Completed Project Can a capacitor store AC voltage?

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
I found this A/C compressor AC capacitor. Its rated for 450VAC 40uF. It says its filled with vegetable oil as the label says. I had a 120V to 220/440V industrial transformer. So I plugged it into my 120V 300W UPS and charged the capacitor with 420VAC from the transformer and measured it with my multimeter on AC voltage and it showed 400+ VAC. Doesn't capacitors store + & - charge on either plates? How can it store AC current which has alternating polarities?
 

Attachments

wayneh

Joined Sep 9, 2010
17,496
The capacitor was charging and discharging with the AC at the moment you disconnected it. You just happened to catch it at that particular level of charge and voltage.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
The capacitor was charging and discharging with the AC at the moment you disconnected it. You just happened to catch it at that particular level of charge and voltage.
Hi,
So the capacitor is holding that AC for a few seconds. Right? Because I just repeated it and tried measuring the voltage across the cap and it kept falling really quick. Pics attached below.
 

Attachments

MrAl

Joined Jun 17, 2014
11,389
Hi,

It only stores the instantaneous voltage

Yes caps dont really store AC voltage they store instantaneous voltage which constantly changes with an AC applied voltage.

If you disconnect the cap when the instantaneous AC voltage is 200v, it will read 200v DC not AC.
If you disconnect when the voltage is 100v it will read 100v DC.
Note that using the meter on AC does not mean it is measuring AC. Many meters read DC on the AC scale too because of the way they are built, but the reading will not be accurate at all.

There's only one case where we might say that the cap is "storing AC" but it requires an inductor to work with it. It doesnt store in the usual way either though, it just keeps transferring energy to and from the associated inductor. This is more of a theoretical case only though because if there is any leakage or any other loss the energy dissipates and then neither the cap or inductor has any energy left.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
Hi,

It only stores the instantaneous voltage

Yes caps dont really store AC voltage they store instantaneous voltage which constantly changes with an AC applied voltage.

If you disconnect the cap when the instantaneous AC voltage is 200v, it will read 200v DC not AC.
If you disconnect when the voltage is 100v it will read 100v DC.
Note that using the meter on AC does not mean it is measuring AC. Many meters read DC on the AC scale too because of the way they are built, but the reading will not be accurate at all.

There's only one case where we might say that the cap is "storing AC" but it requires an inductor to work with it. It doesnt store in the usual way either though, it just keeps transferring energy to and from the associated inductor. This is more of a theoretical case only though because if there is any leakage or any other loss the energy dissipates and then neither the cap or inductor has any energy left.
Thank you. So, can I use the above cap with 450VDC too?
 

Tonyr1084

Joined Sep 24, 2015
7,853
Capacitors do not store AC voltage - it stores voltage. It's rated to handle 450 VAC; that means it can withstand an AC voltage being applied to it. In other words, the capacitor is non-polar (it has no positive or negative lead). Polar (or polarized) capacitors are best known as "Electrolytic" capacitors. They have a specific lead for positive voltages and if you apply a positive DC voltage to the positive lead the capacitor will take and hold that charge. Electrolytic caps are notoriously leaky, meaning they don't hold their voltage very well. Depending on how fast they leak, their plates will eventually leak down to zero volts (where the voltage on each plate is equal to the other). If you apply a negative voltage to the positive lead of the capacitor it can be damaged. Electrolytic's don't do well with reverse polarity and are not suited for AC use. Caps rated for AC can withstand the applied voltage in either direction equally.

Think of capacitors as being like rechargible batteries. You charge them then they give back their stored energy. Since AC is "Alternating Current" that means the voltage is also alternating between positive going and negative going. Capacitors can't do that on their own. As others have explained, when you charged your capacitor on an AC circuit, the moment you disconnected it from the cycle source it retained that voltage and polarity. If you were to hook that capacitor to a load the capacitor can only give what is stored. If it was positive 100 volts then that's all you're going to get out of it. The current is not going to alternate.

No. Caps do not store AC.
 

tcmtech

Joined Nov 4, 2013
2,867
Thank you. So, can I use the above cap with 450VDC too?
Actually with DC they can go way higher. The peak voltage of a 450 VAC sine wave is ~ 636 VDC.

Then add in the fact that they are designed to operate continuously in AC application which requires a fairly low ESR and considerable over voltage capacity and a capacitor like that would likely handle 1000 VDC without issue.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
Actually with DC they can go way higher. The peak voltage of a 450 VAC sine wave is ~ 636 VDC.

Then add in the fact that they are designed to operate continuously in AC application which requires a fairly low ESR and considerable over voltage capacity and a capacitor like that would likely handle 1000 VDC without issue.
Hi,

I absolutely agree with you tcmtech. I just added a high current bridge rectifier to the output of the 440V transformer and tried charging it with DC. The voltage across the cap after 3 secs was round 598V. But the voltage drop was real quick compared to electrolytic caps. But the label on this AC cap says its filled with vegetable oil. So, isn't this also an electrolytic cap?
 
Top