RPM rating for VFD conversion

[DukeOfSavoy]

I'm sure this has been discussed previously, but I couldn't find the exact answer.
Planning on ripping out the MG on my '48 10EE and installing a new motor(thinking between 5-7.5hp). What RPM should I use? Plan on ditching the backgear, would there be an issue with a 3600rpm motor?

 

[Karl_T]

I would suggest you follow the exact route i went.

Monarch 10EE rebuild 10Hp VFD no backgear

I bought my second 10EE a while back https://www.machinesused.com/lots/Monarch-10EE-Toolroom-Lathe-with-Taper-Attachment-and-Chucks-25097#bidNow After playing with the existing DC motor and drive, I gave up on repairing it. I did not try super hard, not too excited about maintaining an antique...

www.hobby-machinist.com

 

[markba633csi]

If I was doing one, I'd do it just like Deek did his, and keep the backgear:

10ee VFD Conversion

Hi All, I wasn't going to post my conversion, but my buddy talked me into it saying he thought it might help others facing the same issues. Here's my story... I've been wanting a 10ee for some time. Out here on the west coast they are few and far between. I haunted auctions, used equipment...

www.hobby-machinist.com

 

[rabler]

I'm sure this has been discussed previously, but I couldn't find the exact answer.
Planning on ripping out the MG on my '48 10EE and installing a new motor(thinking between 5-7.5hp). What RPM should I use? Plan on ditching the backgear, would there be an issue with a 3600rpm motor?

Especially without using the reduction (back) gear, a 3600 RPM motor will give you poor torque at lower RPMs. Typically lower RPM is where you need more torque. You can play with pulley size to make a 3600 RPM motor work, but small pulleys will be harder on the belt life, although that isn't much of a factor for hobbyist use.

You'd be much better using a 1800 RPM motor or even lower, and generally want a good one to get good performance across the full RPM range. Details in the above links are useful. Whether or not to keep the reduction gear is a debate, but if you don't keep it you'll generally want a larger motor. (7.5 -10HP vs 5 HP). You can of course get away with lighter cuts if you have less HP so what works best for you is subjective.

If you do think about keeping the backgear, pull it apart and check it's condition before getting set on that. The backgear/reduction gear units can wear out, I need to make a couple new gears for mine.

 

[Beckerkumm]

No back gear is a lot to ask for a motor and vfd conversion. I'd sacrifice some high end as I seldom need speed in excess of 2500-3000 rpm. I run carbide insert tooling but also have HSS inserts to replace my CGMT which allows for less speed on aluminum. Given that you will be working with a 213-215 NEMA frame, my first choice would be a 5 hp 1200 rpm vector duty type motor which will provide basically the same low end torque as a 7.5 hp 1800 rpm motor but arguably run very smooth at low speed due to 6 poles and higher hz. They are difficult to find at a reasonable cost so if a deal on a 7.5 1800 came along I would compromise. The TENV Baldor IDNM, ZDNM, Marathon black or blue max are impressive and handle the speed variation very well. No way do you want a 3600 rpm motor on a lathe of that type. Dave

 

[DukeOfSavoy]

I saw your build Karl, which is excellent documentation, but I was honestly just curious why one would choose a lower RPM motor vs higher rpm?

 

[jwmelvin]

…curious why one would choose a lower RPM motor vs higher rpm?

Because a motor can be run faster than it’s rated speed on a VFD and maintain constant power output but loses power at lower speeds. It seems to be the consistent recommendation of VFD experts (not me) to use the slower motor so that one retains better low-speed torque.

 

[Karl_T]

I saw your build Karl, which is excellent documentation, but I was honestly just curious why one would choose a lower RPM motor vs higher rpm?

more torque at low rpm. An 1800 has twice the torque at any speed under 1800 than a 3600 RPM.

Of course gearing and pulleys do it also.

The game here is to get as wide a useful range as possible. Larger motors give more torque at low RPMs also. But cost is more.

The VFD rated motors have a much wider usable range of RPMs. A standard 3600 RPM is maybe 4500 max RPM. The 1800 RPM VFD rated motor I used has a max RPM of 6000. With all that HP this motor would run smooth down to 5 Hz or 150 RPM. I had the machine pulleys gear it down 2:1 for 75 RPM min speed. that's a nice speed to thread at. Top motor speed of 6000 gives 3000 RPM to end.

Careful motor selection really pays dividends here.

 

[Beckerkumm]

More poles also result in a smoother running low speed. A 2 pole motor can transmit pulses to the workpiece at some speeds. The 10ee was constructed to deliver a great finish. Most used a flat belt to the spindle and bypassed the threading gears in favor of a flat belt for feeds which also allowed for a wider feed range while not transmitting gear vibration to the tool. When discarding the original DC motor drive you want to keep a many of the benefits that made the 10ee unique. High torque at low speed and smooth running throughout the range. As I said earlier, I'd give up some high end, particularly on the oil bath early models. the later ones had a drip delivery which was better at 4000 rpm. I have never needed anything above 3000. Most of my turning is in the 1000-2500 range for that size machine and it is only threading that gets much lower. The 10ee's ability to run slow feeds at high speed also reduce the need for the 4000 range in my world. Dave

 

[Karl_T]

I agree with Beckerkumm - a six pole would be better. i shopped ebay and did not find a six pole 10 horse VFD rated. It would cost HUGE bucks brand new from a name brand vendor. If you look at my first thread, i started out looking for 8 pole motors - they are too big to go in the bottom of a 10EE.

Now VFD motors are constant torque and HP is torque*RPM. So, at 5 hz, my machine has 5/60*10 = .8 hp

IMHO, if you go 5hp no backgear, you cannot go down to 5 Hz. Not enough power. probably could do 10 HZ, but now you lost a huge amount of speed range. if you want the same 75 RPM bottom, now the max speed is only 1500 RPM.