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Author Topic: Mach3 Turn VFD Setup Help  (Read 2913 times)

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Mach3 Turn VFD Setup Help
« on: August 24, 2013, 09:09:05 PM »
Hello,
I hope that you can help me.
I hope to learn how to turn threads under CNC control.

I have setup mach3 r3.043.066 on a completely isolated win 7 box. (No network access, no other programs loaded... just windows and Mach3)
The driver test passes.

I have a G540 VFD controlling a sherline 4400 lathe spindle... using the G540VFD setup XML
(It works!!!!! - with (mostly) default settings!)
I have a C3 index board also running correctly, using a 70mm disc with 1 hole in it.
(I can not read speeds over 2500RPM, but I really don't need to...)

The problem is in getting slow RPMs... The slowest I can get the machine to run is ~700RPM.
As I change speeds, the lathe appears to slow down (stopping for lower requested speeds) and then gently speed up to the desired speed.
at speeds under 700, the lathe slows down and stops, but never speeds back up.
If I command S700, the lathe stops and then eventually I can see the spindle move slowly (like 1 or 2 RPM) and then the spindle speeds up to 700RPM.

...I believe that the reported RPM is correct as I also use a handheld tach to measure speed.  It matches the DRO.

I am assuming that since the stepper motors I have on the machine can only do 20IPM (on a good day!) - I usually have them set to 15IPM - that 700RPM is waaaay too fast for threading.... I had thought that speeds in the 100-200RPM range were desirable.

I have the PID controller selected in the feedback loop, with the default values (0.25,1,0.3)

From watching the behavior of the machine, it _should_ be capable of these slow speeds, as it appears to achieve these speeds when finding a higher speed.

Any suggestions?
Any ideas would be welcome. I hope I am just doing something stupid, which is typical for me  ;D
Thanks for reading!
Bruce
Re: Mach3 Turn VFD Setup Help
« Reply #1 on: August 25, 2013, 04:21:56 PM »
One of the problems with VFD (AC variable frequency drive) is they run in volts/hertz mode, which means losing torque at low RPM due to reduced output voltage as the frequency drops to reduce motor rpm. More expensive VFD's use vector mode to prevent this problem. So as you are seeing... at low rpms can cause the motor to stall or have unstable speed. Much depends on the motor rated RPM. The option is using a gear reducer or pulley to keep the motor rpm in a higher range, go with a lower rpm motor or get a VFD that has vector mode capabilities.
Re: Mach3 Turn VFD Setup Help
« Reply #2 on: August 27, 2013, 06:07:47 PM »
Just to complete the thread....

i figured it out.
1.  I changed the belt pulley to the slower one on the DC motor.(Biggest change)
2.  I tuned the PID in Mach for the best startup and stability (took the most time)
3.  I reduced debounce to 0

Running the wizard for thread cutting, (and cutting air), the stability of the motor appears to be great (withon 2RPM of target at all times)
I've tested down to 250RPM, and appear to be stable, and within the good portion of the torque curve of the Sherline motor.
I had to add a 'G4 P10' after the 'S250' command to give adequate time for startup and settling at desired RPM.

I will report more when I understand enough about threading to actually cut something that is testable against a standard.
VR,
Bruce
Re: Mach3 Turn VFD Setup Help
« Reply #3 on: August 31, 2014, 06:26:13 PM »
Just curious as to how you tuned PID in Mach3 for closed loop spindle control. How quickly does the spindle rpm settle on the commanded speed in Mach? I know it has been awhile but also you've had some time under your  belt.