# Variable Speed Control



## Bamban (Sep 5, 2014)

I have a single phase 1 1/2 HP motor in my 12x36 Taiwanese lathe. While there is nothing wrong with the gear box, is it possible to convert the machine to variable speed control. Would it be just replacing the motor with 3 phase and run a VFD? What setting do you leave the gear box levers to run the fun range speed of the VFD controlled motor?

TIA


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## randyjaco (Sep 5, 2014)

The best way would be to go with the 3 phase motor and the vfd. Most vfd's have a read out that indicates the frequency. 60 cycles would be standard and anything over 60 would indicate (with mathematics) the increase in speed. Anything under 60 would be your decrease. I would not go over 90 or below 30.

Randy


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## Rbeckett (Sep 5, 2014)

Following on in Randy's explanation, then you would have a range of speeds for each gear you selected.  AT 90 Cycles the max speed in the highest gear would definitely be spun up faster than just the standard single phase and standard gear box selections.  And having the adjustment on all of the other gears means you can hit the sweet spot for any particular metal and cutting combination out there.  It also might slow the lowest gear at 30 Hz enough to do single point internal and external threading to a shoulder much more comfortably and in a bit more control.  Hope this helps illuminate that option a little better for you.

Bob


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## JimDawson (Sep 5, 2014)

+1 what Randy and Bob said.


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## Bamban (Sep 5, 2014)

Thank you all.

With my limited experience, the little mini lathe I had for years spoiled me on the speed control. No levers to change, just spin the knob. I am hoping there is lever gearing combination that I can just set on the 12x36 and leave alone for all the work I wanted to do and just control the speed with the VFD. I will still do the 3P and VFD set up and take advantage of its features and capability.

And I thought shooting in competition is expensive.....


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## Hawkeye (Sep 5, 2014)

As the others have said, 3-phase with a VFD is the sweetest way to run a lathe or mill. It won't take long to find your favourite gear to park in. You can always change gears for special circumstances.

I would strongly advise having some kind of tachometer to read the actual RPM of the spindle. I have a laser tach that I got on sale, but i added MachTach readouts to four of my machines. I've ruined far less materials and tooling since I added the tachs. Before that, I usually went with "close enough". It wasn't always.


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## Bamban (Sep 6, 2014)

What information can I use from this motor data plate when I am ready to order a 3P 2 HP motor to replace the 1 1/2 HP 1P motor currently in use in my 12x36 lathe.


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## JimDawson (Sep 6, 2014)

Since you are changing to a 2HP motor, the only thing that is useful is the RPM.  1720 seems a bit odd, but not to worry, that is a nominal 1800 RPM motor.  4 pole, at 60 Hz = 1800 RPM, but there is always slip which is why you see RPM ratings at less then the nominal speed.

I don't see any frame size on the data plate.  I'm sure the motor is metric so the best you can do is match the shaft size or plan on changing the pulley.  Look at the new motor dimensions to make sure it can physically fit in the space you need to put it in.


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## Skarven (Sep 6, 2014)

I agree with the VFD and 3-phase.  I have a VFD on my gear head mill spindle and one on the mill z-axis.  I run both of them down to 10Hz.  That is for our normal 50Hz.
I use the spindle for tapping at the lowest speed with a white index mark to count revolutions. 

As long as you set the max current in the VFD i do not think you will not have a problem with short-time running at low RPM.
For prolonged operation with a fan-cooled motor you would have to provide some extra fan cooling.

I have already bought a VFD for my 13x40 geared lathe mostly for thread cutting. Mounting it is on the to do list.

A very nice thing with a VFD is the soft start, and the jogging function to let you run very slow to turn just a fraction of a revolution.

Kai


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## British Steel (Sep 17, 2014)

I don't think you'll be able to leave it in one gear and just use the VFD alone for speed control, you'd lose the torque multiplication the gears give you so you'd find low-speed, heavy cuts would suffer from a lack of torque, you'll need to at the least use the back-gear! I normally use 30Hz to 85Hz on my 50Hz lathe motor, giving a roughly 3:1 speed range in each gear / motor speed combination (it has a 3-speed motor, not so easy to match a VFD to... plus a 8:1 back gear and a roughly 4:1 clutched gearbox, 12 speeds in all). 

I do run the VFD as low as 5Hz but purely for jogging to position and using live tooling (milling spindle and TP grinder, for instance) where I want the chuck rotating at 2 RPM... The high end gives me the possibility of 2500 RPM for short periods (ok, pump-lubricated but still plain headstock bearings) when turning small-diameter parts, up from 1500 as standard, which is useful and helps surface finish a lot!

The suggestion of a tacho on the spindle is a good one, if you can use a 60-tooth disk (or the spindle gear if you're lucky?) you can buy cheap LED/LCD frequency counters on Ebay and get a better readout as they sample more frequently that the tacho's with a single pulse per turn and hence give a more accurate / "smoother" display. I was blessed with a 70-tooth spindle gear, so ended up swapping the counter's 24MHz crystal for a 28MHz - hey presto, scaled to a 70-tooth gear, same could probably be done in the opposite direction if you had 40/revolution, drop in a 16Mhz etc. etc.

MAJOR CONSIDERATION - if you have a fan-cooled motor, it WILL RUN VERY HOT at low frequencies, the airflow's more or less proportional to the square of the speed! I've added an external fan blowing into and through the motor cooling vents so I don't cook it, if your VFD has a couple of relay outputs there's probably the choice of making one of them OPEN above a "supervisory" (ABB speak) frequency where the motor's fan takes over, so the external fan kicks in below that and also stays running and cooling the motor when it's stopped too - a Good Move! 40Hz is a reasonable figure for this.

HTH, I could go on (near-constant surface speed, anyone? Dynamic braking?)
Dave H. (The other one)


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