new NCEES #113

I thought the diagram is a full wave rectifier wich has no negative cycle (on the graph). However, the answer has both positive and negative cycles.

Rei said:

I thought the diagram is a full wave rectifier wich has no negative cycle (on the graph). However, the answer has both positive and negative cycles.

Click to expand...

I think it's because they're asking for the neutral trace, not the voltage across the load. I think the voltage across the load resistor will be rectified and smoothed out depending on the capacitance value because without the cap it's a straightforward full-wave rectifier.

Without the capacitor, the voltage across the load resistor should look like answer B.

BUT the neutral current will look like A. The rectifier affects the output voltage, not the input.

That's my reasoning but I'm not confident in it. I think this is one of those "trick" NCEES questions.

Rei said:

I thought the diagram is a full wave rectifier wich has no negative cycle (on the graph). However, the answer has both positive and negative cycles.

Click to expand...

I believe this is a harmonics question. It's asking for the current flow on the neutral wire (normally zero, but harmonics will flow through the neutral). That's also why the frequency is the frequency for the first triplen (i.e. third harmonic). 1/180, 1/90/, 1/60.

My question is - why is the current positive and negative?

This paper explains what is going on. The current Iac flows out for a half cycle, and in for a half cycle. It is rectified as well. http://www.scribd.com/doc/21375173/Diode-B...ifier-Capacitor

Downloadable and printable
/>http://users.ece.utexas.edu/~kwasinski/_3_EE362L_Diode_Bridge_Rectifier_Spring2010.pdf

CGlade said:

QUOTE (Rei @ Mar 13 2010, 01:55 PM)

I thought the diagram is a full wave rectifier wich has no negative cycle (on the graph). However, the answer has both positive and negative cycles.

I believe this is a harmonics question. It's asking for the current flow on the neutral wire (normally zero, but harmonics will flow through the neutral). That's also why the frequency is the frequency for the first triplen (i.e. third harmonic). 1/180, 1/90/, 1/60.

My question is - why is the current positive and negative?

Click to expand...

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Because, neutral will have one direction for positive half cycle and opposite direction for negative half cycle. Please note that Load (RLoad) current direction will note change (always same direction).

Thanks