Question on lathe speed ranges

[calblacksmith]

Hi all,

I need some tech advice.

The PM 90 is itching to be updated (or is that me that is itching to update it?) I want to put a VSD on it. It has a 1hp 3phase motor, fan cooled and not rated for a VSD (they didn't have them in 1957 when it was made)

 

The lathe has the stock variable belt setup. I would like to replace that whole thing with perhaps a step pulley setup, two ranges as I really have doubts about running the motor over the whole speed range without stepped pulleys.

 

I was thinking of going from about 100 rpm to 2000 rpm on one setup and 2000 to the machines 4000 on the second step. Part of the problem is that the motor is 1750 rpm. If it were 3600 rpm, it would be no big deal, just put a single set of pulleys on so that the shaft is driven at 4000 rpm when the motor is at about 6000 rpm, this would lower the bottom end to an acceptable range to keep torque. I really do not want to buy another motor if I can help it.

 

Is this on the right track? Are the steps too big or too small or not needed at all?

 

I have been looking at the VSDs for about a week and think I have a unit picked out from Automation Direct. I have a stand alone digital tach already in the mail, accurate to 1 rpm to 9,999 rpm so I will have the actual readout of the spindle covered.

[wtnhighlander]

You didn,t specify what model of drive, but bear in mind that torque drops off in a sort of bell-shaped curve on either side of the motor's rated speed. With a volts/herts regulated drive I seem to recall +/- 30% of rated speed being the usable torque range. If you have a closed-loop drive, one that REQUIRES the pulse encoder to operate, I have seen manufacturer's claims of maintaining the motor's rated torque down to zero RPM. Don't recall what the upper range was. However, that was a complete drive and motor system, designed for variable speed. Don't expect to get the same performance frm the 1957 vintage motor. Unless you add a seperate means of cooling the motor, I wouldn't recommend it at all.

Not meaning to rain on your parade, just wanted to point out some nasty surprises you may encounter. Take appropriate precautions, and reasonable expectations, and I'm sure you will be happy with your new speed control.

[calblacksmith]

I understand about aux cooling requirements as the fan on the motor will not be turning fast enough to provide proper cooling, got that covered with forced ventalation.

I am looking at a sensorless vector control

http://www.automationdirect.com/adc/Shopping/Catalog/Drives/DURApulse_-_GS3_%28230_-z-_460_VAC_Sensorless_Vector_Control%29/DURApulse_-_GS3_Drive_Units_%28230_-z-_460_VAC_SLV%29/GS3-21P0

 

which does maintain torque to a larger range of speeds than controls without the feedback. I am just asking what is the useful speed range that I need to shoot for, it seems that it would be the ideal situation if I could reach the 100 to 1800 zone in one step. Though that is a 10 to 1 reduction at the bottom end from name plate rating. That could be made up for somewhat by a drive ratio that puts the motor into over nameplate RPM at the top end of 1800, this would shift the torque curve down so that a lower range of spindle speed would still have a  higher RPM and thus higher up on the bell curve for motor torque.

 

Though it may not be possible to achieve the desired results due to a wide spread of speed requirements, what is the goal? As I have really NO use on a lathe other than to make pens on a HF vari speed unit, and some small, under 4" bowls on the same lathe, what is the useful speeds need for work on a 12" lathe that can really work on 12" and larger in the drop ways section, 17" if I am correct from memory?

[Rick A McQuay]

My lathe is only 9" swing but low speed is about 240 RPM which I find useful at times. So 150 - 200 seems like a useful goal; if you can get it a little lower then so much the better but not critical.

[wtnhighlander]

I think shifting the speed range upward is a good idea. With a sensorless vector control system, you will still have significant torque loss if you try to go to 10% of rated speed. I'm not a turner, so I can't say how important that is to the task. Sensorless controls perform much better with a motor that is designed for that application. If you haven't purchsed the drive yet, you might consider a 'sensored' vector control instead. It will probably perform much better with that old motor, and you wanted a speed feedback anyway, correct?

[calblacksmith]

Well, I just laid down the credit card on the controller, the same one in the link above because it is a vectorless unit. I contacted the seller company tech support and discussed the details. I found a motor rated for Inverter duty at 10:1 constant torque, 1 hp for $128 shipped. The tech support said that I can go to 10% of the name plate rating of that motor RPM, and still keep full torque (because of the vectorless drive and that the motor is rated as it is). The motor name plate rating is 1200 RPM and max speed is 2700 RPM ( the whole world changes when you get into these drives. As the speed goes over name plate, you start to loose torque on the other side of the bell curve) So with a 1:1 and 2:1 step pulley setup, I can run the spindle from 120 RPM to 2000 RPM on the lower step and only start to loose torque after 1200RPM. The second setup will take me from ~ 600 RPM to 4000 RPM, with the same torque loss above 2400 RPM.

 

Anyway, it looks like this is going to work. I have the digital tach at home now and I only need to make a motor mount for the new motor on the lathe and wire it all up. All in, it is going to be about $1K, I suppose I might be able to buy a used one in that ball park but I may need to wait a while to do it.