Lathe Torque vs Speed Comparison, Powermatic 3520 data request

[howardkinkade]

I'm compiling a comparison of lathe torque vs. speed curves. This is taking a bit of research and reverse engineering. I'm hoping someone who has a powermatic 3520 could provide this data (big thanks in advance for your help). I'll make my work visible for others who would like an objective comparison of competeing designs.

motor nameplate data (preferably picture), rated speed, volts, amps. . .

pulley pairs (od of each pulley)

Shaft diameter of motor (this has little to do with torque vs speed data, but helps me assess their design life considerations for the machine)

Also, somewhere on our forum I saw someone had the factory VFD settings for the 'Delta S1' (listing of vfd settings). Can anyone point me in the direction to find this data. This is critical to fully understand the power they are allowing the motor to deliver (normal and allowable over current). It would be interesting if they are also over driving the motor to get more power than nameplate (I suspect they are). My best guess is they are also utilizing the max. mech. speed of the motor (probably 4,000 rpm, rated is 1750 rpm). I can figure this out with VFD settings.

Again, your help is appreciated. I'd get the data myself, but don't have local access to this lathe.

 

[Bill Boehme]

You don't need the shaft diameter if you already have a picture of the nameplate or all of the data on the nameplate since the motor frame size is on the nameplate. Well, most of them ... there are some cheapie imports that don't have complete nameplate data.

You will also need VFD data in order to whether it is operating in constant torque mode or variable torque mode as well as other important information such as sensorless vector, volts per Hertz, or encoder feedback ... or in one case, the drive is a stepper motor with a microprocessor controller (Nova DVR) ... I wish you well on computing torque and/or power vs. speed on that one. 😕

The belt type is also important because power is lost in the drive train. Some belts, J section have a very high efficiency while Reeves drive belts can have a comparatively low efficiency, but your model could get very complex here because things like wrap angle, pulley diameter and distance between centers can have an effect on efficiency.

You should also consider plotting maximum power available at the spindle because torque alone doesn't tell you much.

 

[howardkinkade]

Bill, Thanks. Yep I'm aware of the many parameters that impact the calculations. A few of the big unknowns I have already solved. The VFD is a 'Delta S1' V/Hz drive (Not the Delta machinery company). If I can get the parameter settings for the VFD and the nameplate of the motor I can calculate the no loss torque vs. speed data (with the pulley diameters). I saw somewhere on this forum the parameters for this VFD as factory set by Powermatic (I hope someone can point me in the right direction for this data). For someone familiar with VFD's they could read the settings directly from the menu's on the VFD (that is a lot of work though). The VFD is factory locked, but the settings are readable. I have also seen the unlock code identified too (not needed). The belts are Poly V PJ. Estimates for all the other loss factors are easy to estimate and errors are typically not significant. The issue with power or torque always comes up in these discussions. If we plot torque vs. speed the power is a moot point (HP is function of speed and torque). I personally believe the torque vs speed is more meaningful, since it will give a direct assesment of the potential cutting forces (cutting diameter as a variable) for different speeds. High speeds will typically require less tool forces (small diameter turnings) than larger diameter pieces. The laws of physics help with obtaining this characteristic in lathe design too, not necessarly a one to one correlation though.

I'm interested in the shaft diameter, because the pictures appear to have a none standard shaft diameter for that motor frame size. I could be getting tricked from the picture though. Driving the motor above it's base speed (1750 rpm) does create some issues and potential bearing life derating. They may have had a custom shaft / bearing size to help with bearing life (I doubt it, but maybe). Many motor suppliers limit the mechanical speed of the motor to 1.25 times base speed (due to bearing life issues). In order for the powermatic to reach 3200 RPM they are most certainly driving the speed well above this point. This has additional design advantages by operating in the constant HP region of the motor, too.

 

[Bill Boehme]

Frame size is more than just the length and diameter of the motor housing. It includes shaft length and diameter as well as mount type are also part of the frame size so a manufacturer can't state a frame spec if it doesn't comply with all of the requirements. Normally there are enough suffixes to take care of all of the possible configurations.

BTW, resetting or clearing a password is no big deal (well it might be if you need to reenter all of the parameter settings). Most of the parameters on the Delta inverter used on the Powermatic won't have an impact on your analysis. There are lots of housekeeping things like ramp up and down settings, and miscellaneous things like external switches and pot rather than the ones on the inverter. Things like motor parameters are essential for optimized operation, but don't effect what you are doing (unless you are doing a closed-loop model of the motor). Some things would be worth noting such as minimum stable operating speed. If you find an inverter that is operating in variable torque mode, that would be a very important bit of information.I don't know of any lathe that does use that mode.

 

[robo hippy]

Well, you are speaking Geek to me. I would think there are tons of variables, from VFD settings to pulley sizes, to current variations. It was interesting with my Robust to see how the settings on the converter could be changed for more torque and different speeds.

robo hippy

 

[Bill Boehme]

Well, you are speaking Geek to me. I would think there are tons of variables, from VFD settings to pulley sizes, to current variations. It was interesting with my Robust to see how the settings on the converter could be changed for more torque and different speeds.

robo hippy

Well, a motor is what it is and unfortunately it can't be made more powerful by changing a setting on the inverter. However, a motor can and should be optimized for best performance with an inverter.

 

[howardkinkade]

We are falling pray to showing off what we know. At the risk of continuing this technical chest pumping and in the hope that some confusion and learning is helpful, here goes (also, I really am hopeful someone has the info I had requested ;-)

Yes, a motor can't be made more powerful. But, you can get more than rated power from a motor. This does have negative trade-off of reduced life. Depending on the duty cycle the reduced life could be insignificant compared to gains in performance. With the advent of VFD's you can allow the motor to draw more amps before shutting it down, and you can drive the V/Hz curve to give you more low end torque/power. Lots of little tricks to get more performance. Most people don't realize many motors driving power tools with VFD's operate much higher than there name plate speed. The reason, above name plate rated speed the motors is constant HP, below this speed it is constant torque (reduced power at reduced speed). If you want to vary the speed of the motor for a specific application it's best have your top speed well above this break point. This way when you reduce speed there is not a porportionate reduction in power. Looking at motor name plate data is no longer the only indicator of the power capabilities of the equipment. The design of the whole system (VFD included) needs to be taken into consideration.

 

[Bob Howell]

I bought a VFD drive and motor last year from Automation Direct. Since, I have been reading and discussing motors to better understand what I have. One big thing jumps out is the cost of the motor. I got a medium duty Motor($123) which starts losing torque below half speed. I could have gotten a better motor ($155) that starts losing torque at 1/4 speed but the shaft size was 7/8 and frame size was different from what I thought I needed. All this I now see I could have worked around but I studied it for a month and had to get a motor and move on to turning.

Now I know that the best motor is an inverter rated motor which loses very little torque but 1 hp cost $300+. Most guys are getting the medium duty 2 hp motor ($166) and paying $40-50 more for a bigger VFD. This also requires a 220v plug.

The cost is in the copper which makes the motor run cooler at slow speed as I understand. Also more copper takes more space, so a bigger motor.

But I got back to turning sooner which was very important.

The best forum for these issues is OWWM.org in the Electrical area. Someone here recommended it to me and I pass it on. A lot of members program VFD's at their Day job. I found way more information than I wanted to follow. I enjoy turning more.

I just roughed out two 16" bowls and have to stick to shallow cuts but I can live with it over buying another bigger setup. But if anyone around Atlanta wants a motor...

 

[Bob Hadley]

You can download the VFD manual from Delta Electronics. The model is VFD015S21A. It provides numerous charts and equations to understand how the settings and motor combinations work.