# VFD Drives



## metaldestroyer (Jan 30, 2011)

Just wondering how popular these are with users in this group. I have a 1.5 HP drive on my 1 HP 745 Wells Index milling machine and really enjoy it, beats the heck out of changing belts all the time. It also let me increase the maximum RPM by 50% which is nice for carbide cutters or very small HSS tooling. Next will be a 2 HP drive and 1.5 HP 3 phase motor to replace the 1 HP single phase drive on my gear head lathe. Again I look forward to that 50% increase in RPM. What innovative ideas have other users come up with that can only be done with a VFD conversion?

Jack


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## mu38&Bg# (Jan 30, 2011)

I have one on my lathe and mill. I had a DC variable speed on my previous lathe. The best thing is having 3phase 220 motors you can get a VFD that is 220 single phase in and 3 phase out. This is the main reason for me. It's cheaper and more versatile than replacing the motor.

Greg


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## nh_eng (Jan 30, 2011)

Interesting topic and very near and dear to me. IMHO VFDs are a double edged sword. Although they increase the speed range for any piece of equipment, they defeat one of the greatest benefits of gears and pulley systems which is a proportional increase in torque at low speeds. With any gear or pulley system, any reduction in speed is accompanied by a comparable increase in torque. This is lost on VFDs as any decrease in speed also carries with it a comparable decrease in torque. 

Given these limitations, however, in many cases, a machining operation at a lower speed does not necessarily require an increase in torque, so in these cases, the VFD provides a solution and fills a need, but it is not a substitute for changing a gear head or moving a pulley drive belt. Like everything else in life, use common sense. A VFD drive, properly used in conjunction with the existing mechanical speed advantage mechanism on a piece of equipment provides the best of both worlds. Go for it!

Paul


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## shred (Jan 30, 2011)

I have 110-to-220 VFD's on everything with a 3 phase motor-- Mill, Surface Grinder, T&C. Love 'em. These are converted belt machines, so if I need more oomph, I still have the belt-swap option-- the mill needs it every so often, the grinders not so much


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## wareagle (Jan 30, 2011)

I have a VFD on my mill. Probably the best enhancement made to the machine to date. The lathe has a single phase motor currently, but when it goes the way of the Dodo then a three phase motor and VFD will be put in its place. Until then, it isn't broke...


For what it is worth: I have read that three phase motors will yield a better finish than a single phase motor will. Whether this is true or not, I don't know and with my meager skill level I wouldn't know the difference anyway. It would make sense that the power is smoother on a three phase versus a single phase. 

When I added the VFD to the mill (from a static converter), my finishes improved drastically, however I attribute that to having a better range and control for feed/speed.


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## mu38&Bg# (Jan 30, 2011)

I set my low frequency to 15Hz, but still change belts when necessary. The loss of torque is evident when a load is present. I don't bother with belts when cutting aluminum.


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## bearcar1 (Jan 30, 2011)

I rather like the ramp up/down features of these devices. The VFD I have for the mill has it's programming/function buttons built into a small removable control panel that I plan to mount on the factory supplied switch box. It can be mounted up to 20' away from the main unit.

BC1
Jim


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## John Rudd (Jan 31, 2011)

My Chester 9*20 lathe has a vfd...Big improvement over the stock motor..

I generally run with the main belt connected direct to the motor pulley on the 2nd step...Gives me 0 to around 2500 which is more than fast enough for a 125 mm chuck...( not that I run it at that speed...!! )


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## dvbydt (Jan 31, 2011)

Don't foget the convenience of reverse. On my 12 x 36 lathe, even with a big 4 jaw chuck, the VFD reverse will cope with 1000 rpm. On the mill, it makes tapping under power a cinch.

Ian


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## Majorstrain (Jan 31, 2011)

Yep I run a 415V 3 phase input 3 phase output Danfoss VFD on the Bridgeport and retained the belt drive for low RPM torque. 
It also has a brake resistor option which stops the old girl real quick. 
The 12 x 36 lathe runs a Danfoss as well but is 240V single phase input. I'll probably put a cheap Chinese VFD on the 6x9 band-saw when the motor gives up the ghost.
Cheers,
Phil


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## Ken I (Jan 31, 2011)

You can also use a VFD to crank out more power via more revs.

For some reason the VFD manufacturers (and I am an agent for these things - Yaskawa brand) flat line the voltage at the rated frequency - so as your revs increace over nomininal there is a reduction in torque because of the increaced impeadance - ie the power remains constant.

I can see no good reason for this other than (maybe) exceeding the insulation classification (someone is probably about to give me one) as current and torque are synonymous so I see no problem in doubling a motors output by doubling its revs at its rated torque.

I have gone as far as to take a 4 pole 5kW Siemens motor (50Hz - 1450 rpm) and goosed it up to 20kw @ 200Hz 5900 rpm - I had to rewind it for 98V @ 50Hz (effectively 380V @ 200Hz) - this has been running in daily production for 10 years - so where's the problem ? (Siemens "wouldn't gaurentee it" - no surprise there as I was taking a brand new motor and rewinding it I didn't expect them to.) The reason being we needed more power in a restricted space.

To do this you need to know the y=mx+c slope relationship (although 10% for c seems to be a reasonable rule of thumb)

So if you have a one horse 3 phase 220v 60Hz 2 pole 3580 rpm (just short of synchronous 3600 because of slip) you can go say to about 6500 rpm at 380V @ 100 Hz for about 80% more power.

The current has not increased so no increase in heat and the cooling is boosted by fan speed in any case. (Yes there are increaced iron losses with the increaced frequency - don't fret they're relatively small.)

You obviously have to change your primary belt ratios to get the increaced torque otherwise you are only going to get more revs.

Certain cautions obviously apply - can your system handle the increaced power ? Will the insulation handle the increaced voltage and will the rotor / fan handle the increaced revs - in most cases you can safely double the revs.
Asking the motor manufacturer is a waste of time - they will simply say no - if you probe them technically you will find they don't know any better - however - if you can find competent technical person (not a bloody salesman) heed their advice.

Aircraft typically use 400Hz motors and there are high frequency industrial tools available - all for the same reason - to get more out of a smaller motor.

Ken


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