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Loaded motor vs bogging motor

dpb

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#1
I have a PM1440GT, with the 3hp 3ph motor, running off single phase through a VFD.
For the vast majority of work, I see a 0-2 drop in spindle rpm when the tool contacts the workpiece. When trying heavier cuts, especially at higher rpm's, I have occasionally seen larger rpm reductions. I don't have any exact numbers, as I chicken out, and stop the cut, but let's say a reduction of 20-100 rpm, on a 1750 rpm motor, turning the spindle at 1000 rpm.
Any guidelines on determining where load related rpm reduction stops, and bogging starts?
Hopefully this made sense, thanks for the help.
 

JimDawson

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#2
You might want to take a look at your amp draw on the VFD readout when it slows down. If it's less the the motor nameplate rating and it's slowing down then some VFD parameter may need adjusting. Another possibility is that the motor is wired for 460V, not likely but something to check.

3HP is a lot of power unless you are removing a massive amount of material on one pass. My motor is 3HP and can run a 1/2 rougher an inch deep, full width, in steel and the motor does not slow down. The highest I have ever seen my spindle load meter is 57%

Maybe revisit the setup parameters on your VFD and make sure they match the motor nameplate data.
 

dpb

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#3
I will look at the amp draw, thanks for the suggestion. I'm 99.9% the motor parameters are set correctly, as I went over this several times during initial set up.
In your response, it sounds like you are using your mill as an example, my question concerns my lathe, I don't know how to compare the workload of an endmill to single point tooling.
An example of the situation in question would be a .150" diameter reduction on a 1" piece of 304, turning approx 1000 rpm, using a TNMG insert and a .009" feed.
I know an electric motor will tend to run faster under no load, than when fully loaded, but I don't know how to relate this to what I see happening.
 

JimDawson

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#4
my question concerns my lathe,
Ahhhhh, OOPS :) OK, yes it would be possible to load up a lathe like that. If you are getting that much slowdown, it could be that the VFD is current limiting or you may need to bump up the slip compensation.
 

petertha

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#5
Another thing to consider assuming you are power feeding under a heavy cuts like this. A healthy power load is also being transferred to your power feed assembly to traverse the carriage, not just the rotational stock removal load. So the power shaft, worm gear, carriage transfer gears... depending on the lathe configuration & maybe ultimately working its way to smallish surface wear contact areas. I'm saying your motor/VFD may not be the first component to say Uncle & replacing those bits are more involved & expensive. I've got an older gen 14x40 Taiwan lathe & having to replace myself.
 

dpb

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#6
Excellent points. I'd be interested to hear opinions about wise working limits for this class of machine.
 

Asm109

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#7
You have problems somewhere. I have a 10 inch lathe with a 1/2 hp 3 phase motor running on a static converter. (Meaning I can't get to nameplate hp)
I can take a .1 inch depth of cut on 1 inch diameter SS bar without the motor slowing appreciably.
 

dpb

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#9
What VFD do you have? Does it have an AutoTune feature?
It is a Hitachi WJ series, I can't look at it right now to tell you the exact model. It does have auto-tune, and I have used it.
 

mksj

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#10
WJ200-022SF. May be the motor, JBOLT runs the same VFD with the Marathon E470 and has no problems, and I am sure he takes some very deep cuts.
1. Make sure you are running sensorless vector control A044 - 03
2. You might change the motor voltage to 230V from 220V, most motors are +/-5%. A082 = 230
3. Make sure the motor Hp (H003) is set correctly and also set the motor overload to something like 140% B022 =140
4. Rerun Autotune (with the belt off) and make sure that H002 is set to 02 which selects the autotune data for the VFD.
5. There is a slip compensation gain (A047) for the automatic torque boost, but I believe this has more to do with acceleration and deceleration.
6. Check the belt tension that nothing is slipping.
7. You could call Hitachi Tech, they may suggest other parameters to tweak

You may just be taking a deeper cut then the motor can handle, particularly at that speed/gear ratio. Also you want the motor to be running at 60 Hz or higher for deeper cuts, the gives more Hp to the spindle. I normally will see a monetary drop in RPM (like 2-5) entering the cut and the VFD immediately corrects, but I use an inverter motor. I do not take super deep cuts at speed, the most I can do on my 1340GT is around 0.1" diameter cut, beyond that you can feel some rigidity issues with the lathe. I can do 0.2" with very low feed rates. With repeated passes and deep cuts I have had my WJ200 stop, with no error message. It will restart, but I have a feeling that I am pushing it into overload for too long and it shuts down.

The other issue is probably the insert and the profile, you are using negative rake tooling TNMG, which takes a lot more Hp to run. I would try running CCMT tool holder with a 32. 51 or 32.52 insert. Make sure the insert is rated for the material you are turning. SS work hardens quickly, so there is probably some sweet spot for your machine. I do not work with it much so can't really comment, but in your scenario above the feed seems to be a bit high. I find that with these "lighter" machines I run my carbide at 1/3rd-1/2 SFM published.

http://www.hobby-machinist.com/threads/the-pm-1440gt-has-landed.50194/page-5
 
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dpb

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#11
Thank you everyone for the suggestions. It actually did end up being loose drive belts. They weren't squealing, and didn't show any signs of glazing, but, having eliminated a number of other things, I tightened them, and presto! No more slow down during heavier cuts.
Special thanks to Mark Jacobs (again) for giving me several suggestions, and answering my questions.
 
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