https://www.youtube.com/watch?v=_qa0OksIcR4

417746

[Lon Crosby]

A switched reluctance motor will provide full torque at stall. That is probable what drives Nova's DP & lathe.

[David Buchhauser]

My question is: are any of you guys using a single phase 120Vac motor for your VFD conversion or have you switched out your old motor for a new 3 phase motor?
David

[Larry Edgerton]

I put one on my Clausing so I did not have to start the converter every time I want to drill a hole. Works good. With the two speed motor and the Reeves drive I can get down to 5 rpm, not that I need to but making plugs for Azek I get it down to 30. If one puts one on a two speed you have to shut it down before you switch back and forth.

[Eric Danstrom]

My question is: are any of you guys using a single phase 120Vac motor for your VFD conversion or have you switched out your old motor for a new 3 phase motor?
David

Requires a 3 phase motor. If you're searching on the 'bay I've found the search term "3/4 hp 3 phase motor" works very well. Right now there's several items under $100 shipped. Three phase motors don't have a centrifugal switch or capacitor and are easily rebuilt.

[Jared Sankovich]

They make single phase capable vfds but the specific requirements rule out most single phase motors

https://www.wolfautomation.com/blog/vfds-for-single-phase-motors/

[Mike Henderson]

They make single phase capable vfds but the specific requirements rule out most single phase motors

https://www.wolfautomation.com/blog/vfds-for-single-phase-motors/

You can use a standard single phase induction motor with a single phase VFD if you're willing to do a few special things.

Let me discuss this a bit. The problem with a VFD on a single phase induction motor is the starting circuit, normally a capacitor and coil. I'm going to specifically address a capacitor start induction motor.

When a single phase capacitor start induction motor starts, the start capacitor is in series with the start coil. This generates a second phase which creates a torque on the rotor, causing the motor to begin rotating. When the motor reaches about 70% of full speed, the centrifugal switch opens and removes the starting circuit. The problem with using a VFD on a single phase motor has to do with this starting circuit and whether it's in the circuit or not.

You want the starting circuit to be in the system when you're first starting up the motor, but to then remove it in such a way that even if you reduce the speed below 70% of the full speed RPM it does not come back into the circuit.

Here's what you can do.

1. Get a momentary close single pole, single throw (SPST) push button switch.

2. Get access to your starting capacitor. It's usually on the outside of the motor so it's easy to get to.

3. Cut one wire going to the starting capacitor and, using some wire, hook in your SPST push button switch. Put the switch next to the VFD.

4. Hook the power wires of your motor to the VFD.

When you go to start your motor, set the VFD to close to the line hertz (usually 60 hertz). Hold your SPST switch down and turn the motor on through your VFD. This will start the motor normally, just as it it was connected to wall power.

Once the motor is up to speed, let go of the SPST switch. The starting circuit is now completely disconnected from the power. Go ahead and reduce the speed of your motor. When you reach 70% of the rated RPM the centrifugal switch will close but that will not affect anything because the starting circuit is disconnected by the SPST switch.

Us the motor and VFD normally, just as you would with a 3 phase VFD.

Induction motors on a VFD are generally constant torque systems when you're at an RPM less than the rated RPM. But since horsepower is a factor of torque and RPM, the motor will not be able to do as much work at lower RPMs because the HP will be lower.

Also, most general purpose induction motors are cooled by an internal fan. At lower speeds the fan does not move as much air so you have to be concerned about heat (true for both single phase and 3 phase motors) so monitor the heat and put an external fan pointing at the motor if it starts getting hot.

Mike

[David Buchhauser]

You can use a standard single phase induction motor with a single phase VFD if you're willing to do a few special things.

Let me discuss this a bit. The problem with a VFD on a single phase induction motor is the starting circuit, normally a capacitor and coil. I'm going to specifically address a capacitor start induction motor.

When a single phase capacitor start induction motor starts, the start capacitor is in series with the start coil. This generates a second phase which creates a torque on the rotor, causing the motor to begin rotating. When the motor reaches about 70% of full speed, the centrifugal switch opens and removes the starting circuit. The problem with using a VFD on a single phase motor has to do with this starting circuit and whether it's in the circuit or not.

You want the starting circuit to be in the system when you're first starting up the motor, but to then remove it in such a way that even if you reduce the speed below 70% of the full speed RPM it does not come back into the circuit.

Here's what you can do.

1. Get a momentary close single pole, single throw (SPST) push button switch.

2. Get access to your starting capacitor. It's usually on the outside of the motor so it's easy to get to.

3. Cut one wire going to the starting capacitor and, using some wire, hook in your SPST push button switch. Put the switch next to the VFD.

4. Hook the power wires of your motor to the VFD.

When you go to start your motor, set the VFD to close to the line hertz (usually 60 hertz). Hold your SPST switch down and turn the motor on through your VFD. This will start the motor normally, just as it it was connected to wall power.

Once the motor is up to speed, let go of the SPST switch. The starting circuit is now completely disconnected from the power. Go ahead and reduce the speed of your motor. When you reach 70% of the rated RPM the centrifugal switch will close but that will not affect anything because the starting circuit is disconnected by the SPST switch.

Us the motor and VFD normally, just as you would with a 3 phase VFD.

Induction motors on a VFD are generally constant torque systems when you're at an RPM less than the rated RPM. But since horsepower is a factor of torque and RPM, the motor will not be able to do as much work at lower RPMs because the HP will be lower.

Also, most general purpose induction motors are cooled by an internal fan. At lower speeds the fan does not move as much air so you have to be concerned about heat (true for both single phase and 3 phase motors) so monitor the heat and put an external fan pointing at the motor if it starts getting hot.

Mike

Thanks for that explanation Mike! That's good to know for future reference.
David

[Jim Peck]

One other pulley question for anyone still paying attention: The shaft on my motor is 2.25". I'm looking at a step (pyramid) pulley that is 3" high. So, .75" would overhang the end of the shaft. Important: I don't see any situation in which the 2 outer sheaves (that is, those beyond the end of the shaft) would be used.

That said, do you think it still would be safe to use this pulley? I have my doubts (obviously, since I'm asking the question) but maybe they are misplaced; I'm not an engineer, nor do I play one on television.

Thanks,

Jim

[David Buchhauser]

Hi Jim,
I don't see any problem using it with the 3/4" overhang, particularly since you won't be using the outboard groove. Even if you did, you should still be fine.
David