# Thread: 3-Phase rectification & Ripple

1. ## 3-Phase rectification & Ripple

I need a formula to calculate the AC ripple for an unfiltered 3-Phase Bridge ( 6 pulse ). The only formulas I can find are for circuits with a smoothing capacitor.

Maybe I could just use an ungodly small capacitor for the calculation...?? :confused:

The final intent of this exercise is to be able to measure the ripple to determine the health of the diode bridge. The supply is variable 0 - 300 VDC.

2. Originally Posted by SG-1
I need a formula to calculate the AC ripple for an unfiltered 3-Phase Bridge ( 6 pulse ). The only formulas I can find are for circuits with a smoothing capacitor.

Maybe I could just use an ungodly small capacitor for the calculation...?? :confused:

The final intent of this exercise is to be able to measure the ripple to determine the health of the diode bridge. The supply is variable 0 - 300 VDC.
So what is/are the formula/s with the smoothing capacitor?

3. Originally Posted by SG-1
I need a formula to calculate the AC ripple for an unfiltered 3-Phase Bridge ( 6 pulse ). The only formulas I can find are for circuits with a smoothing capacitor.

Maybe I could just use an ungodly small capacitor for the calculation...?? :confused:

The final intent of this exercise is to be able to measure the ripple to determine the health of the diode bridge. The supply is variable 0 - 300 VDC.
If it's unsmoothed, perhaps an easier way to check if all the diodes are conducting would be to measure the DC output voltage of the bridge.
Assuming it to be a full-wave six-pulse bridge, the output DC voltage should be 1.35 times the AC input voltage. Any non-conducting diodes will reduce the output voltage significantly.
You can measure the AC ripple content but bear in mind that it predominantly at six times the fundamental frequency (probably 360Hz in your case) and is a fairly low voltage compared to the DC. The meter may not respond well to that. Anyway, for what it's worth, the RMS ripple for a six pulse bridge operating correctly is a bit over 4% of the DC voltage.

One other thought. I've been in the power electronics game since before the Dead Sea reported sick. I can't ever remember a power diode to fail by going open circuit. The usual failed condition in my experience is short circuit. And that you can easily check with most standard multimeters.

4. Originally Posted by Besoeker
I've been in the power electronics game since before the Dead Sea reported sick.
That's cute!

5. Originally Posted by SG-1
I need a formula to calculate the AC ripple for an unfiltered 3-Phase Bridge ( 6 pulse ). ...
Originally Posted by Besoeker
.... Anyway, for what it's worth, the RMS ripple for a six pulse bridge operating correctly is a bit over 4% of the DC voltage.

...
Don't know how accurate the formula is, I derived the formula myself... I did not reference any publications.

FWIW, it yelds a waveform of about 6.7% p-p, which I imagine is pretty close to a bit over 4% rms.

6. Originally Posted by Smart \$
Don't know how accurate the formula is, I derived the formula myself... I did not reference any publications.

FWIW, it yelds a waveform of about 6.7% p-p, which I imagine is pretty close to a bit over 4% rms.
Altogether unnecessarily complicated and not right anyway.

Peak voltage, Vp, as always is Vp = root(2) * Vrms
Minimum is Vp sin (60) or Vp*root(3/2)
So peak to peak is about 14% of maximum voltage.

I suppose I have posted this before. It gives the waveform, the ripple, and the 6th harmonic content.

7. Originally Posted by Smart \$
So what is/are the formula/s with the smoothing capacitor?
I was using this website:

http://www.electronics-tutorials.ws/diode/diode_6.html

V (ripple) = ( I(load) / ( f(60hz) x #P (6) x C )) x V

8. Originally Posted by Besoeker
One other thought. I've been in the power electronics game since before the Dead Sea reported sick. I can't ever remember a power diode to fail by going open circuit. The usual failed condition in my experience is short circuit. And that you can easily check with most standard multimeters.
This one diode ejected it's anode. ( I have a picture, but my uploads fail since the forum changed. I am probably doing something wrong.) Others were shorted. This resulted in arcing on a variable auto-transformer.

10 bridges ( 9 now) with 12 diodes per bridge & I would rather not open the cabinet, but I can.

I am digesting the rest of your posts, good stuff. ( Yum )

9. Originally Posted by Besoeker
Altogether unnecessarily complicated and not right anyway.

Peak voltage, Vp, as always is Vp = root(2) * Vrms
Minimum is Vp sin (60) or Vp*root(3/2)
So peak to peak is about 14% of maximum voltage.

I suppose I have posted this before. It gives the waveform, the ripple, and the 6th harmonic content.
When I stated 6.7%, my reference value was AC voltage peak to peak. Referenced to one bus of rectified voltage it would be double at 13.4%.

Additionally, your formulas do not yield an instantaneous waveform. SG-1 did not specify what type of formula he was wanting. Plotted out, my formula's waveform is identical to yours.

10. Originally Posted by Smart \$
When I stated 6.7%, my reference value was AC voltage peak to peak. Referenced to one bus of rectified voltage it would be double at 13.4%.
Actually, not quite right. It is 13.4% referred to the peak of the AC voltage. Not the DC.

Originally Posted by Smart \$
That sounds like an oxymoron.
The formula yields an instantaneous value. The waveform is a time varying function, not an instantaneous value.

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