Making Spindle Speed on the NM145 more accurate

[cteufert]

After upgrading to the new VFD I found the spindle speed on my NM145 seemed a bit slow. I ran some tests to see how accurate the spindle speed was compared to what was programmed.

I connected a contact RPM meter to a collet on the NM145 and ran a test. What I found was that when I programmed for 6000RPM I only got 3990. More interestingly the speed was too slow over the entire range of 400 to 6000 and it jumps in steps. For example programming for 400 RPM got 400 but so did programming 550 RPM. 600 got 480 as did 800. 850 jumped to 660.

I found it was possible to change the configuration in Mach3 to allow higher RPMs and make the spindle speed close to what is programmed. It doesn't resolve the fixed steps the spindle spins at though. That seems to be a either a limitation of the Sangmutan board or how Mach3 generates the PWM signal to the spindle. I suspect it is a limitation on the Sangmutan VFD since I tweaked numerous settings on Mach3 and always got the same RPM steps no matter what I changed.

To reconfigure Mach3 to cause the spindle to spin at higher speeds and be closer to the programmed speeds you will need to modify the PWMBase Freq setting on the Ports & Pins : Spindle Setup screen. Mine was initially set to 1400, I changed it to 1000. While there, verify the Minimum PWM is set to 5% to allow programming of speeds from 400 to 6000. (Actually 5% will allow from 300 to 6000 but the VFD only goes down to 400) Don't forget to APPLY the changes and choose OK.

I have attached an image of a spreadsheet and chart that shows the difference the change makes. There are three lines on the chart, one is the command or the speed programmed. Ideally the spindle would follow this line exactly. The other two lines are the readings I took from my RPM meter with the PWMBase Freq set to 1400 (original) and 1000 (new). You can see the stair steps in the actual spindle speed. While it would be nice to have more precise control of spindle RPM, especially at lower speeds where the speed jumps 40% from 480 to 660 it really isn't any different than using a Bridgeport mill with stepped pulleys. You just need to know the actual RPM and make sure your feed rate matches.

Charlie.

[cteufert]

I did some additional testing and found that the spindle speed consistently steps in increments of 180 RPM except the fixed minimum speed of 400 RPM. I found 31 speeds from 400 to 5880RPM plus 0 for a total of 32 different speeds... 32 happens to be a nice binary number so the steps are very likely caused by the program running in the VFD.

A bit more tweaking in mach3 will likely make the programmed speed choose the closest speed step on the VFD. Just takes too long to run the full range RPM test and sometimes I actually use the mill to make parts...

Charlie.

[ihavenofish]

cool. is this with the old vfd or new one?

ill check mine later today to see what the pwm has been set to.

[cteufert]

My results are with the revised VFD.