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Thread: VFD Suppliers
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19th May 2011, 01:54 PM #16GOLD MEMBER
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Morning Ray,
Ok I think I am even more confused. At 0 rpm even infinite torque delivers 0 power. At 1 rpm the torque would need to be 1440 times what it is at 1440rpm(right?)
Originally Posted by Stuart
Are you saying a 1440rpm motor can make 400% torque at 350rpm?
"Ok, another answer is yes, of course,..... (pause for effect)... if you have a 4:1 reduction gear box. "
This confuses me more, if you have a 4:1 gearbox the shaft is now going 87.5rpm with 4 times the torque. But the power is the same. The torque of the motor at 350rpm still needs to be 4 times what it is at 1440rpm to deliver the same power.
Do the drives increase the V(which there must be a limit on) or do they change the wave from so it has steeper raise and fall or both?
But even a square wave is only going to give so much torque...... but then where is at power going if not to the output? I'll think some more, maybe I'll get my head around it.
Thanks for trying. Sorry if it feels like you are hitting your head against a wall.
Stuart
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19th May 2011 01:54 PM # ADSGoogle Adsense Advertisement
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19th May 2011, 02:45 PM #17
Hi Bryan,
Going for a 2hp motor was a smart move.
I think it will work just fine, it all depends on what Hz the vfd is running at, let's say you are running at 50%, (25Hz) then you will still have 1hp available (roughly).
Where you might notice the drop in power is say you have the gearbox selected to 1000 rpm, and dial the vfd down to give 100 rpm, the vfd would be running at only 5Hz, you would find it hasn't got much torque, might still be usable for light cuts..
Next time you are on the lathe, give it a try, and see how slow you can go, with a high gear selected. I'm sure that there are a few others here, (myself included) who would be interested in what your thoughts are.
Regards
Ray
Originally Posted by Stuart
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19th May 2011, 02:56 PM #18
FWIW my lathe is a single speed belt driven with a 4:1 back gear. I am replacing its obsolete 2 hp. motor drive with a 4 h.p. induction motor. At 145 motor rpm (35 spindle rpm) it will be at around 5Hz. I figured I would need at least a 4 h.p. motor to get the required torque at that low speed. (.4 hp resulting X 4 (back gear ratio)= 1.6 h.p
I also have a 2.5 kW AC servo motor that I want to migrate to, but that has its own issues (inertia mismatch between the servo and the load).
(The above is the 1:1 motor:spindle ratio case, and I don't want to give up too much top speed. The lathe originally had 45-4000 rpm range. I don't want to go to 160 Hz+, but I would go to 120 Hz if the drive train balance is OK)
Greg
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19th May 2011, 03:19 PM #19.
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Too late - I ordered it on Tuesday afternoon and it was waiting for me when I arrived at work this morning. Techo Mark and I are going to try it later this afternoon - I'll report back as soon as we have something worth saying.
[EDIT] Nothing much to report other than it works as it says. We've played around a little with the setting of functions and that seems straight forward enough.
Mark is going to help me setup a switch box and bits and pieces so I can't quite plonk it on the lathe yet.
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19th May 2011, 04:05 PM #20Distracted Member
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That might actually be an advantage for parting. Better to stall than break stuff.
Next time you are on the lathe, give it a try, and see how slow you can go, with a high gear selected. I'm sure that there are a few others here, (myself included) who would be interested in what your thoughts are.
These are the upper and lower spindle speeds for each pulley combo (at 5Hz and 70Hz):
1: 42-628
2: 56-845
3: 71-1116
Lots of overlap. I reckon I can leave it in any of those ranges and get most things done, only belt swapping for the extreme cases. It's a lightweight machine and I don't push it hard.
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20th May 2011, 01:55 PM #21
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20th May 2011, 02:51 PM #22
Hi Ray...I have a pair of Allen-Bradley servos and drives, cables etc.
I collected them over a couple of years on ebay, and I am planning* to install the small one on the Deckel feed shaft to replace the chain driven shaft and change gears (Which I do not own).
*Plans change though, and there's a chance of another mill on the horizon. If that happens I'll keep the FP-1 for manual duties and leave the feeds as they are.
The whole subject of inertia mismatch came up in a PM thread. I may be OK, but the servo is pretty small compared to an induction motor (although just as heavy). I was lucky enough to buy everything new at less than 10% of new price, including some massive braking resistors. My lathe has a camlock chuck, so quick stopping won't be a problem.
Greg
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20th May 2011, 09:02 PM #23GOLD MEMBER
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Will,
try these guys:
Electrical Motors,3 Phase Motor,Gear Motor,Industrial Electric Motors by Royce Cross[cat1]=-1&Variable[cat2]=-1&Variable[cat3]=-1&Variable[searchfilter]=-1&Variable[sqlFilter]=-1&pager=1&startpage=1&Variable[cat1sel]=-1&Variable[cat2sel]=-1
Their prices are on the website. I spoke to them today. They said "about" $25 postage for a 10kg motor (1.5HP 3 phase) to Melbourne.
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21st May 2011, 11:38 PM #24
Can you find a link to that thread, it would be interesting to hear why anyone would think low inertia was a problem. Servo motors are specially designed to have low inertia, that why they are often long skinny things, it's so they can accelerate and decellerate faster.
The trick is to be able to tune the PID, I would expect a top of the line AC servo like Allen Bradley would have some auto-tune function.
Regards
Ray
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22nd May 2011, 06:57 PM #25Distracted Member
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22nd May 2011, 10:35 PM #26
Hi Bryan,
I'm finding similar things on the surface grinder cross feed, the torque drops away around 7-8 Hz, so I think for a rough guide to gear selection, you might get (say) 10 Hz to 70 Hz range for each gear ratio. That's a 7:1 ratio, still plenty of flexibility and would mean far less gear changes overall.
Regards
Ray
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23rd May 2011, 09:52 AM #27Distracted Member
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Ray, yes I agree it's a vast improvement. However it's not a total solution for a lack of change gears. Not the way I've done it anyway. If I add another motor pulley to get 6 speeds instead of 3 it will be better.
BTW have you played with PD145 - auto torque compensation? I've set mine to a cautious 6 out of 10 so there's a little room to move.
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23rd May 2011, 10:28 AM #28.
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Ray, where does 70Hz as an upper limit come from and is that for a 1450 or 2850 rpm motor?
I was discussing this with an electrical engineer at work and he was suggesting that almost all electric motors are designed to cope with at least 3000 rpm and if we think about 60Hz motors then 3450 rpm since I don't see motors made for 60Hz being any different mechanically than our 50Hz motors. So a 1450 rpm motor mechanicals should theoretically be able to cope with 1450/3450 x 50 = ~120Hz.
The mechanicals of a lathe are them probably the limiting factor of actual maximum rpm but if the lathe is operated at say a lower gear ratio then this effect is reduced.
Anyway - just exploring the origin of the 70Hz.
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23rd May 2011, 10:57 AM #29Distracted Member
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Bob, 70 Hz if an arbitrary figure I've used as an upper limit for 2 reasons: I don't need more top end, I need more bottom end. And the motor is in retirement so I'm just being conservative. Jim Cox suggests 1.5 - 2 x nameplate speed should be quite safe. And I'm guessing he's being a bit conservative too. See "Electric Motors", Workshop Practice Series No 16, p62.
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23rd May 2011, 11:11 AM #30.
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