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Force of neodymium magnet inside an electromagnetic coil

by alastairgig
Tags: coil, electromagnetic, force, inside, magnet, neodymium
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Jul9-11, 10:58 AM
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Hey guys,
Im trying to design a electric gear shiftier for a motor bike. Basically I require about 200N of force to change gear with a 10mm throw (in both directions). I would like to use a tube neodymium magnet. (23mm diameter 25mm high with a 5mm hole throw the centre with a N48 grade)
What I need help with is calculating the number of turns of wire, the current and gauge of wire of the coil. I will be using 12V power supply and Im limited to 60amps current draw.
So the magnet will be placed in the middle of the 45mm long coil and the direction of current flow will be changed to make the magnet travel in one direction or the other to change up and down. The magnet will return to the centre of the coil via a recoil spring.

Any help will be greatly appropriated.
Thanks so much guys
Alastair - (undergraduate mechanical engineering student, Adelaide university)
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Jul10-11, 09:51 PM
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These problems are so dependent on the exact geometry of the setup that it is worth doing some measurements and then scaling up the results.

If you can, get a digital balance. Put your magnet on a piece of polystyrene foam and place this on the balance. This is to avoid the magnet affecting the accuracy of the balance.

Zero the balance.

Wind up a coil of say 50 turns and place this over the magnet in the configuration you described but not touching the magnet or the foam.

Using a DC power supply with a rheostat in series, gradually apply current to the coil and observe the change in reading on the balance. The magnet will experience a force up or down which will be reflected in the reading on the balance.

You should be able to find a current that would be required to get the force you need.

You should be aware, though, that the magnet has two poles which will be affected in opposite directions by any magnetic field in the coil. So, short magnets are difficult to get working in such arrangements because their poles are very close together.

Also, Neodymium magnets are notoriously brittle and tend to get fragmented into small chips every time they hit something. This may make it difficult to get a force to drive a lever, for example.

The action you describe is usually performed by pulling a soft iron core into a solenoid against the action of a spring. These devices are used extensively in industry and in domestic appliances like washing machines.

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