large & efficient electro magnet

[therian]

Im building pulse motor, after construction small scale I decide to make large one, I will hand wound 2 km of thin wire to create electromagnet it a lot of wire to do manually so I want to make most efficient magnet from first time.
The question is what is more important , coil bobbin length or thickness , should my coil be tall or short and fat, I will use 1 inch diameter iron rod.

Formula from wiki
B = (N I)/L


* N is the number of turns of wire around the electromagnet
* I is the current in amperes
* L is the length of the magnetic circuit
* B is the magnetic field in teslas


show that fat coil will be more efficient (less L) but personal experiment show opposite result , long coil stop moves compass needle from 7 cm when thick coil twice shorter of same wire length stop move needle only from 4 cm away




Another question should I wind many wires together and then connect them (in series or parallel) or single wire is more efficient



maybe picture will explain better what I mean.

[Melanie]

All you ever wanted to know...

http://info.ee.surrey.ac.uk/Workshop/advice/coils/force.html

Wiki I would not trust.

[therian]

Thank you that was very useful information

just one more question, can I have an advice how to safety connect digital scope to read the coil, I have 100x probes but they home made and really noisy so I want to use 10x one and voltage might exceed them and I don't know what will happen to scope then.

[Melanie]

and I don't know what will happen to scope then

You will destroy it. A coil with the kind of inductance you are winding will probably have an emf of a few kV... most modern scopes only have a 50v tollerance.

Use your probe set to 10:1 and use a Resistor divider chain of another 10:1. If the Bottom Resistor is 1M it won't load the circuit. That should give you a 5kV operational tollerance.

[Luciano]

Hi,

Pulse motor evolution. (Moto-perpetuos made in Seattle USA).

http://www.youtube.com/watch?v=lHpikCPKPiE
(Make sure your PC speakers are on).

Best regards,

Luciano

[Melanie]

Yeah, right... more like moto-preposterous...

Would be more meaningful if he showed CURRENT-IN versus CURRENT-OUT as well as the voltage which on it's own is irrelevant.

What he's trying to make you believe is that his Power-Input, less frictional losses, less conversion losses, less electrical losses, is still a NET GAIN?

[therian]

will this wok as 100:1 probes ?



------/\/\/\99K\/\/\--------------------> to scope
******************|
******************/
******************\
******************/
*****************1K (1.001К)
******************\
******************/
****************** \
******************|
----------------------------------------> to scope
all resistors 1W

what about AC is it safe for AC


I already build this probes but they so noisy, pick up all wiring

------/\/\/\25M\/\/\-------------------->to scope
******************|*****|******************
******************/*****| ******************
******************\*****|******************
******************/*****\/******************
*****************1M****--******************
******************\*****| just in case 2 zener diodes( which will newer switch because of low current)
******************/*****--******************
****************** \****/\******************
******************|*****|******************
-------/\/\/\25M\/\/\------------------> to scope

[Melanie]

Looks good if in your opinion 100k isn't going to adversely load your circuit.

Safety.

If you are connecting a scope to an AC supply (eg Supply Mains out of the wall), then the 0v on your scope (unless it's a Battery scope) will almost certainly be Ground/Earth. If you live in a country like the USA which controls population growth by allowing it's citizens to plug their Power Cords into the wall either way round, then you can have a situation where the scopes 0v (Earth) is being connected to the Live in your device under test.

Either...

(a) For Test purposes feed your device through a 1:1 ISOLATING Transformer (this is the preferred method as your device will no longer be LIVE - it'll still be dangerous but not LIVE relative to EARTH), or...

(b) Isolate your scope through a 1:1 Isolating Transformer (not the brightest option as your scope can then become LIVE and rubber-soled shoes and Latex gloves are then recommended).

Why then even suggest option (b)? Well your project/device just might suck 20kW and you only happen to have a 200VA Isolating Transformer!

[Darrel Taylor]

If you live in a country like the USA which controls population growth by allowing it's citizens to plug their Power Cords into the wall either way round

Yes, zapping people with Polarized 220VAC is much better than polarized 110V.
That's right, we're Polarized now. (according to code)


err- Can you polarize 220V? They're both hot aren't they?

_

[Luciano]

Hi,

Use wrought iron for the nucleus of the coil.

More about that here: (Read the book from page 1).
http://books.google.com/books?id=4KWGq6zZAo4C&pg=PA85&lpg=PA85&dq=%22wrought+iron%22+magnetic&source=web&ots=nr7xStdloZ&sig=mv6HPrAQuxmxsjLb0wQpLXST66c

The whole book in a 44MB PDF file.
(Scan of the 3rd edition of the book printed in the year 1900).
http://ia310925.us.archive.org/1/items/magneticinductio00ewinrich/magneticinductio00ewinrich.pdf

The reprint of the book in 2003:
(Same as 3rd edition of the book printed in the year 1900).
http://search.barnesandnoble.com/booksearch/isbninquiry.asp?ean=9781929148110
(113 years after the original book printed in 1891).

The author: James Alfred Ewing (1855 - 1935)
http://en.wikipedia.org/wiki/James_Alfred_Ewing

* * *

Why do you need to connect an oscilloscope?

Possible alternative solution to measure the strength of the coil's magnetic field:

http://img119.imageshack.us/img119/2382/coilig0.png

Best regards,

Luciano

[Melanie]

err- Can you polarize 220V? They're both hot aren't they?

Actually No. One side is grounded at the Building Incommer... so only one pin of the three is Hot and the Power Plug is not reverseable. You still can have a Neutral/Earth Voltage, albeit small, which is usually the load voltage drop across the cable length from the last physical bonded Earth to where you happen to be measuring it.

Two pin adaptors once widely available haven't been legal for quite a while, and those still using them are slowly diminishing through natural selection.

Any purchased appliance running from the mains must have a factory fitted connector to prevent miswiring by the great unwashed.

However, regardless where you are in the world, if you run your device under test through an Isolating Transformer, then the circuit you're playing with is not live relative to Earth (until of course you Earth it through your scope), so that means you can be barefoot using your Central Heating pipework as a footrest safe in the knowledge that if any other part of your anatomy that happens to be hanging about touches a high-voltage point in your device you're not going to die... maybe... or at least you're not going to tell the forum you have...

[Darrel Taylor]

Thanks, I did not know that.
I thought that 220 over there would be the same as ours (2 phases of 110).

I should check with "Ugly's" before I do any traveling.

_

[therian]

Thanks for help.

interesting about isolation, my scope work thought transformer and use only 12V so I dont think I need any other isolation, there is optional exit to ground which i connect to water pipe, is if fine ?

[christopherh]

I learnt so much through these threads. thanks every one for enhancing my knowledge by sharing your posts.