Date: Thu Nov 2 14:49:53 2006

Author: Dan Beeker

Subject: Re: jumping rings "launcher" and load (fwd)

Post:
Most breakers in household wiring (20 amp circuits and less) are thermal
breakers. They require some time to heat up before they will trip. Most of
the breaker manufacturers have charts and figures with the characteristics
of different breakers. A 20 amp breaker with 20.1 amps running through it
may take forever to trip. It takes several tens of amps, perhaps a hundred
amps to get the breaker to trip more quickly. I don't remember the exact
values but they ultimately come out of the NEC for us Americans. I suppose
it is the CSA or some other such regulatory agency in other places. One of
the design considerations for breakers is their ability to trip on humongous
overloads. If the current is large enough the contacts will weld and the
breaker will never trip. Eventually, I suppose it will trip when it erupts
into a ball of plasma. That is why breakers are put into protective
enclosures that are supposed to contain the arcing so people in proximity to
breakers don't get injured. That is also why there are multiple breakers in
electrical supplies. If the branch breaker fails from too much current,
hopefully the feeder breaker will trip. Worst case, the power station shuts
down and the lights go out everywhere.

Breakers upstream of the branch circuits and breakers in high current
equipment may have electromagnetic actuation. Then, current causes the B
field to increase in a trip coil until the circuit trips.

Finally, it should be noted that most breakers are not designed to be used
as switches. They are designed to open a limited number of times under very
limited conditions. That is why we use switches to turn power on and off.

Dan Beeker
Undergraduate Physics Lab Coordinator

Swain West 115
Physics Department, Indiana University
727 E. 3rd Street
Bloomington, IN 47405
Phone: (812) 855-5903
Fax (812) 855-5533
debeeker@indiana.edu

-----Original Message-----
From: tap-l-owner@lists.ncsu.edu [mailto:tap-l-owner@lists.ncsu.edu] On
Behalf Of Dale Stille
Sent: Thursday, November 02, 2006 12:57 PM
To: tap-l@lists.ncsu.edu
Subject: Re: [tap-l] jumping rings "launcher" and load (fwd)




---------- Forwarded message ----------
Date: Thu, 2 Nov 2006 11:31:11 -0600 (CST)
From: Dale Stille
To: "John Cockman, Jr."
Subject: Re: [tap-l] jumping rings "launcher" and load


John and Mike,

You got my curiosity going now.....so I did some real measurements on my
coil.
The demo we routinely do with this is to show this as an electromagnet with
and
without the coil. We routinely turn our variac up to about 80 volts for
both
of these demos. You can't do it for extended periods because the coil heats
up
pretty fast. So, when I measured the current when used without the coil it
shows we are pumping 60 to 70 amps through this for the short time we use
it.
Curious, because the room circuit breakers are 20 amp breakers and the
breaker
on the variac is a 15 amp breaker......and yet we have blown neither of
these
doing this demo in the 20 years I have been here. And before you say the
breakers aren't working.....we know they are because we have other demos
where
we do break them on a routine basis. When we insert the coil and repeat the

demo, then our current is down to 10 to 15 amps.

According to your reports it shouldn't work......and yet it does. And it is

also not just a single special coil that does this. I have built 4 other
coils of this design since I have been here and they all work the same.

Later,
Dale
U of Iowa

On Thu, 2 Nov 2006, John Cockman, Jr. wrote:

> L = r^2*n^2/(9*r+10*l)
>
> r = radius of coil
> l = length of coil
> n = number of turns
>
> Inductive reactance XL = 2*pi*f*L, where f=60hz
>
> Impedence Z = (XL^2 + R^2)^1/2 where R is the resistance
>
> You probably want V/Z to be less than an amp.
>
> John
>
> ----- Original Message -----
> From: Krishna Chowdary
> To: tap-l@lists.ncsu.edu
> Sent: Thursday, November 02, 2006 10:09 AM
> Subject: Re: [tap-l] jumping rings "launcher" and load
>
>
> I knew I was forgetting something obvious. Thanks to all who
> reminded me
> about the inductive reactance of the solenoid+core itself contributing to
the
> overall impedance of the circuit.
>
> Followup question: clearly if the inductance is too small, I _would_
> blow a
> fuse or melt something. The inductance is related to the number of turns
on
> the solenoid, solenoid geometry, and any conducting core filling material.

> If I were to do this with just a simple air core solenoid, does anyone
have
> experience with dimensions, number of turns per unit length, wire gauge,
> etc.to get a reasonable inductive load at 60 Hz? Again, my goal is to
plug
> an air core solenoid into a wall outlet safely.
>
> Again, thanks for your input.
>
> --
> regards
> -Krishna
>
> Krishna Chowdary
> Department of Physics & Astronomy
> Bucknell University



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