MadSci Network: Engineering

Re: Can bismuth be used as a core for an electromagnets?

Date: Sun Aug 19 13:40:45 2001
Posted By: Barry Kamrass, Consulting Engineer
Area of science: Engineering
ID: 997382399.Eg

This is a *very* interesting question, and it was a lot of fun to get the 
answer. I see two questions:  are there electromagnets in motors and are 
permanent magnets used in motors?

Just to define a couple of terms:

Coercivity:  The magnetic field, expressed in Oerstead 
(ampere-turns/cm), required to magnetize or *demagnetize* a 
substance (aka resistance to demagnetization).  This is a major factor in 
electric motor use since the current thru the motor coils produces a 
magnetic flux opposing that of the permanent magnets;  if this flux gets 
too high the motor magnets will demagnetize and you have a dead motor.

Gauss:  Magnetic Flux Density, magnetic "lines of force"/sq.meter.  Also 
Tesla, where 1 Tesla = 10000 Gauss.  This flux is what is responsible for 
magnets attracting/repelling things.  All materials have what is called a 
saturation flux density, the highest flux density you can get with the 
material no matter how much coercive force you apply.

Permeability:  The 'magnetizability' of the material, gauss/oerstead.

For electric motor use you want the magnets, permanent or electromagnetic, 
to have the highest 'energy density' = coercivity x flux density.

Now, the short answers to your questions:
Bismuth is not used in electromagnets--no advantage to using it.
Bismuth *is* used in permanent magnets.

For electromagnet use, you want the highest saturation flux density you can 
get.  You also need good permeability and the stuff has to be cheap.  Here 
the standard material used is very very pure iron, this gives a saturation 
flux density of about 14.1 KGauss.  Coercivity is not an issue here if you 
can pump enough current through the electromagnet coils giving you  
'unlimited' coercive force. So in theory you can get as  powerful an 
electromagnet as you like, for any purpose.  The limiting factor here is 
the saturation flux density, limiting energy density in a given volume.  So 
if you want more force, say for a more powerful motor, you have to make the 
magnets very large.  The problems that result here are the electromagnet 
size, mechanical strength and coil support, and power required, often 
megawatts.  I've seen one electromagnet-based motor of 15,000 horsepower, 
about the size of a locomotive, required ~15 megawatts electric power.

Re Bismuth use in permanent magnets and permanent magnet electric motors:

Around 50 years ago, maybe more, the US Naval Ordnance Laboratory developed 
a permanent magnetic alloy called "Bismanol" which is a
Bismuth-Manganese-Iron alloy.  Bismanol has very high coercive force and 
moderate energy density, making it pretty good for small electric motors.  

[To digress slightly, 'ordnance' means bombs, cannon shells, torpedos.  
Ordnance sometimes contain guidance systems which use small electric 
motor-powered gyroscopes. Hence the military need for a good motor magnet, 
and I suspect that that was the reason that the US Naval Ordnance 
Laboratory was behind this Bismanol work.]

However, checking several magnetic materials companies did not turn up any 
bismuth alloy magnets.  Nor could I find any data on any bismuth-containing 
magnetic materials.  [That does not mean that the data does not exist--it  
has to--but that the data is buried deep within physics archives].  I've 
been working with magnetics since 1973 and have never heard of Bismanol.  
So I suspect that Bismanol was replaced long ago by other alloys.  Thus I 
suggest that you not waste any time trying to find Bismuth alloys.

But there is a solution:  currently the highest energy density permanent 
magnetic material in general use is a Neodymium-Iron-Boron alloy.  This has 
an extremely high coercivity and a moderate remnant flux;  the energy 
density (coercivity x remnant flux) is the highest around.  Also, the stuff 
is relatively cheap so NdFeB magnets are used in everything from automobile 
accessory motors on up.  This is what I suggest you look into.  Suppliers:

Arnold Engineering  214-821-4654 
Magnetics Incorporated  412-282-8282
Magnetic Metals  714-892-6627

You should be able to buy NdFeB from them.  Often, for small quantities, 
just ask for a sample and then you'll get the stuff for free.  Then, since 
NdFeB is relatively easy to machine (unlike other high energy magnets) a 
magnetic materials distributor or a local machine shop will almost 
certainly be able to form the material for you.

For electromagnets use iron but be prepared to spend lots of money building 
one and paying the electric bill.
For permanent magnets use NeFeB.

Good Luck!

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