3 Phase Class D amp for DIY BLDC motor Drive
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 10th July 2018, 10:49 PM #331 Pyramid   diyAudio Member     Join Date: May 2006 Location: Behind the Cheddar Curtain I haven't been able to find a usable formula for 3 phase motors. The closest I remember is a post by Mark Kelly: Effective load=coil resistance x forward voltage / (forward voltage - back EMF). I doubt this is accurate for 3 phase. I've tried modeling something in an Excel spreadsheet, but the results weren't even close to the measurements I was getting and the complexity of the spreadsheet was starting to get unmanageable. I'm sure there are a number of DSP apps that address this, but the math is not trivial. Probably the most direct approach is the empirical method: Measure the DC current between the power supply and amp driving the motor; multiply by the efficiency of amplifier (~50% for class AB, 90-95% for class D) and the result should give a close approximation of the power consumption of the motor. Adjust the output voltage to keep the power consumption constant at both speeds (or what ever speed you want to run at). Because the back EMF generated by the motor opposes the drive signal, the impedance of the motor effectively increases with speed so the amplitude of the drive signal must increase accordingly. As you push the speed higher and the drive level increases, you must take care not to stall the motor or the current will suddenly spike as the back EMF disappears completely.
 11th July 2018, 03:52 PM #332 jobst   diyAudio Member   Join Date: Jan 2010 Location: Chicago Thanks for the explanation Pyramid. For some reason (Dunning–Kruger? Domain dependent thinking?) I always assume that things in the electrical world are neatly reducible to theory, with nice analytical solutions. Guess at that point it would no longer be an engineering discipline, and the fun would be over Maybe I'll try and add a live power display to my unit, as a self-education project...
vgeorge
diyAudio Member

Join Date: Mar 2004
Location: poros island
This is my latest setup and I think that I like the 3 belts better than two.
Also this is the second motor I have and if you remember I used to complain a little bit, about noise. This one is almost silent. Go figure.
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 26th September 2018, 08:46 AM #334 icsaki   diyAudio Member   Join Date: Feb 2005 Location: Phoenix, AZ I'm looking for an extra MA-3D PCB. Let me know if you have one. Thx.
 11th October 2018, 05:57 AM #335 icsaki   diyAudio Member   Join Date: Feb 2005 Location: Phoenix, AZ Thanks Pyramid for making this project available. I built the SG4, the MA-3D and I'm using a BLWS motor. I'm planning to use them in a Thorens TD 125, but with the current pulley dia (0.738") I need 279RPM/376RPM for 33/45. The frequencies are 9.29Hz/12.54Hz. What should be the nominal and reduced motor voltages for these RPM values?
Pyramid
diyAudio Member

Join Date: May 2006
Location: Behind the Cheddar Curtain
Quote:
 Originally Posted by icsaki Thanks Pyramid for making this project available. I built the SG4, the MA-3D and I'm using a BLWS motor. I'm planning to use them in a Thorens TD 125, but with the current pulley dia (0.738") I need 279RPM/376RPM for 33/45. The frequencies are 9.29Hz/12.54Hz. What should be the nominal and reduced motor voltages for these RPM values?
Nominal
9Hz: 7VPP (2.5VMRS)
12Hz: 7.6Hz (2.68VRMS)

Reduced
9Hz: 6VPP (2.14VRMS)
12Hz: 6.58VPP (2.33VRMS)

 12th October 2018, 04:20 AM #337 icsaki   diyAudio Member   Join Date: Feb 2005 Location: Phoenix, AZ Thank you Pyramid.

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