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I would like to ask about an estimate for inverter DC link capacitors, where this inverter will supply 48V 20A 3phase Brushless DC motor and it's driven using a 5 kHz PWM.

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The bus link capacitance will be primarily determined by the requirement to supply reactive current to the load. That will presumably be not very large for a PM synchronous or brushless DC motor. However you have a range of load conditions and speeds to consider and there is no magnetic excitation adjustment. The un-filtered three-phase rectifier does not have much ripple, but it may be desirable to provide some degree of capacitive filtering. There are probably some text books that cover this. A simulation would also provide an answer. Someone else here may be able to provide a better answer, but there is one vote to close by someone that doesn't understand the question.

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  • \$\begingroup\$ Charles, we have many VFDs and servo motor drives in my place of work. I can understand the need for DC link capacitors on the single-phase powered drives but I've always imagined that very little capacitance is required on the three-phase drives as there is always "one-phase up". I'm interested in your comment on the "requirement to supply reactive current" in certain applications. How would the capacitor help and why would it not just be pulled through the rectifiers? \$\endgroup\$ – Transistor Jan 11 at 20:12
  • \$\begingroup\$ @Transistor let me assure you when I say they are not "very little capacitance". While 3phase provides continuous power, the DClink has a fair amount of ripple and this is not good for either power quality OR torque ripple. This capacitor needs to take the freewheel current as well as act as a "tank" of charge during switching events. The L-C to meet power quality also drives the size. it is easy to find 1mF capacitors on 415Vac inverters \$\endgroup\$ – JonRB Jan 11 at 20:15
  • \$\begingroup\$ @JonRB: Good points, My thinking had not considered torque ripple at all and I wasn't taking into account the need to sink regen current. (I'm familiar with the concept of the latter and the need to dump energy when the bus voltage rises in braking. I have made emergency repairs to burnt out braking resistors which were, basically, under-spec'd toaster elements.) I'll read up on the power quality issues. Many thanks. \$\endgroup\$ – Transistor Jan 11 at 20:22
  • \$\begingroup\$ @Transistor: The rectifier prevents any reactive VA from being exchanged with the grid. The fundamental current intro a rectifier is in phase with the line voltage, only harmonic current causes the input power factor to be low. As a result, all reactive energy exchange with the load must be between the load and the DC link cap. Therefore the link caps quite large for an induction motor. For a PMSM, I assume it can be smaller. \$\endgroup\$ – Charles Cowie Jan 11 at 20:31
  • \$\begingroup\$ @Charles: Believe it or not, I'm training guys at work on the use of the harmonic analysis screens on the factory energy meters. Remember that "in the valley of the blind the one-eyed man is king". I am familiar with the double-bump three-phase current waveforms caused by the three-phase bridge. I have some more to add to my tutorials now. Thanks. \$\endgroup\$ – Transistor Jan 11 at 20:37

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