Nomad Spindle Speeds are Incorrect

I am in the process of converting my Nomad to use LinuxCNC (machinekit) with a BeagleBone Black + CRAMPS stepper interface board. As part of this, I had to wire the spindle motor driver to a PWM from the BeagleBone, and wanted to confirm correct operation. So, after debugging and getting everything working, I have discovered that the measured spindle speed ranges from ~1500 RPM to ~7000 RPM, not the specified range of 2000 RPM to 10000 RPM.

Here is a link to a video showing my setup using a hall effect sensor and a oscilloscope:

and here is a Google sheet with my measured values:

Has anyone else seen similar problems?

Never bothered to check before. Just did (opto interrupt disk with ten slots and a fluke freq counter, so no load speed).

at S2000, about 1600RPM, at S10000, about 9100RPM. Not as close as I would like, but about as good as I have seen on other machines (large and small) without closed loop control.

EDIT: measured at the spindle… the disk is on a 3mm shaft. It normally is on my lathe for the closed loop speed control. It is accessible for changes because I made it that way…

That’s because the pulley’s between the the spindle and the spindle motor are not the same size. The reported spindle speed on the nomad is the speed of the spindle, not the motor.

Here’s the motor datasheet, by the way:

And the power chart for the stock motor:


I took the gear ratio into account; motor side has a 50 tooth pulley, spindle is 20 tooth, for a ratio of 2.5:1, and therefore the actual motor speed ranged from 600 RPM to 2856 RPM (see my spreadsheet)

I took a tach to mine based on your comment above, and I see ~ 9350 at S10000. Close enough for me I guess.

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I think it is closed loop - the second “slice” on the back of the motor is a position encoder.

Closed loop with the motor control (the BLDC controller), but the input to that is PWM from the GRBL board, with no direct speed setting information, unless I am missing something.

There is a setting for duty cycle to rpm. It’s not precise, but it’s not awful.

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