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Step 5Magnets and Magnet Wire
Until now, I've just been hand-waving the existence of "MAGNETS". End of story. There exist permanent magnets.
...Yes, there definitely are, and you can actually spec and buy them according to your needs. The type of permanent magnet used in most small BLDC motors today are Neodymium Iron Boron chemistry magnets. They lie within a group of magnetic materials called rare earth magnets, because Nd is a "rare earth metal". These are not actually all that rare, which helps explain why NIB magnets don't cost you an arm and a leg.
Actually, back up. They can. NIB magnets can be so powerful that they leap across a foot or more or open air and slam together - if you are trapped in between, you could be in for a world of hurt. Everybody by now has seen the aftermath of someone's hand being caught between two colliding 4 inch square NIB magnet blocks - I'm not linking that. As a tip for the future: Take extreme caution around magnets!
A typical NIB magnet is rated as Nxx, where xx is a number between 28 and 52 (as of this writing). The number is that magnet's magnetic energy product. Without diving into E&M physics, higher is better.
At a cost, of course. NIB magnets are notorious for being high temperature sensitive. The Curie Point of a permanent magnet is the point at which stops being a permanent magnet. No, they don't regain their magnetism after they cool down. For ultra high strength NIB magnets, this could be as low as 80 degrees Centigrade (or about 150F) .
That's not very high at all - you can easily trash a motor by running it too hot.
Here's a link that explains magnet ratings pretty clearly. The same person is also a reputable dealer of all sorts of magnetic mayhem.
A typical NIB magnet as used in a motor will have a remnant surface flux of 1 Tesla. If you get the Good Magnets, it is safe to assume that B in the torque expression T = 4 * m *N * B * L * R * i is equal to 1.
Hence, the equation reduces to T = 4 * m * N * L * R * i.
The takeaway fact for magnets is that stronger is better until your motor gets too hot. It doesn't hurt to have a stronger B-field. Getting the latest and greatest in N52 magnets can boost you B to 1.1 or 1.2.
I will address how to spec our your magnets shortly, but meanwhile...
Magnet Wire
A permanent magnet sitting there doesn't do anything. It's not very interesting to watch. What makes the motor work is switching electromagnets. If you've been through a physics class with any gusto, you've made an electromagnet out of wire and a nail.
Do not plug this into the wall like yours truly.
Each of the 12 teeth on the stator function as an electromagnet. From the same physics class, recall that for every turn of wire you wrapped around the nail, the electromagnet got stronger. Same deal with the stator teeth - this is why N is a factor in the equation.
So you can just make a 20,000 turn motor and be done with it, right? Sure, if you want to run 10,000 volts to actually push enough current through your windings to mean something.
There are a few constraints to consider when designing your windings. Magnet wire takes up physical space - essentially, given a set of space constraints, the more turns you want to wind, the smaller the wire has to be. This makes sense from a physical perspective. Eventually, when you use nanowires, you can have a 10 billion turn motor that packs all the slots to near 100% fill for maximum magnetic mayhem.
Except your motor resistance Rm would be astronomical. This is another constraint. Choosing the number of turns is a careful balance between getting the Km that you want but minimizing Rm. The motor resistance can only contribute to loss. It can only hurt you. Therefore, the goal of almost all hobby motor winders is to minimize the resistance.
This means using as few turns of the biggest gauge wire you can to get the Km that satisfies you. The One Wiki has a great table of AWG copper wire resistances.
Magnet wire comes in many flavors - they are all, at the end, conformally coated solid copper wire. This coating can be enamel, polyurethane, epoxy, or in exotic / high temperature motors, fluoropolymers and wound fiberglass sheaths. The cheapest grades are generally enamel insulated and will work up to about 150 degrees Centigrade.
By this point, your expensive N52 magnets would have vaporized already - unless you are dead set on taking your motor to the limits (which means this tutorial won't help at all), don't splurge on expensive HT wire.
Can you physically handle it?
Don't underestimate the strength of a strand of copper. You might be used to 28, 24, or 20 gauge magnet wire, which is small enough to be negligibly soft. Maybe annoyingly soft. Now try bending a 16 or 14 AWG solid wire, which is pretty close to the thickness of a piano's bass strings. Now imagine you have to bend this around a corner only millimeters in radius, possibly 100 times or more.
If you are having a hard time with one stand of monster wire, you can consider splitting it into equivalent parallel strands of smaller wire. RazEr's motor was wound with double 22 gauge after I had difficulty wrestling 18 gauge around for 25 turns. Use the wire gauge table to compare diameters!
...Yes, there definitely are, and you can actually spec and buy them according to your needs. The type of permanent magnet used in most small BLDC motors today are Neodymium Iron Boron chemistry magnets. They lie within a group of magnetic materials called rare earth magnets, because Nd is a "rare earth metal". These are not actually all that rare, which helps explain why NIB magnets don't cost you an arm and a leg.
Actually, back up. They can. NIB magnets can be so powerful that they leap across a foot or more or open air and slam together - if you are trapped in between, you could be in for a world of hurt. Everybody by now has seen the aftermath of someone's hand being caught between two colliding 4 inch square NIB magnet blocks - I'm not linking that. As a tip for the future: Take extreme caution around magnets!
A typical NIB magnet is rated as Nxx, where xx is a number between 28 and 52 (as of this writing). The number is that magnet's magnetic energy product. Without diving into E&M physics, higher is better.
At a cost, of course. NIB magnets are notorious for being high temperature sensitive. The Curie Point of a permanent magnet is the point at which stops being a permanent magnet. No, they don't regain their magnetism after they cool down. For ultra high strength NIB magnets, this could be as low as 80 degrees Centigrade (or about 150F) .
That's not very high at all - you can easily trash a motor by running it too hot.
Here's a link that explains magnet ratings pretty clearly. The same person is also a reputable dealer of all sorts of magnetic mayhem.
A typical NIB magnet as used in a motor will have a remnant surface flux of 1 Tesla. If you get the Good Magnets, it is safe to assume that B in the torque expression T = 4 * m *N * B * L * R * i is equal to 1.
Hence, the equation reduces to T = 4 * m * N * L * R * i.
The takeaway fact for magnets is that stronger is better until your motor gets too hot. It doesn't hurt to have a stronger B-field. Getting the latest and greatest in N52 magnets can boost you B to 1.1 or 1.2.
I will address how to spec our your magnets shortly, but meanwhile...
Magnet Wire
A permanent magnet sitting there doesn't do anything. It's not very interesting to watch. What makes the motor work is switching electromagnets. If you've been through a physics class with any gusto, you've made an electromagnet out of wire and a nail.
Do not plug this into the wall like yours truly.
Each of the 12 teeth on the stator function as an electromagnet. From the same physics class, recall that for every turn of wire you wrapped around the nail, the electromagnet got stronger. Same deal with the stator teeth - this is why N is a factor in the equation.
So you can just make a 20,000 turn motor and be done with it, right? Sure, if you want to run 10,000 volts to actually push enough current through your windings to mean something.
There are a few constraints to consider when designing your windings. Magnet wire takes up physical space - essentially, given a set of space constraints, the more turns you want to wind, the smaller the wire has to be. This makes sense from a physical perspective. Eventually, when you use nanowires, you can have a 10 billion turn motor that packs all the slots to near 100% fill for maximum magnetic mayhem.
Except your motor resistance Rm would be astronomical. This is another constraint. Choosing the number of turns is a careful balance between getting the Km that you want but minimizing Rm. The motor resistance can only contribute to loss. It can only hurt you. Therefore, the goal of almost all hobby motor winders is to minimize the resistance.
This means using as few turns of the biggest gauge wire you can to get the Km that satisfies you. The One Wiki has a great table of AWG copper wire resistances.
Magnet wire comes in many flavors - they are all, at the end, conformally coated solid copper wire. This coating can be enamel, polyurethane, epoxy, or in exotic / high temperature motors, fluoropolymers and wound fiberglass sheaths. The cheapest grades are generally enamel insulated and will work up to about 150 degrees Centigrade.
By this point, your expensive N52 magnets would have vaporized already - unless you are dead set on taking your motor to the limits (which means this tutorial won't help at all), don't splurge on expensive HT wire.
Can you physically handle it?
Don't underestimate the strength of a strand of copper. You might be used to 28, 24, or 20 gauge magnet wire, which is small enough to be negligibly soft. Maybe annoyingly soft. Now try bending a 16 or 14 AWG solid wire, which is pretty close to the thickness of a piano's bass strings. Now imagine you have to bend this around a corner only millimeters in radius, possibly 100 times or more.
If you are having a hard time with one stand of monster wire, you can consider splitting it into equivalent parallel strands of smaller wire. RazEr's motor was wound with double 22 gauge after I had difficulty wrestling 18 gauge around for 25 turns. Use the wire gauge table to compare diameters!
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