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 Post subject: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Fri 26, 2019 5:09 pm 
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Just one of those more curiouser things that I have to deal with. Why, why, why?

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Fri 26, 2019 6:56 pm 
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Most are intended for use on DC; some are used mainly on AC, like motor capacitors, line filters (aka "safety capacitors"), and resonating capacitors (as in ferroresonant transformers and induction heaters).

I like FKP polypropylene capacitors in my 160 KHz. induction heater. It operates from rectified 120 VAC, but I use 1200 or 1600 V caps due to the high peak voltage when the tank circuit is tuned.

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Rich


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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Fri 26, 2019 8:10 pm 
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Some oversimplification.......
Consider the old truism: "warranties are written to protect the seller". In many cases, it's the same answer for specifications. It is quite possible for the identical part to be sold with or without an AC spec---simply based on the intended market. A cap maker might have a successful line of caps using the very latest activated beeswax dielectric. One day, they get a call for an AC application. They test some parts, decide on a spec, print some new labels, and there are your AC-rated parts.

Within reason, ANY capacitor can be used for AC. But no one is going to commit to a spec without some testing.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Fri 26, 2019 8:35 pm 
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Try using AC rated breakers on DC!

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Fri 26, 2019 9:07 pm 
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DC what? Power distribution?

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Sat 27, 2019 12:31 am 
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The reason is, if DC is applied to a capacitor, the dielectric is stressed electrostatically in one direction, but then it stays put. Little or no heat is produced. In most electronic circuits, there are AC signal or ripple voltages riding on the DC, but the AC components are small compared to the DC stress.

When AC is applied to a capacitor, the molecules in the dielectric are constantly stressed back and forth. This causes heating. Also, considerable current may flow in AC capacitor circuits, leading to additional heat due to the resistances of the leads and foils. Consequently, capacitors for AC duty must be built to withstand more internal heating, making them bigger, heavier, and more expensive than ones designed to operate on circuits where the voltage is mostly DC. The alternative is to use DC capacitors on significantly lower AC voltages. For plastic film caps, it is not unusual for the AC voltage rating to be only 20% of the DC rating.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Sat 27, 2019 1:36 am 
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For plastic film caps, it is not unusual for the AC voltage rating to be only 20% of the DC rating.

Now that's interesting! I wasn't aware of that aspect. But, there's always AC whether it's for coupling or bypass.
I was looking at high voltage ceramic disc caps and saw they were all rated DC, except one that was rated AC which made me curious

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Sat 27, 2019 3:33 am 
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DC what? Power distribution?


I used to work for Westinghouse and got involved in DC systems for mines and railway traction. I am old enough to have been involved in the conversion of downtown Chicago from remaining DC distribution to 100% AC. Commonwealth Edison furnished high current rectifiers located in office building basements to convert incoming AC to DC to power elevators and motors left over from the early 1900s.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Sat 27, 2019 5:59 pm 
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So, would the DC rating of an AC rated capacitor be 80 percent higher than the marked voltage?

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Sat 27, 2019 7:31 pm 
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pixellany wrote:
DC what? Power distribution?


Yup. I live off the grid and you have to use DC rated breakers for the DC side of the system. The reason is that DC requires the contacts to be further apart and pull away from each other quickly. Otherwise you can pull an arc and prolong the fault. With AC since it is crossing 0V 60 times a second the arc will extinguish much more easily.


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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Sun 28, 2019 9:28 pm 
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The capacitor voltage is not the only consideration, in some applications the current is the most stressing factor.
When DC is applied to a capacitor no steady current is present only the initial charge current.
When AC is applied an AC current flows in the capacitor.
For example with 600V AC (60Hz) across a 1uF cap, it will have 0.226 Amps flowing. If the manufacturer wants to limit the current to a lower value the AC voltage rating will have to be reduced.
At higher frequency the AC current will be greater.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Mon 29, 2019 12:07 pm 
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Quote:
Now that's interesting! I wasn't aware of that aspect. But, there's always AC whether it's for coupling or bypass.


Of course there is. But there are two differences. First off, in typical electronic circuits the AC component is usually much smaller than the DC voltage rating of the capacitor. For example, three volts of AC signal voltage is not much for a 600-volt capacitor to deal with in a coupling circuit. The other thing is, in low power electronic circuitry there simply isn't enough energy available to cause much heating. The situation is very different in high power circuitry like transmitters and induction heaters where capacitors have to be carefully designed for the currents and frequencies involved. The same thing is true of motor run capacitors, capacitors for lighting, power factor corrections, and other applications where continuous operation on AC in low impedance circuits is a requirement. They will typically have thicker plastic films which may have different chemical additives, thicker metallic layers, better lead attachments, larger cases to dissipate more heat, and overall better construction than capacitors designed for use on predominantly DC voltages. An AC voltage rating is a sign of high quality. Not all capacitors are rated for AC operation at any voltage, nor do their manufacturers recommend it.

Ceramic disc capacitors are a special case. They tend to resonate at certain frequencies like piezoelectric transducers. This can radically change the waveforms passing through them (= distortion). Again, it's not a problem if the DC voltage rating is much higher than the AC signal voltages. If you put 30-VAC on a 50-V ceramic, it will likely distort the waveform a lot. But if you put 30-VAC on a 500-volt ceramic, there should not be any measurable distortion.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Mon 29, 2019 1:03 pm 
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Quote:
For plastic film caps, it is not unusual for the AC voltage rating to be only 20% of the DC rating.


Tim Tress wrote:
So, would the DC rating of an AC rated capacitor be 80 percent higher than the marked voltage?


I know what you are saying, but I think the inverse would be 500% higher? (1 / .2 )


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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Mon 29, 2019 7:40 pm 
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The idea of using ratios is fraught with peril.......AC and DC ratings are determined by different things.

The way I would approach it is that there is probably a minimum likely AC rating where the DC is known. e.g.--if that 20% is plausible, then we might say that it is "relatively safe" to assume 20% of the DC rating.

To best understand the correlations, i would study the relationships in specific part types---eg mylar film, polypropylene file, Russian Mil-surplus paper in oil, etc.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Jul Tue 30, 2019 12:23 pm 
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You have to read the spec sheets for the particular capacitors in question. It varies considerably with the type of capacitor and how it is applied. Here's an example of one where some 1,250-VDC polyester film capacitors are de-rated to 125V on AC. https://www.mouser.com/datasheet/2/315/ABD0000C179-947645.pdf

It might be mentioned that while heating was mentioned as the main reason certain capacitors are de-rated when used on AC, there are other reasons. One is that plastic films are inherently subject to aging and deterioration over time, and manufacturers want to ensure sufficient margins that the caps will not fail when the plastic degrades. Another is that a capacitor with an AC voltage rating could be connected across the AC line, as a line bypass, or as a dropping capacitor. In the example cited above, there are some pretty clear restrictions on AC line use. What they are saying is that AC line duty is hazardous to film caps due to the spikes and surges that occur, which is a further reason to de-rate their AC voltages.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Aug Thu 01, 2019 6:14 pm 
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The simple answer to AC vs DC capacitors in power distribution is
to use capacitors rated at the system voltage and frequency, with the
protection of conductors based on the KVAR rating.

We had capacitors used in induction heaters rated at about 4 kW.
Endless trouble. In one of the switch rooms, we had built a leakage
tester into a small bench.

I still have a carton filled with bulged, otherwise useful to me,
rejected capacitors.

The 'Calcoil' manufacturer was in touch with the capacitor makers
to solve the issues. They would ship the latest capacitors, and we
would replace them in the modules on maintenance days.

Many nights the watch electrician had to change out modules on the fly.
Walking under the sheet at the dry end of a paper machine wasn't a cherished call.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Aug Thu 01, 2019 6:24 pm 
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Great stuff. Thanks for all the posts.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Aug Thu 15, 2019 7:41 am 
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Chris108 wrote:
You have to read the spec sheets for the particular capacitors in question. It varies considerably with the type of capacitor and how it is applied. Here's an example of one where some 1,250-VDC polyester film capacitors are de-rated to 125V on AC. https://www.mouser.com/datasheet/2/315/ABD0000C179-947645.pdf

It might be mentioned that while heating was mentioned as the main reason certain capacitors are de-rated when used on AC, there are other reasons. One is that plastic films are inherently subject to aging and deterioration over time, and manufacturers want to ensure sufficient margins that the caps will not fail when the plastic degrades. Another is that a capacitor with an AC voltage rating could be connected across the AC line, as a line bypass, or as a dropping capacitor. In the example cited above, there are some pretty clear restrictions on AC line use. What they are saying is that AC line duty is hazardous to film caps due to the spikes and surges that occur, which is a further reason to de-rate their AC voltages.


That's interesting, the 10 to 1 ratio, DC vs. AC.

One well known de-rating rule of thumb involves the ratio 2.27 (roughly)
So for example a 660VAC rated AC oil cap, usually used for motor run
or motor start, gets rated 1500VDC. I think a lot of hams have used
motor start or run caps in HV DC supplies. I had a 3KV supply with a
bank of 4 ea 20uF/660VAC in series-parallel. It ran great for many
years without failure. These are the older, bigger rectangular types,
not the more modern oval or round types which have gotten a lot
more compact as industry moved to polypropylene types- but those
also seem to be 'up-rated' according to the same 2.27 multiplier rule.

I also have some WIMA FKP1, 0.01uF, 5% film caps obtained new from an
Asian source. They are marked 6000- (6000VDC) and 700~ (700VAC).
So WIMA uses an 8.57 ratio in this case. BTW these types are great
for building hipot testers, that's how I use them. These are quite compact
considering their ratings, potted into small rectangular boxes.

David

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Aug Fri 16, 2019 3:33 am 
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You are talking about two different things. Using a DC capacitor on AC requires different rerating than using an AC capacitor on DC. As mentioned above, use the manufacturer's number for AC use of a DC capacitor. I have never seen anything on DC use of an AC capacitor but I would think 1.414 times the AC rating would be a conservative value. The real value is probably higher but it might be hard to get the number.

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 Post subject: Re: Why are some capacitors rated at Vac and some Vdc?
PostPosted: Aug Fri 16, 2019 9:02 am 
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Yes, 1.414 is pretty conservative.

Various manufacturers have published catalog listings with the dual
ratings mentioned above- for AC oil filled types. A scan of both old and
current literature will often show this. OTOH most of these type caps
are marked with only the AC voltage rating, as this is by far the most
common usage- motor start or run. The 2.27 ratio is very often seen
for the max DC value. Often the cap is constantly working in AC use.
It's often loafing in a DC power supply until you draw DC current
and get AC ripple current. Even then it's lighter duty...

I have found some terrific bargains on new or used surplus
polypropylene types. I usually stick with new parts. I would rarely
run such caps right up to their full voltage ratings. If you keep them
at 80 or 90 percent (or less), you can get way better service life.
Current ratings are generally way more than needed so far as
AC ripple current typically seen in HVDC supply use.

David

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