Newton Tesla Motor units

If you have a Newton Tesla Speed controlled motor on your lathe can you tell me if the RPM read out on the unit is directly proportional to the machine speed?

What I am thinking hear is, can I use a formula to work out the actual vs. displayed rpm or do I need to fit a rev counter on my lathe?

Fitting a rev counter is the best plan, especially if you have to calculate with gears involved/

Emgee

A cheap revcounter is easy to fit, using a Hall effect device. Noel.

A rev counter sounds good but fixing places on a Myford ML7 is limited. That’s why I was hoping to to use maths.

The read out on the vfd is frequency, but machine speed depends on belt ratio and back gear settings, so you'd need a chart with 6 lines. If you can fix a magnet to the mandrel somehow you could make a tacho like I described some time ago. PM me if you're interested. Or does anyone know of a sensor which would detect gear teeth (Myford size)

Do you really play Eb bass, or is that just your avatar? I used to play bass trom

Edited By duncan webster on 17/03/2022 00:03:04

The quick answer is yes, there will be a relationship from which you can estimate the RPM.

But this will depend upon the type of VFD, (vectored or not), the various loaded parameters and likely only valid for no load or constant load conditions.

The other question is WTF do you need to know lathe RPM to any degree of accuracy?- it tells you nothing.

The approx speed is usually more than adequate if you don't want to fit a spindle counter.

This was achieved in my case by using a linear control pot and setting the min and max frequency of the VFD to give the desired min and max motor RPM in line with manual belt speeds at min and max pot positions (usually 1400 to 1500 for a 4 pole).

Then print out a 0 to 100 dial for your control pot, so the approx spindle speed can be determined by the dial percentage times the max speed at any particular belt setting, plus or minus say 10%. Therefor if your pot setting is 40% and the full speed would be 600 RPM at the given belt setting, then 40% of 600 would be about 240 +/- 20 or 30 RPM.

As a caution, I would not drive the motor for high load operations at any less than about 20% of full pot deflection. If you have a fancy vectored thrust unit, you may be able to go to lower frequencies without cooking your motor.

I would add, that my experience has been the lower the frequency, the larger the range of  speed variation under load. But then I am partial to the cheapest and nastiest VFDs money can buy, so your mileage may vary.

Edited By Martin of Wick on 17/03/2022 10:07:59

Edited By Martin of Wick on 17/03/2022 10:12:55

I just bought a hand held tachometer and use that if I need to know the exact speed. I did use it to construct a graph of actual speed vs frequency setting - one line for each belt position.

I do have a tachometer fixed to the milling machine and that is useful, however you soon get used to setting the correct speed by "feel".

Data from my just fitted NT unit on my Myford S7. Dial is calibrated in rpm, from just below 200 up to 1800. However this would be rpm for a synchronous motor - actual max frequency from the unit is 60 Hz (from the manual).

The lathe plate gives spindle speeds for different pulley combinations for 50 and 60 Hz.

Actual measured speeds at max frequency are as follows for the lower speed range..

Actual Plate
288 250
396 360
542 505
748 720

These speeds were measured by Mach 3 from the spindle sensor.

Obviously if you have a lathe with different pulley ratios the numbers would be different but basically if you increase the maximum speeds in each "gear" by 20% (i.e. 60/50 - 1) and scale that by the dial setting/1800 you will get pretty near.

BTW to fix the spindle sensor plate I bought a new "split nut" to fit the back end of the spindle and sandwiched a slotted plate between it and the standard nut that holds the spindle in place. Can post a photo if you want. As I took the tumbler reverse off (not needed for CNC threading) I could mount a spacer into the hole for its shaft to mount the sensor.

Thanks for replies. I don’t need exact speeds, this is why I thought maths would work. Looking at the various replies I think I’ll go for measurements with my hand held tachometer and print a scale for the control. First job, order new motor.

I did something like this and printed it on magnetic paper so that it could readily be removed or modified.

Stephen, not sure if you are thinking about getting the NT unit with motor or "DIY". I always intended to DIY but eventually decided on an NT lathe package, on the basis that they supplied a motor guaranteed to fit, and had already done the hard work of packaging & filtering the VFD etc, and probably once I'd got a motor and a new VFD and filters the price difference mght not be that large. I am pleased that I did, fitting the motor was very fiddly even with 2 people, had the mounting foot been different from the original motor it would have been even worse. Once that was done and the control unit mounted on the front of the cabinet it was plain sailing and the difference it makes in reducing vibration and general smoothness is enormous. NT delivery was held up because the inverters were in short supply, but once it came in they moved like greased lightning.

I just set my VFD to show motor rpm. Then when it's showing ~1450 I know that the spindle speed is as per the plate on the headstock. For different motor speeds a rough mental calculation is going to be near enough for any practical purpose. I did initially fit an eBay hall effect cheapo but I now hardly ever bother to switch it on.

There was an article in MEW several years ago about the speed read out display on a lathe, using a ABS brake counter unit, which was easily attached to my Super 7. Basically it used a VW unit, a simple 12 volt power supply and a display unit. It counts the teeth on the 60 tooth Bull wheel. A friend has a magnet attached to the side of the Bull wheel on his lathe and a counter unit all from Ebay for less than a tenner. Mine cost a bit more at the time. John