Naked Science Forum

General Science => General Science => Topic started by: singh awtaar on 27/02/2009 18:16:39

Title: How does the magnet used at a scrapyard work?
Post by: singh awtaar on 27/02/2009 18:16:39: what is the ON - OFF system in the scrap lifting machine, (based on permanent magnet.) how it work.
Title: Re: How does the magnet used at a scrapyard work?
Post by: Madidus_Scientia on 27/02/2009 18:57:40: It's not a permanent magnet, it's an electromagnet. When a current is passed through wire, it creates a magnetic field. The more current, the stronger the magnetic field. So electromagnets are usually made by wrapping a coil of wire tightly around a peice of iron, or other ferromagnetic material. (Ferromagnetic means something which is attracted by a magnetic field, but is not itself a magnet. Steel or iron, for example.)

The ON and OFF system is achieved simply by turning the power to the electromagnet on or off.
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 27/02/2009 20:14:59: Have you heard of the permanent magnet version which is more 'fail safe'? You use a permanent magnet, which will not 'let go' on its own. You release the load of iron by a burst of electric current in a coil, around the magnet, which nullifies the permanent field for a brief time - then the load falls off.
Title: How does the magnet used at a scrapyard work?
Post by: Chemistry4me on 27/02/2009 22:51:35: Nope, I have never heard of it. Btw...how do they change the poles around?
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 28/02/2009 08:09:23: With the current in the coil. It gives a brief canceling field when switched on.
Title: How does the magnet used at a scrapyard work?
Post by: Chemistry4me on 28/02/2009 08:11:31: I thought that you could switch the poles around on an electromagnet, i.e. the North becomes South and vice versa, how do they do that?
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 28/02/2009 11:13:42: That's unlikely because you would need to change the polarity of a modern permanent magnet - that would require hundreds of times more current and, probably, some heating would be involved.

The system I described uses a supplementary coil, merely to interrupt the field produced from the permanent magnet. (Anyway, what would changing the polarity achieve?)
Title: How does the magnet used at a scrapyard work?
Post by: Chemistry4me on 28/02/2009 11:18:34: I was merely wondering [:)]
So can you or can't you (with an electromagnet)? [???]
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 28/02/2009 15:04:03: Can u or can't you what?
To make a powerfull enough permanent magnet, you couldn't support a suitable electromagnet on a simple crane.
Title: How does the magnet used at a scrapyard work?
Post by: teragram on 28/02/2009 15:52:43: Quote from: Chemistry4me on 28/02/2009 08:11:31
I thought that you could switch the poles around on an electromagnet, i.e. the North becomes South and vice versa, how do they do that?

When the current supplied to the coil of an electromgnet is direct (D.C.), the poles can be reversed by reversing the direction of current flow in the windings. If the current is alternating (A.C.)the poles are continously reversing, ie. at 50 times per second if (U.K.) mains frequency is applied.
Title: How does the magnet used at a scrapyard work?
Post by: Madidus_Scientia on 28/02/2009 16:08:39: No I hadn't heard of that sophiecentaur, good idea though. But you wouldn't be able to have as powerful a magnet would you? or it would be a hell of a lot heavier than an electromagnet equivalent?
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 28/02/2009 17:38:50: Not with modern magnets. You're not after the sort of high flux density of an LHC magnet. You're just after two concentric poles - like a loudspeaker(total flux).
I don't think you could make a micromotor with the power / torque and size of a modern permanent magnet motor using a field winding.
Title: How does the magnet used at a scrapyard work?
Post by: Chemistry4me on 28/02/2009 22:58:48: Quote from: teragram on 28/02/2009 15:52:43
Quote from: Chemistry4me on 28/02/2009 08:11:31
I thought that you could switch the poles around on an electromagnet, i.e. the North becomes South and vice versa, how do they do that?

When the current supplied to the coil of an electromgnet is direct (D.C.), the poles can be reversed by reversing the direction of current flow in the windings. If the current is alternating (A.C.)the poles are continously reversing, ie. at 50 times per second if (U.K.) mains frequency is applied.
Okay, thanks teragram [:)]
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 01/03/2009 15:13:26: You can make a workable electromagnet using AC but it makes a fearsome buzzing noise wken supporting a steel load; it still supports things despite the polarity changing. It is not 'on' all the time and so it isn't as 'strong'.
Title: How does the magnet used at a scrapyard work?
Post by: singh awtaar on 01/03/2009 17:36:06: i do not know these answer giving friends are scientists or some commen men like me. but it is true that their knowledge about permanent magnet is very short. ( except one friend, massage ID no.231997) they know electromagnet only. here are many instruments using permanent magnet like scrap lifter, work holding chucks. needle block for lath machine and so many......avtaar singh
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 01/03/2009 21:49:14: "Scrap lifter"?
How would it let go?
Title: How does the magnet used at a scrapyard work?
Post by: Chemistry4me on 02/03/2009 06:04:58: I'm ~~Spartacus~~ a common man!
Yes, you are correct, my knowledge of electromagnets can be counted on one finger [:)]
Title: How does the magnet used at a scrapyard work?
Post by: singh awtaar on 03/03/2009 08:56:16: thanks for giving answer for my previous question.
my another question is : when the glaciers are melting day by day very speedily due to global warming.
then river should be over flooded. but in India ( how far i know ) the rivers are dry. the ground level is going down day by day. the question is that where the water is going.
Title: How does the magnet used at a scrapyard work?
Post by: Chemistry4me on 03/03/2009 08:57:42: The water is on my front lawn!
Title: How does the magnet used at a scrapyard work?
Post by: Chemistry4me on 03/03/2009 08:59:18: Perhaps you should start another thread in the Environment section? [:)]
Title: How does the magnet used at a scrapyard work?
Post by: Bored chemist on 03/03/2009 20:07:49: What would stop you using a switchable magnetic clamp like this one
http://www.axminster.co.uk/product-Axminster-Magnetic-Welding-Clamp-with-Release-Switch-715039.htm
(bigger than that, obviously) and having a little electromaget flick the switch?
Title: How does the magnet used at a scrapyard work?
Post by: lyner on 04/03/2009 19:40:37: I imagine that it works with a permanent magnet and an iron 'yoke'. The 'switch' will be a lever which pulls the permanent magnet away from the yoke and putting a 'magnetic short circuit' into the reluctance path. It could work on a bigger scale but might require a lot of force to do this mechanical action with a magnet which can lift a car.