NCEES #518

Hey Guys, for this problem the rectified voltage was given as ( root 2 x Line voltage ). Shouldn't the rectified voltage be (1.35 x Line voltage)?

Thanks.

For this circuit the output voltage is a pulse which fluctuates between 1.414 times 208 and 1.225 times 208. The average of this voltage will be 1.35 times 208 (which is the average value of rectified dc output voltage)

The problem asks to calculate Rcl to limit PEAK charging current to 10A. So you need to use the peak voltage of output waveform, which is 1.414*208. If the problem asked to limit average value of output current to 10A, then we would use 1.35*208.

Hope this helps.

You are absolutely right, the keyword was peak. Thanks!

Sorry for the simple question, but I understand where the 1.414 came from ( = sqrt (2)), but where did the 1.225 come from?

Thanks

Each diode conducts for 60 deg, so the min value of output voltage is VmSin60 => Vm*0.866 = 1.225 and Max is VmSin90 = Vm.

Thanks!

How the relationship "Vavg = 1.35X line voltage" is derived mathematically? Any explanation will be appreciated - - Thanks

How about does anyone have any good references for this stuff? Something simple discussing the basics would be good. 3.5 days till count down!

Insaf said:

How the relationship "Vavg = 1.35X line voltage" is derived mathematically? Any explanation will be appreciated - - Thanks

Click to expand...

The Wildi reference isn't really deep but may be sufficient in this area. 1.35 = (3*√2)/π and you should be able to integrate part of a full wave six pulse rectifier. Wildi does cover this in some detail.

VAve/dc= [3*Sqrt(3)/pi]*Vm (by integration)

= [3*Sqrt(3)/pi]*(sqrt(2)(Vrms)); Here Vrms for line to neutral

= [3*Sqrt(2)/pi]*(sqrt(3)(Vrms))

=1.35 VLL

Note: Ques 518 (battery charger) is not related to this relation rather concern with Vmax.

Thanks

"Power Electronics by Muhammad H. Rashid" and "Power Electronics by Ned Mohan" are good sources of reference materials.

Thanks

Which version of ncees are you using? I have the 2011 but my 118 is different