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Author Topic: motor tuning for vfd spindle  (Read 10805 times)

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motor tuning for vfd spindle
« on: September 10, 2008, 05:28:58 PM »
hi all,
 i am attemping to tune my spindle motor, i have a vfd and 3ph motor for the spindle on my lathe, the vfd is bieng fed an analogue 0/10 signal with a step to volts card, which is bieng fed by mach3 steps.

i am experiencing some stuttering with the motor , like an engine mis-firing,  its more prominent at some rpm than others. it could be a motor mount plate issue not fixed proper etc,  or centrifugal balance issue etc.  I want to talk about the software side / mach3 first as i'm new to configuring it and there is alot of stuff to take in. 
the instruction for the step to volts card / C6 is to set 1000 steps per revolution in the config/motor tuning / 'steps per rev' box . so i did.

I tested the voltage at the vfd input at my max rpm and it was 9.96V. producing 2500rpm, great.
it accelerates well, generally seems o.k but for the small knock.  its nothing major, but somehting aint right.

as far as steps produced at max rpm:
1000 steps per rev = 2,500,000 steps per minute! @ 2500rpm  , correct?
so 2,500,000 / 60 = Hz = 42Khz correct?

now if my kernel speed in mach is set to 25Khz, is this going to affect the smooth running of mach or this spindle motor if its pumping out an actual 42Khz to control the spindle. did i get that right?

something else to mention here is i have a 100cpr index pulse  / timing plate on the spindle, this works o.k  as far i can tell relaying a speed to mach it only fluctuates from 2495-2500 when calling out for 2500rpm.
any ideas?


« Last Edit: September 10, 2008, 05:35:59 PM by hutchison »

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Re: motor tuning for vfd spindle
« Reply #1 on: September 10, 2008, 06:15:41 PM »
If your Kernel is set to 25KHz and any axis requires more than that for its top speed then that axis will never reach its top speed. Same for the spindle. If you need 45KHz for one axis you need to set it to that even if the other axis dont require it.
 Have a look at your pulse width and set it to maybe 3 or 4 and see if that makes a difference.
Hood
Re: motor tuning for vfd spindle
« Reply #2 on: September 11, 2008, 02:39:35 PM »
I checked the kernel speed out sure enough it was set at 25000.  how can it run at 2500rpm then?, it sure did!.  I have now changed it over to 45Khz, so all's theoretically fair.
 
I changed the motor pulse time to see if that made a difference on the vibration, it pretty much didnt, all that happened was to increase or decrease the spindle speed,  it didnt change the vibration.

I took off the drive belt from between spindle and motor and tried again,  behold it was deathly quiet, apart from the inverter whistle!.  I think I can concllude that the belt was tracking off and up the side of the timing pulley flange, the lumpy profile of the 5MM HTD belt was just 'biting'  on entry to the pcd form on the pulley, if you get what i mean, a case of mis-alignment.   Its caused from the OEM motor mount plate bieng fixed in only two places, which is quite lame, but then I guess it wasnt intended for a heavier motor and drive, and also was driven by a plain old V belt so there were no notches  or teeth to mis-bite.

The OEM motor mount plate is 4mm thick, it's thick enough but it only has two points of contact inside the machine, one to pivot on and the other one slot/bolt to nip up once you have taken up the belt slack.  I could physically move the motor if i pushed it hard and it wasnt sitting parrallel to the spindle, so the belt was just running off to the edge without the 3rd point of contact to guarantee parrallel.

So next stop is to machine up a larger plate, so I can fix in three place at least, where arrow is!.  I wasnt oblivious to the flexing issue,  i just wanted to check the software side, thanks very much for the advice :-) .
suprising they got away with it before!








« Last Edit: September 11, 2008, 02:45:00 PM by hutchison »

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Re: motor tuning for vfd spindle
« Reply #3 on: September 11, 2008, 03:12:34 PM »
Good news you found the fault and as you say its often hard to decide whether its mechanical or software that is to blame.
Hood
Re: motor tuning for vfd spindle
« Reply #4 on: September 12, 2008, 05:35:33 AM »
i guess then that the spindle pulse time period is directly related to the output speed ,  how then if the pulse time period is increased does the motor speed up in the same time frame?   

is it like an inertia thing? it pulses power on and off, with the off period staying the same and the on period increasing? 
how can this be applied to different motors for tuning...

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Re: motor tuning for vfd spindle
« Reply #5 on: September 12, 2008, 05:52:45 AM »
Basically you have to set the pulse width to suit the drive that needs the widest then you tune all of your motors at this pulse width. I have seen reports lately that the CNC4PC spindle control boards are sensitive to pulse width and if you have it too low then jogging an axis can make your spindle speed fluctuate, for these guys setting the pulse width up a bit solved their problems. I think also there was another speed controller being used that was also very sensitive to the pulse width, setting the pulse width correctly for these devices cured the problems, but cant remember what type it was :(
Hood
Re: motor tuning for vfd spindle
« Reply #6 on: September 12, 2008, 05:42:17 PM »
 at the same time came across another problem,

  I cannot sync the commanded or actual speeds together no-matter what I tried.  I tired various pulse widths, calibrating the spindle, trimming some millivolts on my analogue output card.  Best I could get was 2490 for 2490rpm! but..  at the lower end of the scale betwen 2-300rpm its as much as 100rpm out. for some reason , i think it may be the 100 hole timing plate. 

I noticed that below 2-300rpm the actual spindle speed read/displayed jumped about wildly in the dro when calibrating, it took a long time to move on as the speed would not settle. Sometimes leaping upto 3300 then next increment would be 65 rpm , it was just going haywire.
   The Index pulse plate has 1 ring of 100 holes as displyed in the link, and note the single notch at the top.  The index pulse for mach is taken off the top notch and a proximity sensor as norm, and signals to mach when the notch passes the sensor, its active low. Then the other 100 pulse holes, i had picked up through the other proximity and serial interface but havent plugged them into mach yet, i'm not sure if i could use them for anything.  any more ideas, very grateful :-)   i wonder if a wider slot will help,  what seems odd is i thought it would have problemswith top end speed reading not bottom end where it has a longer time frame to read the pulse.  So far my spindle has broken mach law twice!.  the slots are 2mil.
« Last Edit: September 12, 2008, 05:50:25 PM by hutchison »

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Re: motor tuning for vfd spindle
« Reply #7 on: September 12, 2008, 05:47:56 PM »
Can you replace that with a plate with only a single slot of 10mm or so width, think you will have better luck that way.
Hood
Re: motor tuning for vfd spindle
« Reply #8 on: September 12, 2008, 05:51:35 PM »
sure can, i'll give that a go and see if it helps, |I have faith in a wider slot.  does having 100 slots not make it more accurate for screwcutting, forgive me for thinking traditionally of dropping it on an ainjest /multi-starts..?  i guess mach can cope with accuracy using timing alone? , i worry too much, but its good to know !
« Last Edit: September 12, 2008, 05:54:43 PM by hutchison »

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Re: motor tuning for vfd spindle
« Reply #9 on: September 12, 2008, 05:59:19 PM »
Single slot is probably best for Mach and it needs to be fairly wide, its not so much Mach but the parallel port I think. SmoothStepper should change all that  :)
Hood