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Gary H
Lv 6
Gary H asked in Science & MathematicsEngineering · 10 years ago

How to design a 12-pulse rectifier?

I'm having trouble designing a 12-pulse rectifier with a 3-phase 208V input (120V to neutral). The output is a low voltage (24-30V range, approx 100A), so I'm doing the classical parallel version with a reactor. From graphing, it appears that the output is very close to the peak value of the secondary voltages - is that correct? So do I specify the secondary voltages by the peaks?

Also, how do you choose the inductance of the center-tapped reactor?

I've been trying to simulate the design in Tina9, but the results are suspicious, with minor alterations of values resulting in either convergence faults or overcurrent faults, even though I have nothing floating or mis-connected.

Update:

Rick - I do have a few 0.1 ohm resistors in strategic locations, i.e. in series with each input phase, and a load resistor. The transformers are currently ideal, and the only semiconductors are 6 ideal diodes.

Update 2:

EDIT - excuse me, 12 ideal diodes.

2 Answers

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  • Gone
    Lv 6
    10 years ago
    Favorite Answer

    The output of an ideal 12-pulse rectifier is 12 pulses per input sine cycle each consisting of the peak portion of the waveform, that is the portion between 75 deg. and 105 deg. The the unfiltered DC voltage would therefore have a peak voltage of 100% (100*sin90deg) of the input and a minimum voltage of 96.6% (100*sin75deg). In other words, the output is very close to the peak voltage with very little ripple.

    I believe that the function of the center-tapped reactor is to compensate for impedance difference between the two parallel circuits and equalize the voltage from and the currents through each path. The difference would be mostly due to differences between the two phase-shifting transformers or the two secondary windings depending on the design.

    I would expect that the simulation would show perfect operation with perfectly matched components in each parallel path. A good simulation should include complete equivalent circuits of the components. You might have difficulty with the simulation if the simulated rectifier load is very small. I think that a 12-pulse rectifier needs to have the two parallel sections quite closely matched (or well balanced by the reactor) in order to work properly.

    PS1:

    I think that the reactor does not simply function as two inductors but a sort of buck-boost autotransformer.

  • ?
    Lv 7
    10 years ago

    Regarding the simulation issues, do you have realistic models for the semiconductors? They should include reasonable values for the capacitances.

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