# VFD questions for Clausing 8520 mill



## skipd1 (Jun 7, 2012)

I am contemplating adding a VFD to my 8520 and my question is how big a motor and controller and what speed do you leave the mill on after the VFD in installed??? My mill currently has a 3/4hp 1725rpm motor, but I would like to eliminate changing the belt positions all the time!!

Skipd1


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## burnrider (Jun 7, 2012)

Skip, no mill experience, but this is the same conversion I did on the drill press. 

The 3/4 hp motor has a smaller shaft (5/8?), to bore the step pulley and fit it to the 1 hp which I believe is a std 3/4" shaft- I had a machinist cut it and shave the key. 

Second item is the motor and the rpm. The new motor is 1725 3 phase, but they tend to heat if you run low rpms. The VFD I used recommended use of a computer fan or a TENV (totally closed no ventilation) motor. It's also known as an 'inverter duty motor'. They can run low rpms w/o heating. I installed the computer fan recommended for the self cooled motor installed. It has an internal fan with vents at both ends designed to cool at 1725 rpms. 

Last question, is there an optimum pulley setting strictly for VFD use? No idea. I would reckon the more expensive VFDs with readout can show the load factor and you can set a belt ratio for a range of cutting uses.

A little help from experienced operators on this site can offer more insight.


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## British Steel (Jun 7, 2012)

OK... (opens and stirs can of worms) - I've done a few conversions on belt-drive lathes and mills (mostly Chinese 8250 clones!) as well as my own gear/belt drive lathe, so my opinion:

The VFD/3-phase setup will give you a constant-torque characteristic, rather than constant-power (which you get by shifting the belts). You'll probably end up shifting belts for a couple of good reasons:

When you need more torque (e.g. large cutters, slitting saws, hard / tough materials, pushing a 2" drill through an inch of gauge plate...) you'll want the "mechanical advantage" gained by gearing down the motor;
When you need more RPM (e.g. small cutters, soft aluminium or plastics etc.) you'll end up needing more than the VFD can give you on a medium gear so want to "gear up" using the pulley ratios!

Although a VFD may run from (e.g.) 5Hz through to 100 or 120Hz, at the low end you lose HP (constant torque, power = torque x RPM), and the motor can start to "cog", moving in a succession of jerks rather than smoothly; at the high end power again drops off due to the inductance and iron losses of the motor...

Best approach is to pick a somewhat oversized motor and VFD (e.g. 1.5 HP, 1725 RPM) in the interests of higher torque overall, choose a frequency range that gives you flexibility *in each belt position* - I'll use my own lathe as an example, I run 25-75Hz most of the time from a range of 10Hz up (with 5Hz "jog" for setup and meshing gears) which gives me 11 to 33RPM in lowest back-gear through to 750 - 2250 RPM in highest direct, without too much loss of power at the extremes, and I have an external fan set to cut in whenever the VFD output drops below 30Hz to ensure adequate cooling - it seems to work OK for me 

Just my ha'pennorth,
Dave H. (the other one)


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