3-Phase rectification & Ripple

[Besoeker]

How about, in this case, ...

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

 

[Besoeker]

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.

SG-1
Thank you for the detailed reply.
I don't recognise the part number We still use the 1N series but mostly for power supplies in printed circuit boards. Wire ended axial components.

Unless you want to restore the kit from a historical perspective you might like to consider replacing 12 diodes with something like this:

Good for around 25A and 1200V.

And about 1.25 by 1.25 in inches.
In quantity around $10 a pop.

 

[SG-1]

Would you still recommend that ampere bridge if I were to tell you that it will see bolted short circuit amps from time to time. The input voltage is supplied by (2) 5KVA transformers wired in open delta 1 to 1 ratio. The variable auto-transformers are before the open delta. The diodes appear to be soldered in with super solder, I used an iron with a tip as big as your little finger and could not even soften it up, history will not be preserved.

 

[dbuckley]

The only time I've had to do this in anger was with a rectifier for a cinema carbon arc, and I figured the easiest thing to do was to measure the voltage drop across each rectifier diode.

 

[Besoeker]

Would you still recommend that ampere bridge if I were to tell you that it will see bolted short circuit amps from time to time.

I would compared to PCB mounted axial components.
For relatively little more money you could get a modular bridge capable of sustaining short circuit current indefinitely. The problem then becomes one of protecting the wound components.

 

[SG-1]

Measurements

Measurements

With 50 VDC
3.2 VAC ripple (RMS)
10.4 VAC (Peak to Peak) ripple

With 100 VDC
6.4 VAC ripple (RMS)
20 VAC (P-P) ripple

With 125 VDC
7.9 VAC ripple (RMS)
30 VAC (P-P) ripple

With 200 VDC
12.8 VAC ripple (RMS)
40 VAC (P-P) ripple

The first two measurements DC & RMS were made with a Fluke 289 multi-meter. A Fluke 27 which is an averaging meter displayed the same values.
The Peak to Peak was made with an Fluke 196C Scopemeter.

 

[Besoeker]

/

With 50 VDC
3.2 VAC ripple (RMS)
10.4 VAC (Peak to Peak) ripple

With 100 VDC
6.4 VAC ripple (RMS)
20 VAC (P-P) ripple

With 125 VDC
7.9 VAC ripple (RMS)
30 VAC (P-P) ripple

With 200 VDC
12.8 VAC ripple (RMS)
40 VAC (P-P) ripple

The first two measurements DC & RMS were made with a Fluke 289 multi-meter. A Fluke 27 which is an averaging meter displayed the same values.
The Peak to Peak was made with an Fluke 196C Scopemeter.

Ripple voltage and Vpk-pk are around the ratio for a 6-pulse bridge but rather higher than calculation gives. Noise. distortion....maybe? Sampling rate?

Might I suggest a couple of very much simpler measurements?
Measure the AC input voltage and the DC output voltage. If the rectifier bridge is operating correctly the DC should be about 1.35 times the RMS of the AC input.

Use a clip on ammeter to measure current in each of the three phases feeding the six-pulse rectifier. An open circuit leg will result in unbalanced currents. And the resulting dc component will make the clip on a bit sticky to open.

 

[SG-1]

/

Ripple voltage and Vpk-pk are around the ratio for a 6-pulse bridge but rather higher than calculation gives. Noise. distortion....maybe? Sampling rate? Distortion is possible I noticed the sine waves looked pointy when I was connected L-L. The scope is a 100MHz

Might I suggest a couple of very much simpler measurements?
Measure the AC input voltage and the DC output voltage. If the rectifier bridge is operating correctly the DC should be about 1.35 times the RMS of the AC input. I have measured this and it is spot on. I am just trying to understand ripple better.

Use a clip on ammeter to measure current in each of the three phases feeding the six-pulse rectifier. An open circuit leg will result in unbalanced currents. And the resulting dc component will make the clip on a bit sticky to open.

The DC component causing the clamp-on to be a bit sticky is interesting.

 

[Besoeker]

The DC component causing the clamp-on to be a bit sticky is interesting.

Qualitative rather than quantitative.
But a good clue nonetheless.

 

[SG-1]

Modern bridge experiment

Modern bridge experiment

I wired a 3-Phase DC supply with 3 single phase full wave bridges & some jumpers last week. Using a 250 ohm resistor for load, 250W/250V heater. A Fluke 27 was used to for the measurements.

At 100 VDC output. No smoothing cap. Values rounded to nearest whole number. I started with all 3 phases then single phased the circuit.



NO LOAD

PHASES```Ripple Voltage
1-2-3``````.64 VAC ( no rounding )
1-2```````4 VAC
2-3```````4 VAC
1-3```````4 VAC

.4 Ampere LOAD ( one heater )

1-2-3```````6 VAC
1-2````````45 VAC
2-3````````45 VAC
1-3````````45 VAC

The AC ripple increased x10 from no load to .4 Ampere. Gar was helping me to identify a componet ( turned out to be 220Kohm resistors ) & said the no load values were caused by stray capacitance.

.8 Ampere LOAD ( 2 heaters )

1-2-3``````6 VAC
1-2```````45 VAC
2-3```````45 VAC
1-3```````45 VAC

The old bridges read the same with or without load. I supect the resistors are causing this.

 

[Besoeker]

I wired a 3-Phase DC supply with 3 single phase full wave bridges & some jumpers last week. Using a 250 ohm resistor for load, 250W/250V heater. A Fluke 27 was used to for the measurements.

At 100 VDC output. No smoothing cap. Values rounded to nearest whole number. I started with all 3 phases then single phased the circuit.



NO LOAD

PHASES```Ripple Voltage
1-2-3``````.64 VAC ( no rounding )
1-2```````4 VAC
2-3```````4 VAC
1-3```````4 VAC

.4 Ampere LOAD ( one heater )

1-2-3```````6 VAC
1-2````````45 VAC
2-3````````45 VAC
1-3````````45 VAC

The AC ripple increased x10 from no load to .4 Ampere. Gar was helping me to identify a componet ( turned out to be 220Kohm resistors ) & said the no load values were caused by stray capacitance.

.8 Ampere LOAD ( 2 heaters )

1-2-3``````6 VAC
1-2```````45 VAC
2-3```````45 VAC
1-3```````45 VAC

The old bridges read the same with or without load. I supect the resistors are causing this.

Is there any way you could post diagrams of your circuits and the points at which you are taking your measurements?

 

[SG-1]

Is there any way you could post diagrams of your circuits and the points at which you are taking your measurements?

These measurements were taken from the DC terminals of the bridges. I will work on some diagrams. Trying to set up a photobucket account. At least you might be able to see the beast.

http://s1204.photobucket.com/albums/bb408/Finduilas1/

 

[glene77is]

The diodes appear to be soldered in with super solder,

SG,
Silver Soldered ? :-?
(Welding/brazing with silver rod technique)

 

[Besoeker]

These measurements were taken from the DC terminals of the bridges. I will work on some diagrams. Trying to set up a photobucket account. At least you might be able to see the beast.

http://s1204.photobucket.com/albums/bb408/Finduilas1/

Thanks for posting the pictures.
The construction is similar to what I have seen in the past. Lots of individual components, both electrical and mechanical. I'm still of the view that, unless you have compelling reasons to keep the current assembly, replacement with a simpler modern device would be a better option. Something simple and robust like this maybe.

It's about 3.7 inches long and 1.5 inches wide.

 

[SG-1]

SG,
Silver Soldered ? :-?
(Welding/brazing with silver rod technique)

I have used silver solder before, with no trouble.

I have never seen the other methods used, so very possible.

 

[SG-1]

Thanks for posting the pictures.
The construction is similar to what I have seen in the past. Lots of individual components, both electrical and mechanical. I'm still of the view that, unless you have compelling reasons to keep the current assembly, replacement with a simpler modern device would be a better option. Something simple and robust like this maybe.

It's about 3.7 inches long and 1.5 inches wide.

The most compelling reason to not use a simular bridge is that it no longer is in production. I do plan to replace it with a bridge simular to the one you posted. I still have to estimate the short circuit current, so the bridge is unharmed while the fuses do their job, over & over again.

 

[Besoeker]

The most compelling reason to not use a simular bridge is that it no longer is in production. I do plan to replace it with a bridge simular to the one you posted. I still have to estimate the short circuit current, so the bridge is unharmed while the fuses do their job, over & over again.

The one I posted has a continuous current rating of 160Adc and a single cycle surge current of 1500A. I think that it wouldn't be greatly troubled by a 15A fuse blowing....the i^2t rating is about 6,500 A^2s. A typical (modern) 20A semiconductor protection fuse has a total i^2t let through of 800 A^2s so a huge safety margin.

You mention the two open delta 5kVA transformers. To get 300Vdc, the secondary voltage of each would need to be around 220Vac. That gives a rated current of about 23A continuous. The transformers might have 5% impedance and the variable transformers feeding them might contribute another 2% and there will be some in the supply itself so maybe around 8% total. The available fault current would thus be less than double the continuous rating of the bridge.
Rough and ready but it gives an idea of the scale of the quantities.

 

[SG-1]

The one I posted has a continuous current rating of 160Adc and a single cycle surge current of 1500A. I think that it wouldn't be greatly troubled by a 15A fuse blowing....the i^2t rating is about 6,500 A^2s. A typical (modern) 20A semiconductor protection fuse has a total i^2t let through of 800 A^2s so a huge safety margin.

You mention the two open delta 5kVA transformers. To get 300Vdc, the secondary voltage of each would need to be around 220Vac. That gives a rated current of about 23A continuous. The transformers might have 5% impedance and the variable transformers feeding them might contribute another 2% and there will be some in the supply itself so maybe around 8% total. The available fault current would thus be less than double the continuous rating of the bridge.
Rough and ready but it gives an idea of the scale of the quantities.

As I recall the impedance was about 3% for open delta transformers. The fuses will be type NON, not an electronic fuse. That would bankrupt the company, because of the number of them that blow. There are six 3-Pole fuses in the test set & one two pole.

I was looking at a 100A bridge. It has to have screw terminations.

Thanks for the quick estimate.

And have you seen a diode fail open now ?

 

[Besoeker]

As I recall the impedance was about 3% for open delta transformers. The fuses will be type NON, not an electronic fuse. That would bankrupt the company, because of the number of them that blow. There are six 3-Pole fuses in the test set & one two pole.

I was looking at a 100A bridge. It has to have screw terminations.

The one I posted has screws (not shown) that screw into the captive nuts underneath the flat terminals. A 100A rating sounds good. These units are relatively inexpensive so make sure you have a good margin.

And have you seen a diode fail open now ?

Well, yes. The one you posted.

 

[Open Neutral]

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.

But an automotive alternator's diode fails open. If/when it fails shorted, there's enough battery current through it that it soon goes from a failed-shorted to failed-open device.