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General Mach Discussion / BLDC Motor Control

« on: September 17, 2009, 10:06:34 AM »

I am in the UK and have access to a KX3 Machine supplied from China which is fitted with a 1000W BLDC motor and uses PWM for speed control.  The machine has a 1:1 direct belt drive to the spindle.

 We have had a lot of trouble getting the motor to run over the full  range of speeds.  We were told it ran from 100 to 5000RPM.  Initially with the setup file supplied it would not run or would accelerate up to 6000 RPM.

To cut a long story short we have got it to run by resetting the pin settings and adjusting the PWM Base Frequency higher & higher.  We now have this at 900Hz at 35000 kernel speed this gives about 38 speed settings. The pulley is set for 0 min to 6500 max.

The pulses from MACH 3 are clean (0v to 3.4v) but the pulse train has gaps in it.  (processor?)
The pulses out of the breakout board (Maker not known) are not so good and only 1/2volt high.
The pulses going into the BLDC Motor control look like noisy mains hum! with a 250mv amplitude.

My problem is that the results are not linear and the motor starts at 350RPM.

                      MACH 3     Speed RPM   
                      s600         0
                      s700         350
                      s800         425
                      s1000       650
                      s2000       1600
                      s3000       2750
                      s4000       3750
                      s5000       5000
                      s6000       6000

Does anybody have experience of PWM operation with MACH 3?  Am I being unrealistic in expecting a better result?

Thanks for any help.