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2009-10-02 18:09

CNC Abene





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CNC - Abene VHF-3 - Electrical part 5

Powersupply and returned energy dump.


In parallell with working on the servodrives, trying different models and figuring out that damn encoder trouble I worked on the powersupply. I designed it for 130VDC since that would give me a nice margin regardning PWM dutycycle of the drives and IR-losses etc, yet stay well below the maximum terminal voltage of the motors which is 170V.

I ordered a new 4kVA 3-phase transformer with 90V secondary, this would give me roughly 130VDC after rectifying and filtering. I also designed an powerdump circuit that prevents the voltage in the powersupply capacitors to rise above the rated voltage due to them be charged by the current returned to them when the motors decelerates.

I designed the PCB to fit on top of the two 15000uF/250VDC capacitors and it's made with 105um copper on each side so it can handle the current without problems. I used my DIY machine to cut and engrave a cover in lexan. When the powersupply voltage goes above the setpoint (I have it set at 140V) the dump circuit activates and the MOSFET connects two 15ohm 100W resistors in parallell (not shown in these photos) across the powersupply, effectively loading it with ~18A. This prevents the voltage to rise more than a few volts above 140V even during rapid deceleration.




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