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torque?

Dave Dahlgren

Cast Iron
Joined
Dec 19, 2004
Location
CT.
I have a south bend lathe that i will probably put a VFD on it but.. If the motor has 10 ft lbs now,I am not sure what it has but makes the math easy, and I use a VFD for speed adjustment and keep full torque how does that get me the same torque as originally had? The reason i ask this is the motor originally had a set of pulleys and gears to change speed. This one has what looks like a 6:1 reduction in speed for low gear low speed use. So if i take my 10 ft lbs and multiply by 6 I have 60 ft lbs. With the VFD I thought all you get is full torque or 10 ft lbs. Am i missing something here??
Dave
 
Dave, you are correct that a VFD will not provide a torque multiplication as you lower speed like a mechanical system will. If the motor can provide 10 ft lbs, then that is all you will get using a VFD.

From other posts I've seen on the subject, you will most likely still need to use some of the mechanical gearing to get the full range of performance, but I will let the machinery experts get into that, as my expertise is VFDs.

John
 
Dave, I am no expert but have asked the experts here enough questions and gotten great info on this very subject. I work in a plant that has at least 300 freq. drives. A vast majority just run one speed all the time and never give any problems. However there are several machines that I have become very familliar with in my four years with this company (I'm the guy that gets called @ 2 in the morning and gets to come in and get things going again). What I have learned is that hp is linear and tq is constant to 60hz. A 10 hp motor will put out 5 hp @ 30 hz with full tourqe. After 60 hz your tq goes away fast. With your lathe I would sudgest figuring out what your average speed will be and figure out what your gear ratios need to be to run your drive between 40-60 hz.
This will give you enough range to keep the hp you need for a heavy cut but also the ease of changing feeds and speeds with the pot.

Luke
 
top speed of this lathe is 1800 rpm which is fine with me. So if i read this correctly if i have 10 ft. lbs. at 1800 rpm when i go to 900 with belts and pulleys I have 20 ft. lbs. but with the VFD at 30 Hz and not using the pulley system the best i can hope for is 10 ft. lbs. This seems backwards of what you would want if you are slowing down the spindle to cut a tougher material. Do they normally oversize the motor to use a VFD with direct drive?
Dave
 
On the oversizing motors to run with VFD's, yep, thats the way its done. A lot of CNC lathes are direct drive and they use huge (40hp) spindle motors to maintain torque. If you want to run your SB direct just replace the motor with a 5hp, that should take care of your torque issues.
 
I think i will try to power up the varible speed part of the machine that runs on 115 volts and see if it steps the pulley in and out and go from there.. if that works it might just end up with a 220 single phase 3 hp motor. Would have to buy a lot of electricty for the price of the VFD if that will work..
I don't think a 5 hp motor would fit anyway, it is in a pretty tight spot.
Dave
 








 
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