# Lathe VFD Motor selection



## lucsimoneau (Dec 10, 2022)

Hi all - looking for some insight for my manual lathe.
It's a CT-1440B King Canada Back Gear lathe and I'm thinking about installing a VFD so Ì don't have to fiddle around with belt changes and back gears to get spindle speeds.

Current motor is a single phase 1-1/2HP 220V motor running at 1720 RPM with a 3/4" keyed shaft.





The motor I have on hand is a Weg 2HP 3Phase 220V running at 3450 RPM. Shaft size is 5/8".






The question I have is the following: with double the motor speed, will this cause any problems in obtaining low speeds for threading and other slow speed type applications? I will not get rid of the back gears and pulleys so they could be used if needed.

Has anyone else gone this route to change their main motor?

Thanks!


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## TorontoBuilder (Dec 10, 2022)

Personally I'd not use that motor... it would be great for a 2x72 belt grinder, but the rpm is too high if you want to be able to get low speeds on your lathe...These motors are a dime a dozen, get one with 1745 rpm as it will be much better suited and be less prone to overheating at 50hz or less


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## phaxtris (Dec 10, 2022)

Used 3hp 3ph 17xx rpm motors are pretty cheap, that's the way I would go, save that motor for a grinder like @TorontoBuilder suggested


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## TorontoBuilder (Dec 10, 2022)

OH and would you not want to add some oomph while you are at it?  I'd have thought 3 ph better suited to 1440 lathe


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## TorontoBuilder (Dec 10, 2022)

phaxtris said:


> Used 3hp 3ph 17xx rpm motors are pretty cheap, that's the way I would go, save that motor for a grinder like @TorontoBuilder suggested


You just beat me with the 3 hp.

At first I missed the model being 1440.


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## phaxtris (Dec 10, 2022)

TorontoBuilder said:


> You just beat me with the 3 hp.
> 
> At first I missed the model being 1440.



Ha yea, all good, 3hp just seems to be a real easy size motor to get for dirt cheap around here, I believe they use that size for a lot of HVAC roof fans, just happens to be a good size for a 1440!


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## PeterT (Dec 10, 2022)

Watching with interest. I have a 1440 lathe too with 1.5HP, 220v single phase. Not that I'm planning motor conversion anytime soon, but.....
I'm not sure if @John Conroy documented his VFD/3p motor swap on his 14" Modern lathe, but would be great reference. Beyond the motor/VFD selection, where I get conversion apprehension syndrome is all the other obligatory wiring involved with a typical lathe: the E-stop button, the jog button, the brake (e or mechanical if it has), the forward/off/reverse (carriage handle control) switches all somehow feeding control inputs to VFD. Obviously it can be done, I've seen finished projects that make me green with envy. But electrons are not my forte so personally I would need clear, dummy proof instructions.


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## TorontoBuilder (Dec 10, 2022)

PeterT said:


> Watching with interest. I have a 1440 lathe too with 1.5HP, 220v single phase. Not that I'm planning motor conversion anytime soon, but.....
> I'm not sure if @John Conroy documented his VFD/3p motor swap on his 14" Modern lathe, but would be great reference. Beyond the motor/VFD selection, where I get conversion apprehension syndrome is all the other obligatory wiring involved with a typical lathe: the E-stop button, the jog button, the brake (e or mechanical if it has), the forward/off/reverse (carriage handle control) switches all somehow feeding control inputs to VFD. Obviously it can be done, I've seen finished projects that make me green with envy. But electrons are not my forte so personally I would need clear, dummy proof instructions.


I did my belt drive 12x37 lathe within the past year... gutted the inferior busybee controls and started from scratch. I should have documented the process better and blogged it but I did not.

I'll likely be planning the same for a new 1440 that this tme I'll do complete documentation of. But that said I thought @John Conroy's was one of the conversions I relied on for guidance.... at least I seem to recall some videos I thought were his.


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## PeterT (Dec 10, 2022)

I thinkClough42 did one on his channel, but different/smaller lathe from a control inputs standpoint?
Hobby Machinist forum has some bigger/14" type conversion projects including bedway positionable e-stop switches & some braking resistor stuff. I had a list of links but seem to be MIA.


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## lucsimoneau (Dec 10, 2022)

Thanks for all the input - greatly appreciated. I was looking at a budget of approx $500 since I already had a 2HP 3PH motor. Looking a bit on the web and a 3HP 3PH motor goes for around my total budget. For @PeterT, if I do get to install the VFD I'll post a detailed description of the route taken.  This is my King Canada CT-1440B lathe before installing a Mitutoyo DRO and a backsplash: 




I would probably used the existing panel cover to place the relative switches, mainly the speed control, forward/reverse keeping the E-Stop. Don't have coolant on this machine so the position could be used for the potentiometer.






Was watching the Clough42 conversion of his Grizzly lathe and it seems very simple to do.
Regards,


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## PeterT (Dec 10, 2022)

lucsimoneau said:


> For @PeterT, if I do get to install the VFD I'll post a detailed description of the route taken.  This is my King Canada CT-1440B lathe before installing a Mitutoyo DRO and a backsplash:
> I would probably used the existing panel cover to place the relative switches, mainly the speed control, forward/reverse keeping the E-Stop. Don't have coolant on this machine so the position could be used for the potentiometer.


Yours looks identical to mine. You should get started.... Like tomorrow! LOL

My manual says 70-1350 (spindle) rpm. I'm not sure what the stock single phase motor nominal rpm is, I should check ID plate. I never really contemplated what would be required from a sheave standpoint if VFD motor was significantly different. Maybe that's what limits, or at least factors into 3p motor selection to begin with. I seem to remember one guy selected a 'higher' RPM 3p motor but I can't recall details like by how much - was it a little or a lot. Maybe he just somehow maximized his RPM output level through the VFD to stay within spindle bearing & chuck max limits? Higher speed might be useful in some cases but maybe pushing things in other respects.


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## TorontoBuilder (Dec 10, 2022)

PeterT said:


> I thinkClough42 did one on his channel, but different/smaller lathe from a control inputs standpoint?
> Hobby Machinist forum has some bigger/14" type conversion projects including bedway positionable e-stop switches & some braking resistor stuff. I had a list of links but seem to be MIA.


Yes James was another resource I relied upon, but his was relatively simple compared to 1440 with more controls that must be kept


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## TorontoBuilder (Dec 10, 2022)

lucsimoneau said:


> Thanks for all the input - greatly appreciated. I was looking at a budget of approx $500 since I already had a 2HP 3PH motor. Looking a bit on the web and a 3HP 3PH motor goes for around my total budget. For @PeterT, if I do get to install the VFD I'll post a detailed description of the route taken.  This is my King Canada CT-1440B lathe before installing a Mitutoyo DRO and a backsplash: View attachment 28751
> 
> I would probably used the existing panel cover to place the relative switches, mainly the speed control, forward/reverse keeping the E-Stop. Don't have coolant on this machine so the position could be used for the potentiometer.
> 
> ...


A goof VFD will eat up that budget. 

You really should use a "better" VFD that allows for external braking resister for this class of lathe. That will blow your budget. Unless you don't turn anything heavy at all and dont use a larger motor than 2 HP.


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## cjmac (Dec 10, 2022)

TorontoBuilder said:


> A goof VFD will eat up that budget.
> 
> You really should use a "better" VFD that allows for external braking resister for this class of lathe. That will blow your budget. Unless you don't turn anything heavy at all and dont use a larger motor than 2 HP.


I went through this process about 15 years ago. My 12 x 36 lathe had a 1.5 hp motor. My motor supplier recommended using a 1 hp 1200rpm 3 phase motor. This means 6 poles instead of 4. He said the better low speed torque from the 6 poles would make up for the lower horsepower. It works great. I don't think I have used the gears since I installed this. 

Chris


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## PeterT (Dec 10, 2022)

The smoothness of 3p motor vs 1p & elimination of 1p start/run capacitors & centrifugal switch are other advantages. I want to dive into this VFD stuff one day, but not really on on my main grocery getter where the downtime would drive me nuts. At least at my amateur level. Smells like one of those 'weekend' jobs that runs a year LOL


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## TorontoBuilder (Dec 10, 2022)

PeterT said:


> The smoothness of 3p motor vs 1p & elimination of 1p start/run capacitors & centrifugal switch are other advantages. I want to dive into this VFD stuff one day, but not really on on my main grocery getter where the downtime would drive me nuts. At least at my amateur level. Smells like one of those 'weekend' jobs that runs a year LOL


One thing I learned during the process... you need a clear plan and to acquire everything you need before starting. Dont touch anything on the lathe til then if you want to minimize down time.

I made a layout for my electrical controls cabinet in fusion to plan my layout... only mistake was it was kind of tight enclosure that made working inside it tricky rather than easy. I'd have gone with larger one in hindsight


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## PeterT (Dec 10, 2022)

TorontoBuilder said:


> Yes James was another resource I relied upon


Not to tangent this post but James (Clough42) just posted 2 videos on rotary phase converter for his 3p mill. Not so much the RPC itself which was interesting, but more the broader electrical picture - what's coming in from the wall & what needs to go out to motor & where common misconceptions occur. I'm still clueless on this stuff but I gained some incremental understanding.


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## Dabbler (Dec 10, 2022)

A couple of points to consider in your lathe conversions:  it is a good practice to use  a motor rated at about 1 1/2 times you single phase motor if you use a VFD.  You end with better starting torque.

When I use a GE TECO 510, I match the HP rating and the VFD rating, as they have extra electrical and thermal headroom built in to their design.  When I use a very inexpensive VFD, (offshore, etc) I run a 1HP motor on a 2HP VFD, to get some thermal buffer.  On a 3HP motor, I'd use a 4HP VFD if it were one of the cheap ones.

Putting a VFD on a lathe requires considerable wiring changes - doing it on a mill is a couple of hours at worst.  It's like night and day.


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## YotaBota (Dec 10, 2022)

A search for VFD should get you 4+ threads on lathe installs and a couple on mill installs.

I agree with not using the high RPM motor.

eMotorsDirect.ca is a good source for VFD's and motors. The TECO brand VFD is one used often, it's not cheap or inexpensive but has all the options and support.

I think I'd look for an inexpensive used motor but spend the money on a good VFD, or save up a few more $ and buy both new.

I'm using a 2hp TECO VFD with the factory 1.5hp 3ph motor on my lathe and love it.


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## YYCHM (Dec 10, 2022)

Single phase to 3 phase conversion on Modern GH1440W lathe
					

I've been thinking about doing this since I got the new lathe a little over a year ago but a few things have happened lately to push me toward starting this project. First was the capacitor failure I experienced on the single phase motor that came with the lathe. Second, my wife and I got back...




					canadianhobbymetalworkers.com


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## KeeponDragon (Dec 10, 2022)

Definitely watching this one...I'm curious what, if any issues I'll have converting my 9x20 over to VFD...


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## Rauce (Dec 10, 2022)

Dabbler said:


> A couple of points to consider in your lathe conversions:  it is a good practice to use  a motor rated at about 1 1/2 times you single phase motor if you use a VFD.  You end with better starting torque.
> 
> When I use a GE TECO 510, I match the HP rating and the VFD rating, as they have extra electrical and thermal headroom built in to their design.  When I use a very inexpensive VFD, (offshore, etc) I run a 1HP motor on a 2HP VFD, to get some thermal buffer.  On a 3HP motor, I'd use a 4HP VFD if it were one of the cheap ones.
> 
> Putting a VFD on a lathe requires considerable wiring changes - doing it on a mill is a couple of hours at worst.  It's like night and day.



This is good advice. I did a 2hp motor with a 2hp import VFD on my last lathe and it would shut off on start up until I adjusted the ramp and would also shut off on heavy cuts sometimes. I lived with it but won’t be making that mistake in the future.

I think using a higher RPM motor isn’t really an issue here. The point of the switch is to eliminate changing belt positions right? So I would install the belt in a position that gives you a speed in the neighbourhood of 1000rpm at the spindle with the higher speed motor running at 60hz (between 4:1 or 3:1 reduction).

That way you can go up to 80-90hz to get your 1400rpm top speed and down to 20-30hz to get down to 300-400rpm. For anything slower than that I would still use the back gear since you’ll get a lot more torque that way.

A name brand VFD will have a good manual with diagrams for controls. You can probably reuse some of what’s there and find a spot to put a potentiometer for frequency control.

A built in tachometer for rpm is nice but you can also do what I have done before and use a handheld tach to plot frequency and rpm and print a little chart to stick on the wall next to your lathe.


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## Dabbler (Dec 10, 2022)

Most VFDs have a conversion factor parameter to display a close approximation of RPM instead of Hz.  This is very good if you don't change the belts/gears.


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## YotaBota (Dec 10, 2022)

These work well.





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					www.amazon.ca


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## TorontoBuilder (Dec 10, 2022)

YotaBota said:


> These work well.
> 
> 
> 
> ...


That's what I added to my old lathe...
















I mounted the magnet to a disc, and the disc to the spindle with a taper loc fitting... and the sensor to the headstock... not shown mounted. Sad omission that


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## curmudgeon (Dec 10, 2022)

TorontoBuilder said:


> I mounted the magnet to a disc, and the disc to the spindle with a taper loc fitting...


Nice!

What doesn't work well is hot gluing the magnet to a treadmill motor flywheel while bench testing maximum speed.  The replacement magnet is now epoxied to the aluminium step pulley on the spindle of my wood lathe.


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## YotaBota (Dec 10, 2022)

I made a collar to fit the tail end of the spindle, milled a flat spot and JB Welded the magnet into place.


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## Rauce (Dec 10, 2022)

Dabbler said:


> Most VFDs have a conversion factor parameter to display a close approximation of RPM instead of Hz.  This is very good if you don't change the belts/gears.


I’ve seen that on a friends unit but I don’t think any of mine have that feature? At least I don’t recall seeing it in the manuals. I have an older Teco on my mill and the lathe will have a GE/Fuji.

Not a big deal for me, my mill has a vari-speed head so I leave it at 60hz and it really just serves to provide 3ph. My lathe once it’s done will have the VFD in the cabinet out of sight so I’ll be adding a seperate tachometer, or maybe a DRO with rpm option.


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## Dabbler (Dec 10, 2022)

Rauce said:


> I have an older Teco on my mill



You can get the manual for your TECO online, for almost any model.  Barring that, there's a 1-800 number that the guys at the end of the line are *really* helpful...


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## Susquatch (Dec 11, 2022)

Looks like there are a lot of like minded members here. @TorontoBuilder , @Rauce , @YotaBota , @curmudgeon , @KeeponDragon , @cjmac , @phaxtris , @lucsimoneau ,

@Dabbler and I have chatted about just such a conversion on my 1440 lathe too.

I already have the 3Hp TECO L510 VFD for this job. I've been shopping for a 3HP VFD Rated 3phase motor for quite some time now to replace the 2HP single phase motor that is there. I also plan to install a DRO.

My spindle sticks out of the rear of my lathe head by about 2-3/4 inches.






So I am planning to make a double sleeve collar of aluminium or plastic to sandwich two or four or more magnets between them to get an RPM Readout. Mine will be similar to what @YotaBota described above. It will be an Arduino based system initially, but my future DRO will definitely include an RPM function that will use the same sensor.  By slipping the collars over the rear spindle all the way to the head, they won't interfere with my rear spider and that makes a solid sensor mount on the gear box a no-brainer (maybe pickup the two screws for the cover bolt shown in the photo above). The ABS Pipe in the photo would work. Sandwiching the magnets between two sleeves eliminates the worry of magnets flying around in my shop. If I knew how to drill a sleeve from the inside out I'd only need one sleeve.

The reason for multiple magnets is to speed up the data acquisition, improve low speed resolution (where it is needed most), and provide better balance in the spindle. Although I doubt balance is really an issue, it's free with multiple magnets so why not.

I'm not really sure I really need a VFD Rated motor since I'm mostly interested in obtaining slower speeds. My lowest speed is 70rpm now and I'd like to hit 40 - but 50 would do. A VFD Rated motor would take me to 10 or even lower.

Although my VFD does have the RPM function that @Dabbler described, I won't be using it because I still want to be able to use my gearing. I'd prefer a direct rpm reading from the spindle.

Obviously, this project is only on my to-do list for now. It isn't really more than a plan collecting ideas right now. I'll post more fully in a new thread when I actually do the conversion. I'm just throwing my planning thoughts out there for the benefit of those who are doing it now, or who are planning a future conversion too, or who have better ideas to contribute to my own plan.


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## Proxule (Dec 11, 2022)

YotaBota said:


> eMotorsDirect.ca is a good source for VFD's and motors. The TECO brand VFD is one used often, it's not cheap or inexpensive but has all the options and support.


Thats where I got mine for 200 bucks, picked up local. Easy peasy


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## Rauce (Dec 11, 2022)

Susquatch said:


> I'm not really sure I really need a VFD Rated motor since I'm mostly interested in obtaining slower speeds. My lowest speed is 70rpm now and I'd like to hit 40 - but 50 would do. A VFD Rated motor would take me to 10 or even lower.



Modern general purpose motors are good for 10:1/20:1 which I think is more than adequate. Any lower than that and you’re losing significant HP anyways. At that ratio you would be down to 7rpm with constant torque. 

A true inverter duty motor on your lathe would be rated to go as low as 0.07rpm at the 70rpm setting and hold torque at 0rpm without overheating.


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## TorontoBuilder (Dec 11, 2022)

Susquatch said:


> Looks like there are a lot of like minded members here. @TorontoBuilder , @Rauce , @YotaBota , @curmudgeon , @KeeponDragon , @cjmac , @phaxtris , @lucsimoneau ,
> 
> @Dabbler and I have chatted about just such a conversion on my 1440 lathe too.
> 
> ...



Note: Anyone buying a cheap rpm meter from amazon will likely be limited to single magnet low resolution capability, because well the manufacturer is cheap.  Not a big deal, I tested my old tachometer against several very costly analog and digital tachometers at 100 rpm and so no greater than 3% variance. It only really matters for very low speeds where accuracy is critical. Not often the case on a lathe

Multi-magnets require a DRO with that option, or an arduino or other controller based custom job. My buddy is making us a few to beta test and then we will likely offer some to community members. Ours will have multiple encoder options and multiple magnet option and surface feet per minute calculation. One other item I requested to be built into the tachometer is a revolution counter for winding coils for electric motors and solenoids etc... this will require high accuracy at low rpm.

Yes shameless plug... mods feel free to delete that second paragraph if it offends


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## whydontu (Dec 11, 2022)

TorontoBuilder said:


> Note: Anyone buying a cheap rpm meter from amazon will likely be limited to single magnet low resolution capability, because well the manufacturer is cheap.  Not a big deal, I tested my old tachometer against several very costly analog and digital tachometers at 100 rpm and so no greater than 3% variance. It only really matters for very low speeds where accuracy is critical. Not often the case on a lathe
> 
> Multi-magnets require a DRO with that option, or an arduino or other controller based custom job. My buddy is making us a few to beta test and then we will likely offer some to community members. Ours will have multiple encoder options and multiple magnet option and surface feet per minute calculation. One other item I requested to be built into the tachometer is a revolution counter for winding coils for electric motors and solenoids etc... this will require high accuracy at low rpm.
> 
> Yes shameless plug... mods feel free to delete that second paragraph if it offends


I have Arduino code and basic design for a tachometer / SFM using a 20x4 LCD display. Its configured for a mill, but could be easily modified for a lathe.


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## Susquatch (Dec 11, 2022)

TorontoBuilder said:


> Multi-magnets require a DRO with that option, or an arduino or other controller based custom job.



The Ditron D80 DRO on my mill can be set to use multiple magnets. I plan to get a Ditron for my lathe too. I'd like to keep them the same for commonality. So no worries there. 

A while back I looked at the code that @whydontu provided. It shouldn't be a problem to add multi sensors to that. If it is a problem, I'll have fun working it out. I've already wired up a box to play with.

I'm not fond of the Amazon/Ali tachs so no worries there.


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## TorontoBuilder (Dec 11, 2022)

Susquatch said:


> The Ditron D80 DRO on my mill can be set to use multiple magnets. I plan to get a Ditron for my lathe too. I'd like to keep them the same for commonality. So no worries there.
> 
> A while back I looked at the code that @whydontu provided. It shouldn't be a problem to add multi sensors to that. If it is a problem, I'll have fun working it out. I've already wired up a box to play with.
> 
> I'm not fond of the Amazon/Ali tachs so no worries there.


We paid extra to get the more expensive DRO option for the lathe JUST to be sure that we didn't have to learn anything different between the two machines.


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## lucsimoneau (Dec 12, 2022)

A bit back to my original question - if the VFD set at 30Hz would put the RPM range of the 3450 RPM motor at 1725, that would mean that I'm back with the original motor's speed range with the belts and back gear. What would be the best position of the belt in this situation? Later on I could always switch out the motor for a slower RPM model pretty easily if this caused a problem. Thanks!


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## phaxtris (Dec 12, 2022)

lucsimoneau said:


> A bit back to my original question - if the VFD set at 30Hz would put the RPM range of the 3450 RPM motor at 1725, that would mean that I'm back with the original motor's speed range with the belts and back gear. What would be the best position of the belt in this situation? Later on I could always switch out the motor for a slower RPM model pretty easily if this caused a problem. Thanks!
> 
> View attachment 28839



That's going to depend on what spindle speed you most often use, I would also plan your belt selection with the motor running as close to 3450 a possible while still giving you the spindle speed your looking for

 I honestly don't think you are going to get away from having to move belts entirely, that motor is going to run out of torque at some point as you keep turning the Hz down, even dc motors run out of torque when you slow them down enough


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## Rauce (Dec 12, 2022)

lucsimoneau said:


> A bit back to my original question - if the VFD set at 30Hz would put the RPM range of the 3450 RPM motor at 1725, that would mean that I'm back with the original motor's speed range with the belts and back gear. What would be the best position of the belt in this situation? Later on I could always switch out the motor for a slower RPM model pretty easily if this caused a problem. Thanks!
> 
> View attachment 28839



If it was me I’d probably use “A2”. That would give you 800rpm spindle speed at 60hz. Set the upper frequency limit to the highest speed you want to run your spindle. 

If you set the lower limit to 20hz you’ll be down to 266rpm. Put it in back gear and you’re down to 61rpm. That sounds reasonable to me and at 20hz you will not notice a significant drop in torque, especially if your new 3ph motor has more HP than your old one. 

With that set up you’d get the full range you had before and never have to move a belt. Just back gear for low speed operations.


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## lucsimoneau (Dec 12, 2022)

@Rauce that seems to be my line of thought - can the motor be pushed above the 60Hz frequency to get higher speeds and if so, what would be a "safe" maximum setting? Greatly appreciate the comments


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## Rauce (Dec 12, 2022)

lucsimoneau said:


> @Rauce that seems to be my line of thought - can the motor be pushed above the 60Hz frequency to get higher speeds and if so, what would be a "safe" maximum setting? Greatly appreciate the comments


For sure, I’ve run mine up 120hz. Never needed to go higher than that, I’m not sure what a max for a general purpose motor is. Usually the specifications are more concerned with the low end. 

I’d be more concerned with the spindle. Say at 120hz you’d be at 1600rpm spindle speed and that chart shows a max of 1100rpm. What can the bearings take?


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## TorontoBuilder (Dec 12, 2022)

lucsimoneau said:


> @Rauce that seems to be my line of thought - can the motor be pushed above the 60Hz frequency to get higher speeds and if so, what would be a "safe" maximum setting? Greatly appreciate the comments


For all the work involved in making the chnage I can't see why you wouldn't just get the more appropriate motor, They're easy to come by.

But @Rauce is correct, you can do as he says and likely overclock the motor to 120hz without too much worry.


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## thestelster (Dec 12, 2022)

This is an excerpt from a recent article I read:
"Rules of thumb- a 3600 rpm motor can be run safely up to about 75 Hz (or 25% above its rated speed). An 1800 rpm motor can be run safely up to 120 Hz or 200% of its rated speed."
     "The real limiting factors for over-speeding a motor are not so much electrical as they are mechanical. Bearing wear and rotational instability resulting from changes in mechanical balancing requirements at speeds above the nominal can lead to motor failure and/or unacceptable vibrations when operating a motor continuously at higher than its rated top speed."


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## Rauce (Dec 12, 2022)

thestelster said:


> This is an excerpt from a recent article I read:
> "Rules of thumb- a 3600 rpm motor can be run safely up to about 75 Hz (or 25% above its rated speed). An 1800 rpm motor can be run safely up to 120 Hz or 200% of its rated speed."
> "The real limiting factors for over-speeding a motor are not so much electrical as they are mechanical. Bearing wear and rotational instability resulting from changes in mechanical balancing requirements at speeds above the nominal can lead to motor failure and/or unacceptable vibrations when operating a motor continuously at higher than its rated top speed."


That makes sense, I’ve done 120hz on 1800rpm motors. 

So I guess that’s another reason to go to with the slower motor and different belt setting. At 75hz you’d only hit 1000rpm.


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## TorontoBuilder (Dec 12, 2022)

thestelster said:


> This is an excerpt from a recent article I read:
> "Rules of thumb- a 3600 rpm motor can be run safely up to about 75 Hz (or 25% above its rated speed). An 1800 rpm motor can be run safely up to 120 Hz or 200% of its rated speed."
> "The real limiting factors for over-speeding a motor are not so much electrical as they are mechanical. Bearing wear and rotational instability resulting from changes in mechanical balancing requirements at speeds above the nominal can lead to motor failure and/or unacceptable vibrations when operating a motor continuously at higher than its rated top speed."



First let me preface this comment with the statement that I was the first person to recommend against a 3600 rpm motor for this application. Secondly I still think  it is a poor choice when an alternative can be had for under 100 bucks...

However, I'll add two comments. A hobby machinist's motor is going to be subject to far less wear than a commercial use lathe so assuming starting point is a newish motor it is likely that there willl be no issues with the proposed operation. Most of the bearings in this size motor are rated for much higher speeds than they are operating at. It is typical that the bearings in a decent motor are rated for 9600 rpm, and the lubricant can dissipate the heat from operating at such speeds.  I can't say about the loading issues because I dont know what braking the proposed vfd can provide... 

My point is this is safe enough to try as a stop gap interim solution, but it is far from best practice


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## Dabbler (Dec 12, 2022)

I agree with Ryan.  If you set your lathe to run at 800RPM at 60Hz, it really gives you all the range you need. 

Because the 3450 motor has fewer poles, you will see less torque than for a 1725 RPM motor.  Really I don't think that will limit you much unless you really need to take really deep cuts.  Starting and stopping will heat your motor a bit more than for a 1725 motor.  Again not much of a problem, but something to check if you start and stop a *lot*.


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## PeterT (Dec 12, 2022)

TorontoBuilder said:


> Most of the bearings in this size motor are rated for much higher speeds than they are operating at. It is typical that the bearings in a decent motor are rated for 9600 rpm, and the lubricant can dissipate the heat from operating at such speeds.


I think what @Rauce is pointing out is the spindle is rated/recommended 1100 rpm so would be the more limiting factor even if motor bearings capable of much more?
I suspect belt speed or gear train might similarly factor even if the spindle bearings could take it?
Max rpm of chucks decrease as a function of size although typical hobbyist diameter is still pretty high. But some people have found out new sources of vibration & even jaw grip reduction depending on chuck condition & the machine.


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