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Three Phase Rectifier Capacitor

maheshyadla

Feb 27, 2012
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A 3 phase 240VAC line and I get a max output voltage of 589Vdc.
designing the circuit for 100KVA.

After the three phase rectification my output voltage is 589Vdc, after these I need to place a capacitor branch at dc side. So how to calculate the Capacitor value.

Can any help me how to calculate the capacitor value at dc side of the 3 phase rectification and also if possible with some equations.

Thanking you in advance.

Best Regards,
Mah
 

duke37

Jan 9, 2011
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This is well outside my experience!

If you rectify the three phases with full wave rectification you get six pulses per cycle. At 50Hz (?) this gives 3.3ms between pulses.
The output voltage will be about 550 and with 5% drop between pulses, that will be 27.5V.
P=I*V so I=100k/550 = 182A

The rate of drop of a capacitor dV/dT = I/C so C=dt/dV*I = 0.022F
The capacitor must have a low ESR or it will overheat.

You may want a smoother output than this and you may wish to use three diodes instead of six. Both will need larger capacitors.

I hope someone else can give an indication whether this is in the right area.
 

maheshyadla

Feb 27, 2012
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Hi ,

Thanks for replying.

I am using six diodes in the rectification bridge and my Vrms = 240 V at 50Hz input frequency and Peak Voltage= 339V and my dc line voltage is 589 and after rectification I am getting the voltage about 560V Dc.

I am attaching my circuit diagram and wave forms to please go through that if possible.

Thanking you
 

Attachments

  • ThreePhaseRectification_Resistance_Welding_Transformer.jpg
    ThreePhaseRectification_Resistance_Welding_Transformer.jpg
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maheshyadla

Feb 27, 2012
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Voltage Ripple in Three Phase Rectification

I am designing the three phase voltage rectification with the input of 240 Volt's AC and 50Hz.
I am designing it for 100Kw output power.

So. how do I calculate the ripple voltage in the three phase rectification.

Can you please Explain me with some formulas please. And also I am attaching my circuit t this post.

Thanking you in advance.
 

Attachments

  • ThreePhaseRectification_Resistance_Welding_Transformer.jpg
    ThreePhaseRectification_Resistance_Welding_Transformer.jpg
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duke37

Jan 9, 2011
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The ripple voltage can be calculated with the formula I gave earlier.
Why do you need to control ripple? There will always be a positive voltage with three phase full wave rectification and the inverter could be controlled to give a constant output. The capacitors would then only need to bypass high inverter frequencies.

I do not see any waveforms.
 

maheshyadla

Feb 27, 2012
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The ripple voltage can be calculated with the formula I gave earlier.
Why do you need to control ripple? There will always be a positive voltage with three phase full wave rectification and the inverter could be controlled to give a constant output. The capacitors would then only need to bypass high inverter frequencies.

I do not see any waveforms.
Hi Duke,

One doubt, as you said that 5% of voltage ripple. How did you calculate the voltage ripple as 5%..

Can you please explain me ... If possible.
 

duke37

Jan 9, 2011
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Voltage ripple
I did not calculate the voltage ripple, I just chose a value at random to show how the equation is used.. It is up to you to determine what ripple can be tolerated. As I said, you may be able to run with the ripple direct from the rectifier. Why does a welder need a low ripple?
 
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