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1ph-to-3ph Conversion: Is a higher power motor necessary? (And other questions)

TimNJ

Plastic
Joined
Aug 21, 2021
Hi all,

I'm in the midst of restoring an old Walker Turner 900 series drill press. Although the 600rpm minimum speed is not terrible, I think it's still about 2x too fast for some types of work I might get into. Secondly, I have a nagging fear that I will crack the pot metal spindle/motor step-pulleys during some future (unsuspecting) belt swap.

So, logically, a 3-phase VFD conversion sounded about right. The next step was to find a motor to replace the original 1/2HP "The Driver" motor. The below outlines what I've learned over the last few days. Please fact-check me if I'm wrong about anything!

----

A number of online posts allude to the need for higher HP motors when attempting a 1ph-to-3ph conversion. Initially, I thought this was because the motor lost some capability at lower-than-nameplate speeds. However, in my research, I found that torque is theoretically constant between 0 - 100% rated speed. (I suppose there must be some lower limit there, and that is torque is not truly constant down to 0%.)

At present, my understanding is that the implied "need" for higher HP motors for VFD purpose is really just related to the thermal capability of the motor. For TEFC motors, the motor drives its own cooling fan, so obviously the cooling performance is worse at reduced speeds. I found that "Inverter Duty/ VFD-Ready" motors are typically given with a constant torque ratio, given as the ratio of reduced speed to nameplate speed. Per my understanding, the meaning of this is simply "how slow can the motor spin without overheating". In the case of a 1725rpm motor with a 10:1 constant torque range, the motor can spin at 172RPM @ nameplate torque, while maintaining temperatures (at least) lower than the insulation class rating.

Does that sound about right?

-----

So, now I am less convinced that a higher HP motor is truly necessary, as long as a decent inverter duty ready motor with large constant torque ratio is used. I searched many catalogs (Baldor, US Motors/Nidec, WEG, Dayton), found some OK options, but most "only" had a 10:1 or 5:1 constant torque. Then I found the "W Series" from Book Crompton, which seems like a lot of bang-for-your-buck. 15:1 constant torque ratio, NEMA MG-1 P31 rating, 1.25x service factor and <$250 for most 1/2HP and 3/4HP models.http://www.brookcromptonna.com/Product Catalogs/Aluminum.pdf. Unlike most rolled steel motors, the Brook Crompton is cast aluminum with lots of cooling fins. Intuitively, this type of motor should be able to dissipate heat much more effectively, which probably explains its 15:1 torque ratio and 1.25x service factor.

-----

*Now my main question is: Am I likely to harm the tool (drill press) by over-sizing the motor to 3/4HP or 1HP (for instance)? I would imagine that the drivetrain has some limits. In a drill press, probably the splined spindle shaft is the highest stress point(?) If the full-load torque is increased by say 25 or 50%, then the drill press will undoubtedly "see" that extra stress under heavy load, right?

Given my lack of experience with machine tools in generals, I want to make sure I'm making sense. Anyone have any input or advice?

At this point, I'm leaning towards a Brook Crompton 1/2HP/1725RPM inverter duty motor (as mentioned above). They are about $220, which is not bad

Thanks!
Tim
 

SomeoneSomewhere

Hot Rolled
Joined
Dec 24, 2019
It is generally the VFD that needs to be over-rated for single-phase use - this is because the input rectifier and smoothing capacitors have higher loads placed on them. Alternatively, use a VFD specified for single phase input. Over-rating a three phase motor is necessary when it's directly driven from single phase, with a starting capacitor.

With a good VFD, you would be able to specify a maximum output torque so it won't exceed whatever value you want, but this probably won't be necessary. Much smoother starting will also improve things for the gears.

Drill presses generally don't run long enough for heat soak to be a major concern. No massive heat dump from direct-on-line starting will also help.

Unless you're expecting to run at 172RPM, maximum torque, for ten+ minutes, it's likely that any 1/2HP 3ph motor will be totally fine.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
There are a couple of factors that you are missing, first although torque may remain flat as one decreases the speed, Hp falls off in a linear fashion below the base speed. You also loose the mechanical advantage of belting or gearing down the RPM and the same motor base speed. So is you reduce the motor speed by 50%, you have 50% less applied torque and 25% Hp to the spindle. So that is the main reason for over sizing the motor in a VFD application with a wide speed range. In addition you would gain a mechanical advantage increasing the motor RPM above its based speed, above the base speed Hp is somewhat constant and torque falls off. The degree that full Hp is maintained is based on the motor design, so most inverter/vector type motors will maintain constant HP up to their maximum rated speed. Inverter/vector motors in 1-5 Hp range are usually good for 3X their base speed, standard 4P motors usually 1.5-2X their base speed. Most 4P motors these days have a constant torque ratio in the range of 1:4 to 1:10, inverter/vector style usually 1:1000 or higher. Standard 4P motors are usually good for 2X their base speed although performance can drop a bit when used above 90-100 Hz. Still perfectly acceptable for your application.

As far as cooling, a lot depends on the motor type/design and the application/loading. In general TEFC will run down to about 15-20 Hz and maintain adequate cooling, depending on the loading and ambient temperature. A TENV are designed for even lower speeds w/o overheating. It has nothing to do with the constant torque ratio, that only defines the range that constant torque is maintained. Also how well this is maintained is also dependent on type of VFD used and if you are running it in V/Hz are feedback/vector mode. Motors operating over a wide speed range are often TEBC using a separate electric cooling motor or other forms of cooling.

My mill uses a TEBC inverter/vector style 4P motor, the motor speed range is 20-200Hz and is a direct drive with a 10:1 back gear. The spindle speed range is ~40-4500 RPM. So if your goal is to get a wider speed range with less belt changes, I would up-size the motor probably to 1.0 or 1.5 inverter/vector rated 4P motor a (either TENV or TEFC) and you would have a usable speed range in the neighborhood of 20-120 Hz. Should not be an issue unless you had a gear head mill. New motors of this size from eBay or surplus supply house run $200-300 range.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
Cooling in a setup like this is a no-brainer. Just put a small muffin fan on the bottom of the motor, blowing up. Fan comes on whenever the drive is powered. This drill does actually work at slower speeds a lot of the time.

SB3.jpg
 

TimNJ

Plastic
Joined
Aug 21, 2021
There are a couple of factors that you are missing, first although torque may remain flat as one decreases the speed, Hp falls off in a linear fashion below the base speed. You also loose the mechanical advantage of belting or gearing down the RPM and the same motor base speed. So is you reduce the motor speed by 50%, you have 50% less applied torque and 25% Hp to the spindle. So that is the main reason for over sizing the motor in a VFD application with a wide speed range. In addition you would gain a mechanical advantage increasing the motor RPM above its based speed, above the base speed Hp is somewhat constant and torque falls off. The degree that full Hp is maintained is based on the motor design, so most inverter/vector type motors will maintain constant HP up to their maximum rated speed. Inverter/vector motors in 1-5 Hp range are usually good for 3X their base speed, standard 4P motors usually 1.5-2X their base speed. Most 4P motors these days have a constant torque ratio in the range of 1:4 to 1:10, inverter/vector style usually 1:1000 or higher. Standard 4P motors are usually good for 2X their base speed although performance can drop a bit when used above 90-100 Hz. Still perfectly acceptable for your application.

As far as cooling, a lot depends on the motor type/design and the application/loading. In general TEFC will run down to about 15-20 Hz and maintain adequate cooling, depending on the loading and ambient temperature. A TENV are designed for even lower speeds w/o overheating. It has nothing to do with the constant torque ratio, that only defines the range that constant torque is maintained. Also how well this is maintained is also dependent on type of VFD used and if you are running it in V/Hz are feedback/vector mode. Motors operating over a wide speed range are often TEBC using a separate electric cooling motor or other forms of cooling.

My mill uses a TEBC inverter/vector style 4P motor, the motor speed range is 20-200Hz and is a direct drive with a 10:1 back gear. The spindle speed range is ~40-4500 RPM. So if your goal is to get a wider speed range with less belt changes, I would up-size the motor probably to 1.0 or 1.5 inverter/vector rated 4P motor a (either TENV or TEFC) and you would have a usable speed range in the neighborhood of 20-120 Hz. Should not be an issue unless you had a gear head mill. New motors of this size from eBay or surplus supply house run $200-300 range.

Thanks for your detailed reply. I guess what I'm failing to understand is why a loss of horsepower matters at low speed. As I understand it, and it may be a flawed understanding, "the torque is what does the cutting". It is the centripetal force measured some distance from the center of rotation. Horsepower is defined at torque x rotational speed, so obviously horsepower goes down linearly too as speed is reduced. But if the cutting torque is still there, who cares? (Excuse my ignorance.)

You mention that "if you reduce the motor speed by 50%, you have 50% less applied torque and 25% Hp to the spindle"...Is this statement with regard to "standard duty" motors, but not "inverter duty/VFD" motors? Based on most (inverter duty) motor curves I've seen, there is no loss of rated torque at 2:1 (50%) spin-down.

I guess it makes sense that the "constant torque ratio" is exactly what the name implies, and not really about the thermal performance. But, this leaves a little uncertainty about how to size a motor for adequate thermal performance (based on the datasheet alone.) In any case, this is for home/intermittent use. I doubt it really matters, as was mentioned in the first reply by SomeoneSomewhere.

Regarding up-sizing to 1.0 or 1.5 inverter motor...Do you mean 1HP or 1.5HP, or do you mean 1.5x multiplier to original 1/2HP (3/4HP)? And this is recommendation based on cooling or mechanical advantage, or both?

Thanks.
 

TimNJ

Plastic
Joined
Aug 21, 2021
It is generally the VFD that needs to be over-rated for single-phase use - this is because the input rectifier and smoothing capacitors have higher loads placed on them. Alternatively, use a VFD specified for single phase input. Over-rating a three phase motor is necessary when it's directly driven from single phase, with a starting capacitor.

With a good VFD, you would be able to specify a maximum output torque so it won't exceed whatever value you want, but this probably won't be necessary. Much smoother starting will also improve things for the gears.

Drill presses generally don't run long enough for heat soak to be a major concern. No massive heat dump from direct-on-line starting will also help.

Unless you're expecting to run at 172RPM, maximum torque, for ten+ minutes, it's likely that any 1/2HP 3ph motor will be totally fine.

Good notes, thank you. And, I hadn't thought about the wear-and-tear of the initial start up jolt as the gears mesh , etc. I wonder what percentage of wear (on average) is due to startup and what is due to steady-state usage.
 

TimNJ

Plastic
Joined
Aug 21, 2021
Cooling in a setup like this is a no-brainer. Just put a small muffin fan on the bottom of the motor, blowing up. Fan comes on whenever the drive is powered. This drill does actually work at slower speeds a lot of the time.

SB3.jpg

Good idea! Yes, can simply rig up a small AC fan with power from the VFD wiring box.
 

dalmatiangirl61

Diamond
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
The W-T drill presses came with 2 pulleys, or 3 pulleys, which do you have? I have never broken a pulley when moving a belt, but if you are gifted enough anything is possible. Personally I prefer the torque you get by gear reduction, and do not view moving belts as a chore, and will take the reliability of the stock system over the unreliability of a vfd.
 

TimNJ

Plastic
Joined
Aug 21, 2021
The W-T drill presses came with 2 pulleys, or 3 pulleys, which do you have? I have never broken a pulley when moving a belt, but if you are gifted enough anything is possible. Personally I prefer the torque you get by gear reduction, and do not view moving belts as a chore, and will take the reliability of the stock system over the unreliability of a vfd.

Some good wisdom in that. Yes, I have just 2 pulleys, which gives a 600rpm minimum speed. I'd like to go to 200-300rpm or so...The slow-speed attachment is unobtanium. I have considered trying to build my own, but it is a little complicated without a mill or lathe, although I think it would make a decent third or fourth project once I have proper machine tools. Something can probably be hacked together without those tools, but will probably be a little janky.

I've seen many people have good success with Walker Turner VFD conversions. (It's quite a popular thing to do, apparently).

Regarding the pulleys, mine have a few chips off of them...I'm probably the third or fourth owner of the DP, so I don't know how those chips came about. It does seem unlikely that moving belt position would do that, but who knows.

But anyway, good thoughts...I will think a little harder about whether I really want to go for a VFD conversion.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
...
I've seen many people have good success with Walker Turner VFD conversions. (It's quite a popular thing to do, apparently).
....

It's really prety handy. This one's my home walker turner (southbend is at shop at work) and is operated by a small box with fwd/rev switch, speed control pot, and a foot pedal for the start/stop. This can actually be used to power tap at low speed, the drive is an inexpensive Teco unit in a small vented box. Have not shifted the belt in years.

drillpress_vfd_1.jpg


drillpress_vfd_2.jpg
 

dalmatiangirl61

Diamond
Joined
Jan 31, 2011
Location
BFE Nevada/San Marcos Tx
One of mine has a few chips in it too, so far it has not been a problem. Slow speed attachment can be had, but yes they do run high, my first one came with it, was quite disappointed the second one did not, it lives in the wood shop.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
You are not factoring the mechanical advantage of the pulley/gear speed reduction system, they are there so you have a mechanical ratio as you decrease the speed. A VFD does not have a mechanical advantage as you go below the base speed, the mechanical ratio is a multiplier of torque. So at 30 Hz use loose the mechanical multiplier of torque 2X from the drive system. The Hp is reduced by 50% and you loose the mechanical advantage 2X. Torque on inverter/vector motors is essentially 100% down to 0 speed, but you do not see mills with these motors operating down to 0 speed. You always need some mechanical advantage, you need to significantly up size the motor if you want it to operate at lower speeds.

Good example is the 10EE lathe, some conversion eliminate the back gear and go to a single speed. The original motor was a DC 3Hp, many of the VFD conversions end up stuffing a 10 Hp motor in the base to make up for the loss of the back gear and anemic low end cutting power. It requires an inverter/vector motor to get any decent low speed performance (and also maintain full torque) and an extended high speed range. I have worked with a number of people in lathe conversions spec. the motor/VFD. It all depends on the speed range you need, in general if you want a wider speed range and do not want to use a mechanical speed reduction then I would recommend increasing the motor size 50-100%, reason why I suggested 1-1.5Hp if you are going to go through all the effort and cost. Something like below, you would also need to factor in mounting and shaft size. You run the motor off of a smaller pulley so you maintain decent motor RPM a slower speeds and run the motor to the higher speeds to get the maximum speed you want for the design of the machine. Also be aware that the current motor pulleys may not be rated for the maximum RPM of the motor. Last drill press I did a similar VFD conversion I went from the stock 3 Hp motor to a 5 Hp inverter/vector and from 4 belt speeds to 2. Lots of different ways to approach this depending on the machine and what you are going to use it for.
Marathon Motors 056H17F2022 1 HP 5400 RPM Max Speed 230/460V Vector Motor
Marathon Motors 056H17F2022 1 HP 5400 RPM Max Speed 230/460V Vector Motor | eBay
 

thermite

Diamond
The W-T drill presses came with 2 pulleys, or 3 pulleys, which do you have? I have never broken a pulley when moving a belt, but if you are gifted enough anything is possible. Personally I prefer the torque you get by gear reduction, and do not view moving belts as a chore, and will take the reliability of the stock system over the unreliability of a vfd.

^^^ THIS ^^^ That series W-T (mine is early 1940's) all you need to do is go find - or FAB - the very FEW parts it takes to put the third cone pulley, shaft and such onto it.

Walker-Turner are sweet and durable DP for small money, do punch above their weight class "somewhat"... but..

It wasn't built for, and cannot effectively APPLY high HP or larger drills that want slow-slow RPM in any case. It's just too lightly built for that, one end to the other.

Why make a complex and costly problem out of a proven-simple and CHEAP solution?
 

thermite

Diamond
Not costly. Hardly complex. Why because it works well - note the unused intermediate pulley setup in the one photo above, not used. No need for it.

Surely there is a need!

Even bereft of its complement of parts, it serves as an extremely enduring reminder that Walker-Turner / Kearney & Trecker held an order or three of magnitude greater understanding as to how build an all-around useful drillpress than a late coming Old Troll-Fart with Trump Derangement Syndrome will live long enough to learn.

Run what they built.

It's about as good as it gets for small money and minimal hassle.

Not good enough?

More powerful drillpresses exist. Really. They DO!

ALL parts of them sized accordingly.

Not just the motor.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
Yet again you are preaching to the choir. That WT in my shop is a good deal more solid than my work SB unit. Plus it comes with a JT33 mount chuck, with the lock-ring mount. When I need to drill large holes I use a milling machine, so this one rarely gets above 1/2 inch drills, as that's what the chuck is DESIGNED for. If you note the belt is pretty much fixed on a mid-speed setting. So it's got lots of torque, enough to twist a part out of a vise if not careful, and enough speed to run small drills at speed. Slow enough to tap, and reverse out. I care not what your opinion is, still maintain that a VFD is a great fit for light drill presses like this.
 

thermite

Diamond
Yet again you are preaching to the choir. That WT in my shop is a good deal more solid than my work SB unit. Plus it comes with a JT33 mount chuck, with the lock-ring mount. When I need to drill large holes I use a milling machine, so this one rarely gets above 1/2 inch drills, as that's what the chuck is DESIGNED for. If you note the belt is pretty much fixed on a mid-speed setting. So it's got lots of torque, enough to twist a part out of a vise if not careful, and enough speed to run small drills at speed. Slow enough to tap, and reverse out. I care not what your opinion is, still maintain that a VFD is a great fit for light drill presses like this.

Consumables, VFD are. Finite lives, and not long ones, even if not under power. Not gentle even on the motors built for them.

Lighter the duty, fewer the holes over which to amortize the costs, costlier each hole becomes. F**k-wth time is never really "free"either, as some other project cannot re-use time once wasted.

Done a long-term life-cycle cost per-hole on that, have you?

Didn't THINK so!
 

TimNJ

Plastic
Joined
Aug 21, 2021
You are not factoring the mechanical advantage of the pulley/gear speed reduction system, they are there so you have a mechanical ratio as you decrease the speed. A VFD does not have a mechanical advantage as you go below the base speed, the mechanical ratio is a multiplier of torque. So at 30 Hz use loose the mechanical multiplier of torque 2X from the drive system. The Hp is reduced by 50% and you loose the mechanical advantage 2X. Torque on inverter/vector motors is essentially 100% down to 0 speed, but you do not see mills with these motors operating down to 0 speed. You always need some mechanical advantage, you need to significantly up size the motor if you want it to operate at lower speeds.

Good example is the 10EE lathe, some conversion eliminate the back gear and go to a single speed. The original motor was a DC 3Hp, many of the VFD conversions end up stuffing a 10 Hp motor in the base to make up for the loss of the back gear and anemic low end cutting power. It requires an inverter/vector motor to get any decent low speed performance (and also maintain full torque) and an extended high speed range. I have worked with a number of people in lathe conversions spec. the motor/VFD. It all depends on the speed range you need, in general if you want a wider speed range and do not want to use a mechanical speed reduction then I would recommend increasing the motor size 50-100%, reason why I suggested 1-1.5Hp if you are going to go through all the effort and cost. Something like below, you would also need to factor in mounting and shaft size. You run the motor off of a smaller pulley so you maintain decent motor RPM a slower speeds and run the motor to the higher speeds to get the maximum speed you want for the design of the machine. Also be aware that the current motor pulleys may not be rated for the maximum RPM of the motor. Last drill press I did a similar VFD conversion I went from the stock 3 Hp motor to a 5 Hp inverter/vector and from 4 belt speeds to 2. Lots of different ways to approach this depending on the machine and what you are going to use it for.
Marathon Motors 056H17F2022 1 HP 5400 RPM Max Speed 230/460V Vector Motor
Marathon Motors 056H17F2022 1 HP 5400 RPM Max Speed 230/460V Vector Motor | eBay

Ah. Yes, now it makes sense. I am an electrical engineer by trade. (And it shows!) So, pulleys have the advantage of slowing the rotational speed down by the ratio of the pulley diameters and increasing the driven torque by the (inverse) of that ratio. Yes, yes...that sits well in my head.

On that note, and to this point...

It wasn't built for, and cannot effectively APPLY high HP or larger drills that want slow-slow RPM in any case. It's just too lightly built for that, one end to the other.

The W-T slow speed attachment allows speeds down to 165RPM, roughly 10:1 speed reduction, and then....1:10 torque increase? So, for a 1.5lb-ft motor, at the slowest speed (165RPM), doesn't the spindle have the capacity to be driven at 15lb-ft? Right?? Would you consider this wearing out the machine?

So, if you use a VFD solution with an "oversized" 1HP motor, this motor can drive the spindle with 3ft-lb, assuming driven and driven pulleys are the same diameter. I don't see how this would be higher stress than the factory setup. What am I missing?

The only thing I can think of, is at higher speeds, the factory setup has reduced torque, due to the pulley ratio, whereas when using an 1800RPM VFD setup, the high speed torque would be higher than the factory setup. (I don't know if it makes a difference.) In this case, to more closely factory high speed torque, a 1200RPM motor can be used, and the torque would roll off when at 1.5 or 2.0x overspeed.

From my view, most VFD sets will be weaker than factory belt setups, so I don't see how it would stress the machine (powertrain) more?
 

thermite

Diamond
doesn't the spindle have the capacity to be driven at 15lb-ft? Right?? Would you consider this wearing out the machine?
A Monarch 10EE 3 HP large-frame DC motor only deleivers about 23 lb/ft at nameplate nominal. That pushes a D1-3 spindle.

You have a Jacobs Taper and a max 5/8" chuck?

You never GET there.

The Jacobs chuck slips on the hard shank of the drill, more easily yet on a harder-yet tap shank, and the tool has spun in the chuck, first.

Walker-Turner built in a "balance" amongst the team players.
They did it well. VERY well.

A W-T doesn't LOOk like there is the least damned thing "special" about it.

But as with the legendary one-hoss shay, every least bit of it was in very good harmony with every OTHER bit.

W-T as with "gutless wonder" Maytag washing machines became borderline REVERED in comparison to Craftsman, Rockwell-Delta (who eventually acquired W-T) and legions of imported lookalike - but never quite WORK-alike - lesser value-for-money drill presses as have gone to their worn, bent, cracked, broken graves.

Are/were their BETTER drillpresses? Surely. Easily two dozen makers. Every one of them higher-priced, too. Very "higher".

I consider myself fortunate to have acquired my USED Electro-Mechano W105 for under $400, shipped.

The Alzmetall AB5/S was a skosh over ten times that to get home and sorted out...more than one forklift rental, drop and recovery alone.

But WTH? At 4400 lbs, Avoir for a 1952 vintage legend, that is still only about a dollar a pound.

A dollar a pound won't buy meat for supper, and supper won't LAST a hundred years, either.

:D

Want to alter a Walker-Turner? It isn't hard. They are not sacred religious relics.

But which components of their proven "team" dare you NOT alter as you Engineer towards a WHOLE new balance?

I HAVE spare DC motors and drive that could be even better than a VFD.

Also wasted. Back to the light-duty spindle, bearings, and chuck, y'see.

Single-phase vintage motor on my one is only 1/2 HP. Yet I stall or slip the belt very seldom in any given year, lo fifty years or so in use.

And WHEN that happens? I am grateful it stalled or slipped. Not angry.

'Slow forward" to a 7 HP motor into PIV-Werner-Reimers steel belt variable drive then into a geared-head for the AB5/S.

Track bacwkards to the 3 foor, 5 foot, and eight foot radials I loved enough to have married .... in my innocent youth. I mean HOLES!

:)

Pilgrim? If you jam a drill in one of those dinosuarians . or my Heavy Hams Hans 5 MT spindle - whilst slamming holes into steel?

Drama happens. Shrapnel, even. Fast, too!

But it is the tool. Not the guts of the drillpress, hand-grenaded.

Horses for courses....
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
...
Done a long-term life-cycle cost per-hole on that, have you?

The one at work's been on the job for around 20 years. I'll leave it to the fart smellers, er, smart fellers to calculate its expected demise date. Mybe make book on that.

What's YOUR guess as to when my free VFD [1] there's gonna go belly up, Bill? How about make it interesting, put 20 bucks on your prediction?

[1] was free to me, was a 'spare' that another project stashed away, the original polisher was long gone, so they handed it off to me.
 








 
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