# Replacement motor for lathe



## Reddinr (Jul 7, 2017)

I am converting my lathe to have a VFD speed control.  The motor in it now is a 2 speed import rated 3 or 4 HP depending on speed selection.  It is rated 1725/3450 RPM.  I plan to replace it with a 7.5 HP single speed, inverter-grade motor.  I want to have good low-speed torque so it is over-sized.  There seem to be more choices in 1800 RPM motors vs. 3600 RPM motors.  I have room to change the pulley to a larger pulley and use the lower RPM motor.  I have to change the pulley anyway because NEMA motor shafts don't come in the Chinese /non-NEMA diameter that this one has.

Would you:  Change the pulley diameter and maybe (3) belts and use 1800 RPM or use the same diameter pulley?  Anyone think of an advantage one way or the other?  Cost difference would probably be a wash.


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## Karl_T (Jul 7, 2017)

You can spin an 1800 RPM up to 4000 with a VFD, making it a way better choice.

Can't answer your pulley question but you will really loose low end torque with a bigger motor pulley


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## mksj (Jul 7, 2017)

I would recommend a 3 or 5Hp motor, you should not need anything bigger. Kind of an a strange combination in that I wold expect a constant torque motor would be 2/4 Hp for those speed ranges. Either way I would go with a 1750 RPM inverter drive rated motor and run it up to 2X it's base RPM. I would use the same size pulley that is on it now, dual belt should be fine. Since an inverter motor in the 3-5Hp range will spin up to 6000 RPM, you can easily go with a smaller pulley size and safely go up to 3X the base speed. Although most newer 3 phase motors have VFD rated insulation, they are not designed to run over ~90Hz because both their torque and Hp declines more rapidly after that point. Inverter drive (or vector) rated motors are designed to run off of VFD only and will maintain the motor specs to a minimum of 120Hz, and most maintain their full Hp up to their rated maximum speed. If you went with a conventional 5Hp motor, you could easily run it up to 90-100Hz, so you would run a slightly larger pulley to achieve the same maximum spindle speed. So something like the Baldor Reliancer Super E Motor 5HP 230/460V 1750RPM EM3665T would be the least expensive and work well with running it with to 100Hz with a 1.2X larger motor pulley.  http://www.ebay.com/itm/5-HP-1750-RPM-NEW-BALDOR-ELECTRIC-MOTOR-EM3665T-230-460V-/352084168010

Inverter drive motors, I would run with the same size pulley going to 120Hz, or 0.7 X current pulley size if running to 180 Hz.
This would be a great motor (I use the 2 Hp version), but the shaft is an odd size so the pulley would need to be bored specifically for this shaft size.
http://www.ebay.com/itm/Baldor-Reliancer-Motor-184TC-5HP-230-460V-1760RPM-3PH-IDNM3665T/182487935288
This one is a great price on an inverter motor, but way too big a motor for your lathe.
http://www.ebay.com/itm/7-5-HP-213T...-RPM-3-ph-TENV-Baldor-IDNM3770T-/122523863098
Possible alternate 5Hp, but I would get the BlackMax below.
http://www.ebay.com/itm/Reliance-El...S-Master-Inverter-Duty-P18G1194-/201787218575

If buying new then the Marathon BlackMax series are probably what I would recommend.
http://www.electricmotorwholesale.com/MARATHON-Y541.html
http://www.electricmotorwholesale.com/MARATHON-Y543.html

Also be aware that a larger motor requires a larger VFD, and most single phase VFDs max out at 3 or a few 5Hp. Beyond that you need to double the VFD size and derate it for single phase operation. So a 10Hp for a 5Hp motor, 15Hp VFD for a 7.5Hp motor, which get expensive quickly. I have run 3Hp inverter motors on 14" lathes, they are unstoppable. A 5Hp motor is often used on 2 speed 14" lathes, but if you have more speeds than 3Hp should be more than adequate.


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## Reddinr (Jul 7, 2017)

Guys.  The effort you put into these answers is impressive to say the least.  I don't know how to thank you.  My lathe is a 16" gear-head.  I would like to pick maybe 4 gear settings and do everything else with the VFD speed.  I have a drive that can have closed loop control and I have a very good A,B,Z encoder so I'm thinking about putting that on the spindle.  Overkill I know but that helps with dealing with shifting gears / RPM setting I think.

I have doubts about the real HP of the original motor.  One advantage I have is acres of space in the motor compartment.  The 213T motor actually almost fits my present pulley which is 1.5" bore.  I have a BIG three phase AC source feeding my shop.  It is a 200 HP motor drive converted into an AC inverter that puts out filtered 60Hz. sine-wave (almost) power.  My equipment motor drives get fed by that.  I have not seen a motor with 1.5" (1.495 measured) shaft as yet and may never find one.   Can you see any problem running a 7.5 HP motor with a 5 HP drive when the motor is probably seeing 3-4 HP loading on its worst day?


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## Ulma Doctor (Jul 7, 2017)

for best results and to save an inverter, use an inverter rated for the HP of the motor.
if you put a 3 hp on a 5 hp motor you may soon blow up the inverter or possibly damage the motor or both.
1.375" is the standard shaft size for a 213t motor, you may have a special rotor in the motor if it measures in at 1.5"
often equipment manufacturers will order motors with special specifications, here is a list of common differences
sometimes it's RPM that is special, or undersize/oversize shaft diameter or length, 
or if the shaft is special keyed or threaded, sometimes reduced/extra HP rating, or if the motor is being used in a hazardous location, or even explosion proof motors


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## Reddinr (Jul 7, 2017)

BTW.  I mentioned 3 belts because that is what is on the lathe now.  Sounds like 1800 RPM is a good choice since I can run that up past 3600 RPM anyway.

If I set up the drive max currents and trip currents for a 5 HP motor, is the drive for the most part protected if I connect a 7.5 HP motor but just load that motor to about 4 HP?


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## tq60 (Jul 7, 2017)

Go with 3 belts or maybe 4 by considering swapping both pulleys to work out optimum bottom rpm torque by increasing ratio or larger driven pulley.

Our 16 inch lathe has a 7.5 hp motor and with a SPC only has 5 and it needs every bit of that to slosh around the oil in the gearbox on higher speeds.

Why 4 belts?

Ours has maybe 5 and this is due to the power transfer.
More belts is more surface area and better grip reducing bearing loads by not needing to be as tight.

If a gear head lathe the higher rpm will require more power due to oil drag.

Thinner oil may help but considering you have manufactured by 3 phase available then less need to De - rate a vfd but higher power may be better at higher voltage as current is less and easier to wire.

Pause and consider your total plan before buying anything.

Ours lists a 5 hp motor option from the factory but rpm were lower due to different drive ratios.

Look at the manufacturer options regarding motor size and operating rpm as it may help you determine not what rpm you want but what range you may get with the motor sizes you use.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk


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## mksj (Jul 8, 2017)

Given that it is a 16" lathe and you have the space along with 3 phase changes the picture a bit. You could easily get by with a 5Hp, but that 7.5Hp Baldor listed above is killer, but you do need an inverter/vector motor to achieve the speed range you want. You could use 2 gears all day long and get the range of speed you want. You need to match the VFD to the motor size, you cannot under size it by saying you are only going to use X% of Hp. Since you have 3 phase, just use a 7.5 Hp 3 phase input VFD and be done with it, you will also need a substantial braking resistor to go with it. Get a decent VFD with sensorless vector motor control, in my experience it holds better than 0.1% speed regulations under all loads, my lathe is +/-1 RPM. I have entertained  (and had one on an install that I did similar to what you are planning) using a feedback encoder, but believe me it is truly wasted in this particular scenario and you will not see a performance difference. Feedback encoders are helpful where exact speed/shaft position is required, holding loads, very low speeds or absolute speed regulation, which is not the case. The VFD needs to be set for the motor operating parameters and autotuned to the motor.



You can then set the motor overload parameters to dictate how it operates and at what point it will trip (the default is usually something like 120% current for 1 minute at the particular operating output voltage). You need to keep in mind that below the base speed of 60 Hz, the motor will loose Hp in a linear fashion down to 0 RPM, so if you are running at say 20 Hz a 7.5Hp motor is delivering 2.5Hp. Above the base speed most of these inverter motors remains at full Hp up to 6000 RPM, although toque will fall off. This is one reason on factory installed VFDs  that you often see an operating range of something like 20-180Hz. So most of the time you are operating at full Hp, low speed you have full torque. There are other factors, so above the motor base speed the applied spindle Hp is greater relative to a fixed speed motor and the torque is about the same as a fixed speed motor at high speeders. A fixed speed motor requires gear changes to achieve the higher speeds and the power/torque would be reduced by the ratio of the speed increase.  The bottom line it will be way more than you can possible use, I have done quite a few installs with the Marathon BlackMax, Baldor IDNM, and a few other inverter/vector motors, they are truly amazing and almost not audible (most in this size range are TENV, so no fan). There is some tweaking of the VFD parameters to get the motor to operate in the sweet spot.

I would go with 3 belts of the current machine, should be more than enough to transmit the power needed and if anything I would rather have the belt slip/go as opposed to something else. You may be able to bush the current pulley, but decent motor pulleys are inexpensive and may be better balanced at speed. Pulleys also come in two piece with a locking inner hub that fits the motor shaft. Use a decent belt, I have been using Gates Tri-Power II that have a notched belt (they bend better and are less likely to take a set) with very good results. I really doubt you will have any shortage of power or speed range. I would stay with the current speed range and headstock oil weight, if splash lubrication you want to stay under 2000-2500 RPM. I see no reason to deviate from the manufactures specifications.


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## Reddinr (Jul 8, 2017)

Again, mksj thank you for the detailed information.  I think that takes care of my original motor question nicely.   That 7.5HP motor is a sweet deal.  Hard to pass it up but I already have a Yaskawa A1000, 5 HP drive with the closed loop option already installed an encoder and a sense of adventure.   I was thinking the encoder would help deal with the gear changes in that the drive would need to know what the gear ratios are in order to turn the spindle at the correct set RPM.  I would have the encoder on the spindle, not the motor.   I could have some sort of switch to tell the inverter what range I'm in but that seemed clunky to me.   I may start out doing that though.

I also am rigging my lathe with a servo that can optionally be used for the Z axis feed.  I have long term desire of tying that together with the drive to include sorta-cnc with me controlling the X axis for now and allow things like engraving on a cylinder and that sort of thing, but that's another story.


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