3-Phase rectification & Ripple

[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...??

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.

 

[Smart $]

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...??

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?

 

[Besoeker]

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...??

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.

 

[LarryFine]

I've been in the power electronics game since before the Dead Sea reported sick.

That's cute!

 

[Smart $]

I need a formula to calculate the AC ripple for an unfiltered 3-Phase Bridge ( 6 pulse ). ...

.... 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.

 

[Besoeker]

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.

 

[SG-1]

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

 

[SG-1]

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 )

 

[Smart $]

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.

 

[Besoeker]

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.

Additionally, your formulas do not yield an instantaneous waveform.

That sounds like an oxymoron.
The formula yields an instantaneous value. The waveform is a time varying function, not an instantaneous value.

 

[Besoeker]

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.

Are the 10 bridges in parallel? Does each one of the 10 bridges have two diodes in parallel per leg? What is the current rating of the whole kit?
Questions I know, but I'm just trying to get my head round what sort of configuration would requires 120 diodes.
For a bit of background, we routinely make rectifiers up to about 10,000A with just a single six-pulse bridge with just six devices (usually SCRs) with no paralleling.

 

[Besoeker]

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.

Smart$
You didn't spot the deliberate error?

Minimum is Vp sin (60) or Vp*root(3/2)

Should have been:

Minimum is Vp sin (60) or Vp*root(3)/2

 

[Smart $]

Actually, not quite right. It is 13.4% referred to the peak of the AC voltage. Not the DC.

6.7% of the AC voltage input peak to peak. 13.4% of the output's peak voltage.

That sounds like an oxymoron.
The formula yields an instantaneous value. The waveform is a time varying function, not an instantaneous value.

You are correct. I was multi-tasking when I wrote that and needless to say a bit distracted. :grin:

 

[Smart $]

Smart$
You didn't spot the deliberate error?

Minimum is Vp sin (60) or Vp*root(3/2)

Should have been:

Minimum is Vp sin (60) or Vp*root(3)/2

Actually, I thought it looked a bit odd, but didn't scrutinize past the highlighted part. I errantly assumed you'd at least get an equivalency right, eh matey :roll:

 

[Besoeker]

6.7% of the AC voltage input peak to peak. 13.4% of the output's peak voltage.

Most people would take "rectified voltage" to be the DC value. Not peak.

 

[Besoeker]

Actually, I thought it looked a bit odd, but didn't scrutinize past the highlighted part. I errantly assumed you'd at least get an equivalency right, eh matey :roll:

I did - before anyone commented on the typo, matey!

 

[Smart $]

Most people would take "rectified voltage" to be the DC value. Not peak.

So you now speak for most people :roll:

Experience has shown me, and through introspection concur, that I am not a member of the "most people" category. So I frequently have to adjust so others might understand... even when correctly stated the first time. I'll be first to admit that I may be wrong, and on many occasions am. But what I hate most is when, out of courtesy (or failing memory ), I give others the benefit of [my] doubt, start second guessing myself, carefully reassess the issue... only to find I was correct in the beginning :grin:

 

[Besoeker]

So you now speak for most people

So what would you consider to a reasonable measure of rectified voltage?

 

[Smart $]

So what would you consider to a reasonable measure of rectified voltage?

How about, in this case, ...

... the AC ripple for an unfiltered 3-Phase Bridge ( 6 pulse )...

 

[SG-1]

Answers

Answers

Are the 10 bridges in parallel? One bridge per circuit. 10 circuits total. Each circuit is use as variable power supply 0 -300 VDC to energize control circuits for testing.

Does each one of the 10 bridges have two diodes in parallel per leg? Yes, in parallel & series. Each phase has 4 diodes. Each diode has a resistor in parallel also, 220k ohms.

What is the current rating of the whole kit? Not given, but the Westinghouse part number is 508C586G34. The diodes are 1N3671A, so 12Ampere average current x2 = 24. Circuit is fused at 15 ampers with NON type fuses.


Questions I know, but I'm just trying to get my head round what sort of configuration would requires 120 diodes. No problem, the more you know the better the answer.

For a bit of background, we routinely make rectifiers up to about 10,000A with just a single six-pulse bridge with just six devices (usually SCRs) with no paralleling. These bridges date back into the 60s, they have voltraps in parallel with the bridge. It is conneted with the DC at each end & the 3 phase tapped between. The bridge that is dead also has a dead voltrap. The voltrap was sparkling in the middle.