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vfd,how much torque at reduced speed

js412000

Cast Iron
Joined
Oct 24, 2006
Location
pennsylvania
I am designing a spindle drive for a horizontal machining center and can't seem to figure how much motor torque is available at various speeds. I am looking at a 20 horsepower motor 6000 rpm and need 2500 inch pounds at around 150 rpm. (enough torque to snap off a 1-8 tap, which is the heaviest demand that I expect.) I don't want to use any more gear or belt speed reduction than is necessary.
 
20 hp @ 6000 rpm = .5hp @ 150 rpm . If you have a shaft 2" in diam, its perifery (1"diam) has a load of 2,500 lbs on it, so work done = PI X 2 X 2500 X 150 lbs/inch/min or PI X 2 X 2500 X 150/12 ft/lbs/min. 1 HP = 33,000 ft/lbs/min. My maths works this out to be 6 HP, so my answer is that you need at least a 12 :1 reduction somewhere.
I checked this against wikipedia (http://en.wikipedia.org/wiki/Horsepower#Mechanical_horsepower) where hp = torque(ft/lbs) X RPM /5252
Frank
 
I concur with Chuckee but our numbers differ a little.

You need a reduction to get that torque at that RPM from a motor developing 20 HP @ 6000 RPM. 20 HP @ 6000 RPM = 274 lb in torque. Can't get the torque you desire from that motor using electronics alone.
 
Let's see if I can figure this one out.

T = HP x 5252/rpm

2500 inch lbs is about 208 ft. lbs.

20 * 5252 = 105040/6000 = 17.5 ft lbs of torque

This is according to the formula at http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

So, to get 200 ft lbs you need a 12:1 speed reducer (208/17.5 = 11.88) and indeed a 12:1 reducer is exactly what Chuckey said.

As far as a VFD goes, you can usually get excellent torque at 30 hz using a V/Hz type VFD. You can get excellent torque from a SV VFD at 10 hz or below.

Here are a couple of charts showing the difference.

The one on the left is a V/hz VFD and the one on the right is a SV type drive.

As you can see the SV (Sensorless Vector) drive still produces 100% torque at 10hz.

VFDs will not reduce speed and increase torque like a gearbox or pulley arrangement will. They reduce speed but the torque remains the same (within reason).

A 20 hp motor at 1725 RPM yields about 45 ft. lbs.
 

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Thanks for the help. I found some information on the Yaskawa website. Their spindle motors are good for full torque down to 1500 rpm and then it drops off fairly quickly.
It looks like I won't be able to get the torque that I want with a single belt reduction.
 








 
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