Has Anyone Ever Run a 4-Pole 3HP Motor at 5000+ FPM with a 3HP VFD?

T

The Mighty Ginsu

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My 3HP VFD will not run a 3HP 1800-RPM motor and a 4" drive pulley fast enough to get 5000 FPM. There is just no way. I could put a huge drive pulley on it, but that would cut the torque.

That leaves me with a question: has anyone here actually done it? It's starting to look like it's a fantasy.

I notice that KMG sells 2-pole motors, not 4-pole.
 
FWIW my 2hp VFD and 2hp 1800 RPM motor with 5" drive wheel run at 4700 FPM (120hz). Plenty of torque and speed for grinding my full flat Chef knives. On the 10" wheel I can't even slow it down at full speed when hogging and profiling. I can slow it if I force it on the flat platen, but this doesn't occur in my normal knife making. I would imagine with your 3hp motor you wouldn't have a problem, and if you really feel the need for speed use a 6" drive wheel. Should be plenty of torque in real world use.
 
No "huge" pulley needed. Just change the drive wheel to a 5" or 6" wheel. At that size and double sped from the VFD it will scare you in use. With a 3HP motor, torque isn't an issue.
 
The problem is that my TECO 3HP VFD will not power a 4-pole 3HP grinder at anything above 130Hz. You can set it higher, up to 400 if you want, but it quits. Even at 130, it will react badly when pressure is applied to the workpiece. Apparently it causes a DC overvoltage. Disabling deceleration and braking don't help.

A larger wheel will turn faster, but at 6", I will still need 109 Hz. A 3600-RPM grinder would only require 54 Hz. With a 4" pulley and a a 3600-RPM motor (actually 3450 to account for slippage), I would only need 83 Hz.

I went over to Practical Machinist, and they told me it was crazy to expect a 4-pole motor to run at 167 Hz, which is what you need to turn a 4" pulley fast enough to get 5000 FPM.

It really looks like 3600 is the way to do it, but I am not an electrical engineer.

I would just like to know whether anyone has actually made it work: 3HP motor, 4-pole, VFD, 5000+FPM. I have a feeling no one has.
 
You only set it at 120 Hz.

A 5" wheel, 1800RPM motor and 120Hz from the VFD would run the belt at 4700 FPM.
A 6" drive wheel would be a screaming 5650 FPM.
 
Listen to the folks - they got GOOD advice. Don't worry about power/torque - put a 6" pulley, set freq at 120 hz, with either a platen or 10" contact wheel, if you can slow the motor any hogging, I'd like to see a video of that. More power than you'll ever use.
 
The Mighty Ginsu said:
But...has anyone here DONE it, with 3 HP and a 4-pole motor on a 3HP VFD?
Click to expand...

I run a 3HP rated VFD and a 10:1 inverter rated Leeson 4 pole motor at 150 hz without problem. I think the motor is 2HP.

My Romi machining center has a 7 HP three phase spindle motor that runs at 10,000 RPM. For some reason I'm thinking it's actually a six pole. I'm not real sure how that works. But some large three phase motors can be run very fast with the right electronics.
 
Is there a particular reason you think you need that much speed and horse power? I run the same set up as StuNY and have yet to feel like I'm lacking anything. Granted, I'm not trying to push my grinder through the wall, or grind 20 blades in an hour, but I've yet to stall or slip a belt. Now, I use a direct drive machine, which may factor in a little bit, but still...
 
I have 1,5kW motor (I assume it's 2HP) on My grinder with VFD set up to 100Hz. We have 50 Hz in power grid (CE) and 2-pole motors have about 2860 rpm (My one for sure), 4-pole motors about 1900 RPM. It is possible to run My motor at 5900 RPM but I never needed it. Usually I run it at 50 Hz (about 19 m/s at belt [cca 3780 SFPM]), sometimes at 65-70 Hz (about 25-26 m/s at belt [cca 5100 SFPM]). I have had 145 mm wheel before and I decided that it was too big for My needs so I made another smaller one. Now I have 125mm wheel (I assume it's 5") and there is enough variability to grind steel, wood and any other materials used for knife-making. I know few knife-makers at Czech knife forum that they use 4-pole motors on their grinders (with 3-stage pulleys or VFD) without any problems. I hope it was helpful.

Regards, Rene.
 
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So apparently no one has done it with a 3HP 4-pole motor and 3HP VFD. It appears to be impossible, so I should either go to 2HP or get a 3HP 3600-RPM motor.

It's good to know these things so you don't waste time chasing unicorns.

knife to a gunfight said:
Is there a particular reason you think you need that much speed and horse power? I run the same set up as StuNY and have yet to feel like I'm lacking anything. Granted, I'm not trying to push my grinder through the wall, or grind 20 blades in an hour, but I've yet to stall or slip a belt. Now, I use a direct drive machine, which may factor in a little bit, but still...
Click to expand...

I tried to come up with an FPM figure based on what other people said they were doing. Some guys claimed 7000; that seemed a little crazy to me. Others claimed lower figures. I chose 5000 based on what I read.

Right now I'm just trying to find out whether there is really any point in having an 84-pound motor that takes up most of my grinding cart, when I might be better off with a 40-pound 2HP motor that actually works better. I can make the 2HP motor turn at 167 Hz with no problems.

Today I'm making a new drive wheel for the smaller motor so I can compare its performance to what I'm getting now.
 
Your torque of a 2 hp (or 3 hp) turning at 167 hz with a 4" drive wheel compared to the same motor turning at 120 hz with a 6" drive wheel is going to be close to same. In fact, you might even have more torque at 120 hz because as the freq gets much past 120 hz the efficiency is going to start falling like a rock!

Your idea of making a new drive wheel (6" diameter?) is a VERY good idea.

Ken H>
 
Surely use 3HP motor with 6" drive wheel if You can. You will don't want back to 2HP after several uses.
 
Is there a prize for biggest screwup on the forum? I think I want to enter.

After wasting all this bandwidth, I discovered the motor was wired for 460. That should have been the first thing I checked, but it slipped my mind. All the motors I've bought in the past were wired for 230. When this one arrived, I briefly looked at it, and I assumed the three big wires spliced into the leads were hooked up for 230.

The wiring has been changed. The motor runs great at 120 Hz. It eats metal like a wood chipper eats trees.

Sorry about that.
 
Glad you found the problem - we all make "dummy" type mistakes. That "should" have been one of my first questions was to check motor wiring.

Glad you're now in business.
 
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