PE Exam Question

[kris7o2]

Hello,

Can anyone please help me understand this problem? I am having a hard time understanding how to solve this. Thanks

Consider a 208/120 V, 3-phase, wye-connected, 4-wire transformer. Each 120-V phase feeds a separate and identical dc power supply comprising a simple full-wave bridge rectifier followed by a large filter capacitor. There are a total o f three dc power supplies, one connected to each phase. The following circuit diagram shows one o f the three power supplies:


The outputs of the three power supplies are loaded with identical resistive loads. There are no other loads 011 the transformer. Which of the following oscilloscope traces of the neutral current is most likely to be observed?

 

[DLD PE]

For a full wave rectifier with a smoothing capacitor, you will have non-zero charging current, so you can eliminate D. You have 3 phases, so 6 pulses per cycle, 3 positive, and 3 negative (1/60 seconds). Option C shows 3 pulses, so eliminate C.

Everything I've read on full-wave rectifiers with a capacitor tells me the output signal should be positive, but I know the answer is A. I would have chosen B, but I know that's wrong. This problem is very confusing. Sorry I can't be much help.

EDIT: It has something to do with the neutral current. See link:

https://engineerboards.com/threads/new-ncees-113.11707/