DC Voltage Rating for AC rated capacitor

[N4LTA]

I am putting together a Modified Heising  Modulator  using three 10H Hammond Chokes in series  and a Hammond 125GSE transformer. I need a 3-4 uF capacitor rated at approx 2400 volts DC.

I have some 440 VAC rated 10uF capacitors.  I have heard different stories on how much DC these can handle.

I am sure that they can handle 1.414 x AC . In looking at dual rated capacitor on the net - I see that at lower AC voltages (500 or so) - The DC rating is at least  2 X the AC rating and in some cases 2.8 X.

I am planning to use three 10uF 440 VAC rated caps wired in series in my modulator The transmitter has a little over 600 volts on the plate.

Has anyone got a definite answer to how much DC voltage an AC capacitor can handle?  I would think 2.8 x  DC or - the peak to peak of the AC would be likely.  That is just a guess .

Pat
N4LTA

[KE6DF]

I started a thread on this topic a while back.

http://amfone.net/Amforum/index.php?topic=21858.0

Dave

[N4LTA]

Thank Dave,

I had read that one  0 but still no one has given a definite answer - Is it the peak to peak voltage or the peak to zero voltage?   

DC times  1.414 or 2.828???

Also - 440 volts is a nominal voltage in a 480 volt motor circuit that can see AC voltages approaching 500 volts.

The nominal utilization voltage is 480 volts - the motor rated voltage is 460 volts -

So is a 440 volt AC motor start/run oil cap AC rated for 500 volts minimum - I would say likely as  I routinely measure 490 volts and more at the terminals of 460 volt motors.

That would likely mean that a 440 VAC Capacitor should be fine at a minimum of 500 x 1.414 or  707 volts.

or is it 2.828 x 500 or  1414 volts - a very big difference


Pat
N4LTA

[KE6DF]

I know what you mean about lack of definite answers.

But that's how technology works some times.

It all depends on how conservative you want to be.

I liked the comment someone made in my thread that unless the manufacturer includes a DC rating in the secs, then you should use 1.414 times the RMS AC spec.

In browsing around I saw some 440vac caps that were also marked as 1000vdc. So I would go with that rating if if using those caps.

But, for a 440vac cap with no DC rating I'd used 622 (440 * 1.414) as the limit.

[N4LTA]

I think I was the one that made that comment. ( being the conservative engineer that I am.) I am not worried about using 500 or even 510 volts x 1.414  - because I see 440 volt caps used in AC circuits near 500 voltls AC commonly.

510 x 1.414 = 720  - that is about as far as I would go - but my guess is 1000 volts DC may be OK.

We need an expert to give his opinion?

I think I am going to put some DC on one and see what happens.

Pat
N4LTA

[N2DTS]

I would not push things, and have one explode.
I can see one exploding and spraying oil on fire all over the place, are you ready for that?

I can give you a 4uf 4000 volt cap...

Brett

[N4LTA]

Brett  - If you have one to give - That sure would make things simpler.

Thanks,

Pat
N4LTA

BTW - I used to make serious rocket motors  - like 100 - 500  pounds of thrust - so I have lots of experience blowing things up.

[N2DTS]

I have a few 4uf at 3000 volts, and a 3 uf at 4000 volts, which one do you want.

I always wanted to play with rocket motors, I think it would be great fun.
I remember reading about the saturn 5 motors, the pumps had to pump something crazy like 50 tons of oxygen a minute or something like that, and needed 5000 hp each, and always wondered where that power came from.
Sure would like to study the designs...

I can't help thinking the solid rocket motors were cheating...

Brett

[N4LTA]

The solids are pretty easy at low thrusts but man rating a solid is really tough.

I am hearing that the ARES will be canceled in the next weeks and a new manned booster announced to begin design. A solid propelled crew vehicle was a really bad idea, but NASA is famous for bad ideas.

I sent you a PM.

Pat
N4LTA

[N2DTS]

I don't understand why they don't dust off the saturn 5 plans and modernize it.
Sucks we have to ask the Russians if they can put people in space for us.
Maybe just have the saturn 5 made in China...

What you sent me did not include the address, but I can look it up.

Brett

[N4LTA]

The Saturn 5 was pretty old technology - but the Atlas Heavy or the Delta Heavy versions are already developed and could be man rated but NASA didn't invent them.

I subscribe to the AR discussion group and a lot of the smaller private companies are on there most all the time - Lots of real rocket scientist and some NASA people.
It's interesting to hear info from the inside of the industry.

The ARES program was a joke and was never going anywhere. About 80 % into the design - they found that the rocket was not stable without adding so much weight that the payload capacity was reduced to essentailly nothing!

The test launch at the end of last year was more or less a publicy stunt. They fired a shuttle SRB with a dummy nosecone  and a quickly put together guidance pack using vernier rockets from another vehicle.. It went straight up and came down ballistically with some recovery parachutes to soft land the SRB. Even the parachutes did not function to design levels.

From what I am hearing  - the Shuttle program will be extended until the new vehicle is near ready and the Space Station will be extended to 2020.

This is getting pretty much off topic so I'll shut up..


Pat
N4LTA

[K1ZJH]

Thank Dave,

I had read that one  0 but still no one has given a definite answer - Is it the peak to peak voltage or the peak to zero voltage?   

DC times  1.414 or 2.828???
 
or is it 2.828 x 500 or  1414 volts - a very big difference


Pat
N4LTA

Here's my take. FWIW, and I haven't had a coffee yet..

In an AC circuit the cap would subjected to the full
Peak to Peak AC waveform, it would need to be rated for 2.88 times the average RMS voltage.

In a DC circuit, the capacitor's DC rating is the same as the Peak voltage of the AC voltage, or just 1.414 times the RMS rating.

Since you're using it an AC system, my guess is 2.82 times the caps DC rating.

Pete

[N4LTA]

That is my feeling also - but I wanted to hear it from someone else.

It would probably be a bit greater than the rated voltage because AC  motor circuit circuits  generally operate at a minimum of plus or minus 10% voltage.

Pat
N4LTA

[KE6DF]

"In an AC circuit the cap would subjected to the full
Peak to Peak AC waveform, it would need to be rated for 2.88 times the average RMS voltage.

In a DC circuit, the capacitor's DC rating is the same as the Peak voltage of the AC voltage, or just 1.414 times the RMS rating.

Since you're using it an AC system, my guess is 2.82 times the caps DC rating."

Actually, I think 2.82 times would be pushing things a bit much.

Although the peak to peak AC voltage is 2.82 time the RMS, if you tied one side of the AC line to one terminal of the cap and the other line to the other terminal, then the cap would see a peak potential difference between it's terminals of 1.41 times RMS with one polarity, and then the voltage would swing to 1.41 times  RMS with the opposite polarity.

But the peak voltage difference between the "plates" of the capacitor would be 1.414 RMS and that would be the peak stress on the dielectric for which it must be designed.

Now, I would expect that a motor starting application might result in transients so I'm sure there is a safety margin built in, but IMHO you are pushing things too far if you go to 2.82 times.

Before you built that into a rig, I suggest you hook it up and try the high DC voltage for a while.

Maybe better try it outside the house and keep a fire extingusher handy.

An exploding cap blowing fiery oil all over your shack can ruin your entire day.

Dave

[N4LTA]

Knowing what I know about electrical distribution, I would still guess that 3 x the marked voltage is probably OK.

That is the pattern that the capacitor manufacturers seem to follow.

Fires and and burning oil are not a likely failure mode. The most likely scenario would be a poof as the capacitor blows the safety vent. Likely you would hear it sizzling long before it vents. It all depends on the energy source supplying the capacitor voltage.

A good test would be to use my 0-2500 volt power supply with variac voltage control and a series current limiting resistor. A one meg resistor would limit the total current to 2.5 MA.


Pat
N4LTA