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Capacitor safety

DeSelle

Cast Iron
Joined
Oct 23, 2006
Location
Midlothian, TX
Hi all

Thanks to everyone here, I have put together a working RPC and hope to test it this weekend. I have a general question about capacitor safety. My balance caps are wired per the fitch design and my understanding is they will discharge through the motor windings. The start caps are wired in two groups parallel like the fitch design with a bleed resistor across the last cap in the bank on both. I need to get in and clean up some wiring but I want to verify these capacitors are truly discharged before I touch anything. I have read that you can short the terminals with something and if no spark then they were discharged and you are safe. If they do spark then you have now discharged them and you are safe. This makes me ask “how big of a spark might I encounter?” Can I just use a jumper from the ground buss and touch it to the open cap terminals to bleed them off? Can I trust a meter to measure across the terminals and if zero volts then I am good?

Thanks for everything to this point and forward,

Nathan
 
Hi all

Thanks to everyone here, I have put together a working RPC and hope to test it this weekend. I have a general question about capacitor safety. My balance caps are wired per the fitch design and my understanding is they will discharge through the motor windings. The start caps are wired in two groups parallel like the fitch design with a bleed resistor across the last cap in the bank on both. I need to get in and clean up some wiring but I want to verify these capacitors are truly discharged before I touch anything. I have read that you can short the terminals with something and if no spark then they were discharged and you are safe. If they do spark then you have now discharged them and you are safe. This makes me ask “how big of a spark might I encounter?” Can I just use a jumper from the ground buss and touch it to the open cap terminals to bleed them off? Can I trust a meter to measure across the terminals and if zero volts then I am good?

Thanks for everything to this point and forward,

Nathan

Not much of a spark... but it is all relative, I reckon..

Just use screwdriver or whatever to ground cap terminals and you'll be good.

You can check the voltage in a wall outlet to see if your meter is working.

----

Aside: balancing voltages is going to take trial and error with your multimeter when the rpc is running. Every motor has unique characteristics thus will require unique capacitance values b/w legs.

Wagner.

Sent from my SM-G935P using Tapatalk
 
Using a screw driver to short charged terminals is not a very elegant method. You might also cause the metal to weld to the capacitor terminal.
Use a resistor. The value depends on the capacitor value and the discharge time you want.
 
Agreed. Depending on the capacity and where in the cycle you broke the connection you can get a lot more of an arc than you want on a screwdriver. On high powered transmitters it is standard procedure to have a shorting stick of an insulating material with a ground wire attached to the end. In this case, the wire should have something like a hundred thousand ohm resistor in series with the wire. These oil filled capacitors can store a charge for a long time. You can come back after days have passed and still have a fatal charge stored. Another technique that goes back to Nikola Tesla himself is to do measurements and shorting with one hand in a pocket and never lean against the machine's frame while doing it.

The power levels are not usually high enough to worry about contact lenses, but remember that UV exposure can weld contact lenses to a cornea. If you suspect it has happened, DO NOT try to remove the lens. Go to an ophthalmologist and explain the problem.

Bill
 
The only way to know for certain the thing is discharged is to use a known good voltmeter to check. Every other method makes you "pretty sure", since you do not know FOR CERTAIN if it was fully discharged, or if maybe the resistor failed, etc..

But a decent means is to use a 1000 ohm resistor, preferably a high power, 25 watt size is generally OK.

That will discharge a capacitor in a second or so, up to 100 uF.at 600VDC charge on it. You obviously can only use that as a temporary discharge tool, not leave it connected.
 
One issue that hasn't been pointed out is even though the amount of charge may be small, the surprise and jerk reaction can cause more problems and injury than the shock.

Tom
 
These oil filled capacitors can store a charge for a long time.

Former boss claimed that in some cases, cosmic rays would actually charge *up* better quality
oil-filled units. He always stored them with shorting wires across the terminals.

Back in the day the appropriate way to discharge 600 volt oil filled capacitors (after they had been
charged up) was to either a) toss them at a co-worker, or b) chase your buddy around the shop
with them, pointy end foward.....

Ah youth.
 
Former boss claimed that in some cases, cosmic rays would actually charge *up* better quality
oil-filled units. He always stored them with shorting wires across the terminals.

Just fyi. Cosmic Rays (muons, the derivative that can penetrate earth's atmosphere) are not going to charge a capacitor.... For numerous reasons.

Sent from my SM-G935P using Tapatalk
 
Just fyi. Cosmic Rays (muons, the derivative that can penetrate earth's atmosphere) are not going to charge a capacitor.... For numerous reasons.

Sent from my SM-G935P using Tapatalk

Actually, it is the other way around. I have an instrument that measures radiation by charging a very low leakage capacitor which is then left in the radioactive area for a known length of time and the amount it has discharged measured. The particles cause ionization in the air.

Bill
 
Actually, it is the other way around. I have an instrument that measures radiation by charging a very low leakage capacitor which is then left in the radioactive area for a known length of time and the amount it has discharged measured. The particles cause ionization in the air.

Bill

10-4.

Here we are speaking different densities of particles that cause ionization. Even if the capacitors leakage rates were the same, and we assume that a muon caused ionization sufficiently close the plate terminals, the charge could not accumulate due to the low density of muons vs leakage rate for cap.

Just wondering, how did you come to be in possession of that instrument??

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Recently purchased a 1800uF 450DC capacitor. There was a shorting wire across the terminals. Have seen them stored that way.
 
A simple useful tool I use to discharge capacitors is a regular lightbulb screwed into a temporary pigtail socket. Gives a visual indicator of discharge without a bunch of arcing.
 
Last edited:
FEBF143E-3664-4A8C-BCA4-D38B54F552C5.jpgOk, I am now convinced that they are discharged properly through the resistors. The converter is outputting 248, 244, 249 with the initial stab at balancing. I’m inclined to leave it alone. My last (for now) question before I try to hook up the Hendey has to do with idler rotation. I noticed that the idler is spinning backwards. That’s going to bother me. To reverse the direction it seems like I just switch L1 and L2 from the converter to the idler. Is it really that simple. I’m concerned about the rest of the start caps and such
 
View attachment 247281Ok, I am now convinced that they are discharged properly through the resistors. The converter is outputting 248, 244, 249 with the initial stab at balancing. I’m inclined to leave it alone. My last (for now) question before I try to hook up the Hendey has to do with idler rotation. I noticed that the idler is spinning backwards. That’s going to bother me. To reverse the direction it seems like I just switch L1 and L2 from the converter to the idler. Is it really that simple. I’m concerned about the rest of the start caps and such

Depends on how you have the start caps wired. Remember, the motor starts as a single phase motor with the third set of windings as the start winding. Reverse the start cap and the motor will go the other way.

Tom
 
Thanks Tom

I’ll try that tomorrow. I wired the entire unit like the fitch diagram posted here. The only additions are the volt meters
 
Just fyi. Cosmic Rays (muons, the derivative that can penetrate earth's atmosphere) are not going to charge a capacitor

Didn't say it was correct, just that my boss did that. Besides I suspect he was worried about neutrinos. Or may be magnetic monopoles.
Possibly galactic axions. It was a long time ago.
 
Didn't say it was correct, just that my boss did that. Besides I suspect he was worried about neutrinos. Or may be magnetic monopoles.
Possibly galactic axions. It was a long time ago.

There is a whole galaxy of as yet undiscovered particles Star Trek is always having to deal with. Now doubt some of them will charge capacitors when the ship's shields get below 10%.

Bill
 
10-4.
Just wondering, how did you come to be in possession of that instrument??

Sent from my SM-G935P using Tapatalk

For the same reason that magpies collect bright objects, I collect odd things. Looking around my living room, I have a Lord Kelvin multicellular electrostatic voltmeter, a Russian CRT computer memory tube, a South Congo chief's drum, used in the African drum communication network, A transmitting tube from the British Chain Home radar system used in the Battle of Britain, another large water cooled tube from the Rocky Point station, a couple of tangent galvanometers, two Brown Repeaters, a Karl Winter electrostatic generator, two pre 1900 X-ray tubes, and other items too numerous to mention.

The instrument in question showed up on EBAY with little interest so I got it cheap. I was going to post a picture but I can't find it. I think it is in the cabinet in the shop blocked by a high current power supply. I don'r remember the name except that it is a dosimeter. A friend who used to maintain the accelerators in the high powered X-ray machines at the Barnes Jewish complex used one, but he can't remember the name, either. It was a long time ago.

Bill
 








 
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