Replacing 1750 RPM motor with 3-phase 3450 RPM motor + VFD (Milling Machine)

Hello all- I have a bridgeport-style milling machine with head similar to J-Head, but without back gearing.

The current motor on the machine is 1750 RPM 120/240 single phase, and I want to go with a variable speed setup using VFD and 3-phase motor (have done a lot of reading on this site on that in general).

My question is- if I go with a 3450 2-pole 3-phase motor (there's one available locally) - what RPM can I expect it to run down to with reasonable torque? When using a VFD, does the rated RPM have the same effect/meaning compared to being used without a VFD? (meaning same frequency, etc - are the RPMs double that of a 3-phase 4-pole setup that's rated for 1750 RPM).

Any general thoughts would be helpful, this will be my first VFD setup!

torque is constant all the way down, if the vfd is set up right.

a 2 hp 3450rpm motor produces half the torque a 2hp 1750rpm motor does, so keep that in mind.

Thanks- that’s helpful. Will the 3450 rpm motor also turn twice as fast compared to 1750 rpm motor at same VFD settings?

gavingear said:

Thanks- that’s helpful. Will the 3450 rpm motor also turn twice as fast compared to 1750 rpm motor at same VFD settings?

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Yes, it will

I converted a mini mill to VFD a few years back. Ended up calling WEG for a ton of good advice and free help to match the VFD to the motor.
You did not note which VFD you were considering?
Or how old the 3 phase motor was.
The item I've seen on some other forums as I was reading up were along these lines:
The comments usually went along as noting to be careful with older motors. VFD control outside of the original OEM ratings may cause it to fail (again passing the usual comments I saw). Newer motors built for VFD control are built for the CV and CT type workloads.

I just happened to be checking something related to motors and VFD the other day. These are some of my finds still open.
http://drives.danfoss.us/workarea/downloadasset.aspx?id=17179912780
Setting up a Variable Frequency Drive | iKnow Variable Frequency Drives
http://ecatalog.weg.net/files/wegne...f-electric-motors-50039409-manual-english.pdf

Enjoy and hope your into chips soon!

Go with a 1750 RPM motor and over speed it, they can easily go to 100 or if need 120Hz. Better motors, such as inverter/vector motors are designed for a much wider operation performance window and much higher speeds (6000 RPM for 2 and 3 Hp motors). A standard 3450 RPM can only be over sped by maybe 20%, not recommended. You would loose too much torque using a 3450 RPM motor. I recently dealt with several mill installs with 2 speed constant Hp motors converted to VFDs, in every case they operated better on their 1750 RPM (4 pole) winding, and we set the maximum speed to 120Hz. Go with a better VFD run in sensroless vector mode, it will give you better operating stability across a wider speed range.

mksj said:

Go with a 1750 RPM motor and over speed it, they can easily go to 100 or if need 120Hz. Better motors, such as inverter/vector motors are designed for a much wider operation performance window and much higher speeds (6000 RPM for 2 and 3 Hp motors). A standard 3450 RPM can only be over sped by maybe 20%, not recommended. You would loose too much torque using a 3450 RPM motor. I recently dealt with several mill installs with 2 speed constant Hp motors converted to VFDs, in every case they operated better on their 1750 RPM (4 pole) winding, and we set the maximum speed to 120Hz. Go with a better VFD run in sensroless vector mode, it will give you better operating stability across a wider speed range.

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When you over speed a motor, you lose torque once you go over 60Hz. So running a 1750RPM motor at 120Hz will have almost no torque left.

I can also choose a pulley combination to optimize speed/torque for the 2-pole 3450 RPM motor- put it in a lower range setup. That may work...

Jraef said:

When you over speed a motor, you lose torque once you go over 60Hz. So running a 1750RPM motor at 120Hz will have almost no torque left.

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"No torque" is bit overly pessimistic. You have about half of torque left.

gavingear said:

I can also choose a pulley combination to optimize speed/torque for the 2-pole 3450 RPM motor- put it in a lower range setup. That may work...

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If you want large torque/rpm range(to avoid pulley changes) with VFD select "oversized" motor and vfd and run the motor as fast as you dare and match the rpm range to your liking with pulley set.

5.5 hp 4-pole motor with matching VFD gives you enough torque at low end (30...300rpm) to drill 3/4" holes efficiently with 1:1 pulley ratio and at 120hz you get ~3500rpm top speed without fiddling with belt and pulleys.
Add external cooling fan for the motor if you intend to run fhe motor for long periods at low speed and high torque (ie drilling dozens of 3/4" holes)

Thanks Matt- appreciate the real-world figures and what will work for this kind of scenario!

Jraef said:

When you over speed a motor, you lose torque once you go over 60Hz. So running a 1750RPM motor at 120Hz will have almost no torque left.

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Not sure where this is coming from, at 120 Hz as you would have the same torque you would have as a 3450 RPM run at 60Hz. Torque is less of a factor at speed on a mill, as one would use smaller end mills. You also have other factors depending on the drive ratio. Yes it pays to go with a larger motor, but why don't you take note of the current factory VFDs for mills in this size range. I happen to own the Sharp with the factory digital drive, which uses a 1750 RPM 3 Hp motor run at 20-200 Hz. This is also how many factory installed VFD systems on lathes are designed in the 2-5Hp. I have yet to have stalled a lathe under load, in fact if designed properly they maintain +/- a few RPM no matter what the load, even at 2X the motor base speed. Based on the statement above, just about every factory VFD system on manual mills and lathes would be wrong. A 5Hp motor on the top of a Bridgeport head would be a bit too heavy, a 3 Hp is a very common motor size to be paired with a VFD for this size mill. If you have just a single speed, then yes you would need a much larger motor, but I Assume you have at least different belt drive ratios available.

Jraef said:

When you over speed a motor, you lose torque once you go over 60Hz. So running a 1750RPM motor at 120Hz will have almost no torque left.

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You will still get enough torque to do the job at high speeds. A 1/8" endmill doesn't need as much torque as a 3" shell mill. For a bridgeport on a VFD, I'd go with the 4 pole/1750rpm motor and use a 4-5hp instead of the 2hp it came with. That will give the capability of a sensible torque at low speeds and the ability to run up to 4000rpm at high speeds.

If doing that, I'd also change the drive to a poly-V belt to get the power and speed capabilities instead of the original step pulleys with an A section V belt.

no back gearing be better to have 2 steps I would think.

mksj said:

Not sure where this is coming from, at 120 Hz as you would have the same torque you would have as a 3450 RPM run at 60Hz.

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I am not sure I understand what you are saying here . . . or that you understand what you are saying?

A 60Hz rated (nameplate) motor regardless of number of poles will operate at 120Hz at half the rated torque (i.e. constant HP) just as each of your attached graphics show. Now if you are comparing a 4-pole at 120Hz to a 2-pole at 60Hz . . . fair enough, but your comment isn't clear in this regard.

I have retrofitted quite a number of spindles on knee mills and every single one we have with a step pulley is operated with a VFD. A 4-pole or even 6 pole motor is better than a 2-pole motor especially if no back gear exists. If you have a TENV motor design, all the better as you can run it all day long at 100% torque at 10 RPM and never worry about cooling issues.

We have a 10HP 4-pole Baldor TENV motor on our Tree milling machine spindle wiwth a nearly 1:1 timing belt ratio. We have a 20HP 460V drive connected to the motor 230V leads and we have the drive programmed for a 120Hz, 460V motor . . . we get full torque to 3600 RPM and constant HP to 8000+ RPM - it works great and we can rigid tap a 1" NC tap into A36 Steel at 300 RPM without blinking an eye. It has been working like this for going on 10 years without issue.

motion guru said:

If you have a TENV motor design, all the better as you can run it all day long at 100% torque at 10 RPM and never worry about cooling issues.

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Not really. TENV designs don't get a free pass, thermally, just a larger ration coupon, and physical size according.

Their case material, size, and arrangement (external cooling fins, for many cases, pun intended), are purpose-designed to hold a safe thermal equilibrium within due bounds, same as any other design.

What loading and RPM that means may differ dramatically, one TENV to another, but "infinite" it is never. ALL have a thermal failure point, somewhere.

You wants "indefinitely", you has to ALSO make the power and efficiency sacrifice of impedance-protected (add-on if not inbuilt) and "torque motor" WINDINGS, not just case design.

Lovely critters for many things, torque motors can be. I cudda damn near married some of Bodine Brothers finer ones, the monetary value of the problems they solved.

But sore lousy at making chips effectively. They prefer to act like infinite-length springs.

mksj said:

Not sure where this is coming from, at 120 Hz as you would have the same torque you would have as a 3450 RPM run at 60Hz.

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Based on the statement above, just about every factory VFD system on manual mills and lathes would be wrong. A 5Hp motor on the top of a Bridgeport head would be a bit too heavy, a 3 Hp is a very common motor size to be paired with a VFD for this size mill. If you have just a single speed, then yes you would need a much larger motor, but I Assume you have at least different belt drive ratios available.

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One downside of overspeeding induction motors that you lose a lot more than just a half in peak/short-term maximum torque. 4 pole motor at 120hz constant hp range doesn't have nowhere near the peak or short-term maximum torque compared to 2-pole motor at 60hz.

This means that you stall the motor lot more easily if(when) there is overload.



At around 2x nominal frequency (60hz for americanos) the peak torque is same as rated torque. And after that rated torque is starting to fall of even faster (1/v^2) (constant slip range in some literature)
"Knee point" or frequency where the power is starting to fall off depends on motor type but you can assume its somewhere 1.5x to 2.5x nominal frequency.
(AFAIK and YMMV larger motors (motors with large peak torque) have wider constant hp range)

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And I would assume that once one goes to trouble of buying motors and vfd's you don't want to change belt pulleys unless absolutely necessary
Commercial solutions need to be cost-effective but if you are sourcing your vfd and motor from 2. hand markets the pricing is often about the same for 2hp or 5hp unit.

MattiJ said:

One downside of overspeeding induction motors that you lose a lot more than just a half in peak/short-term maximum torque. 4 pole motor at 120hz constant hp range doesn't have nowhere near the peak or short-term maximum torque compared to 2-pole motor at 60hz.

This means that you stall the motor lot more easily if(when) there is overload.

At around 2x nominal frequency (60hz for americanos) the peak torque is same as rated torque. And after that rated torque is starting to fall of even faster (1/v^2) (constant slip range in some literature)
"Knee point" or frequency where the power is starting to fall off depends on motor type but you can assume its somewhere 1.5x to 2.5x nominal frequency.
(AFAIK and YMMV larger motors (motors with large peak torque) have wider constant hp range)

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And I would assume that once one goes to trouble of buying motors and vfd's you don't want to change belt pulleys unless absolutely necessary
Commercial solutions need to be cost-effective but if you are sourcing your vfd and motor from 2. hand markets the pricing is often about the same for 2hp or 5hp unit.

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Your curves represent a very low breakdown torque (BDT) motor of only 225 percent. This is more typical of a DIN motor design.

Not to stray too far from the OP’s question . . . US domestic supply standard NEMA B design motors generally have 300 - 350 percent BDT and are designed for 600 percent current inrush at locked rotor . . . as such, the BDT as it decays at the square of V/hz doesn’t adversely affect the motor operating torque at as low a frequency as your graphs illustrate. We review motor curves for every application we encounter and when extended speed operation is needed we select motors with appropriate levels of BDT.

Also, one would normally more apt to be torque limited by the available current from the VFD and work design parameters for the system to match the drive and motor peak torque and associated peak current requirements to the application.

With regard to BDT decay, the scenario I outlined above maintains constant flux density to 120 hz with no torque decay and then constant power to approximately ~275Hz before the BDT dropped to 3/2 of rated torque encountering the design limit where stall risk needs to be considered. This was using a standard inverter duty Baldor IDNM series industrial motor and Control-Techniques UniDrive SP. http://www.practicalmachinist.com/v...rofit-almost-done-156021/?highlight=Tree+j425

No second hand market in this case, we integrate several hundred drives and motors a year from very small 1kW units up to 2MW units and as a Siemens technology partner as well as certified integrator for Rockwell, ABB, and Nidec, we get excellent pricing. All of our machine tools, cranes, blowers, etc. benefit from having high performance drive technology installed. We are just wrapping up an install of an 840D sl on a 1942 Cleveland planer mill.

Monarchist said:

Not really. TENV designs don't get a free pass, thermally, just a larger ration coupon, and physical size according.

Their case material, size, and arrangement (external cooling fins, for many cases, pun intended), are purpose-designed to hold a safe thermal equilibrium within due bounds, same as any other design.

What loading and RPM that means may differ dramatically, one TENV to another, but "infinite" it is never. ALL have a thermal failure point, somewhere.

You wants "indefinitely", you has to ALSO make the power and efficiency sacrifice of impedance-protected (add-on if not inbuilt) and "torque motor" WINDINGS, not just case design.

Lovely critters for many things, torque motors can be. I cudda damn near married some of Bodine Brothers finer ones, the monetary value of the problems they solved.

But sore lousy at making chips effectively. They prefer to act like infinite-length springs.

Click to expand...

I respectfully disagree with the premise that “you can’t exploit a TENV design to operate at rated torque at very low rpm (or even zero speed) indefinitely without damage.”

I have done many load balance hoisting applications with TENV motors operating at 100 percent current at near zero speed 24 hours a day over which time the shafts turn only a few hundred revolutions in either direction. Motor cases are warm to the touch, but overheating has never been an issue, even when baking in the sun on a hot day. Applied RMS voltages at low speed operation are in the 10’s of Volts and IR losses and associated heat load are significantly less at low speed operation.

motion guru said:

Your curves represent a very low breakdown torque (BDT) motor of only 225 percent. This is more typical of a DIN motor design.

Not to stray too far from the OP’s question . . . US domestic supply standard NEMA B design motors generally have 300 - 350 percent BDT and are designed for 600 percent current inrush at locked rotor . . . as such, the BDT as it decays at the square of V/hz doesn’t adversely affect the motor operating torque at as low a frequency as your graphs illustrate. We review motor curves for every application we encounter and when extended speed operation is needed we select motors with appropriate levels of BDT.

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Curve was just to illustrate the idea. Better use the motor specific data IF it's available.

And AFAIK typical "european or IEC" motor also has 300-400% BDT and >600% Is just like NEMA B motors.
Last 3kW motor I was digging around had 410% BDT and 700% Is