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Old 12-05-2012, 12:44 PM   #1
Pulsar
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Default DC motors and armatures

Ok, it seems like a lot of websites and people who try to explain DC motors are unable to explain why an armature should be made out of metal. See if you follow my thinking.

Current passes through the windings around the armature. That produces the e-field that fights the magnets that makes the whole thing turn.

So what part does an armature made out of metal play? If anything, the e-field in the windings will create currents in the armature, and that in turn will create a back field that fights the direction-change of the current in the wire. If anything, it seems like a conductive armature would make a motor less efficient, not more efficient.

In fact, they make armature-less motors. Most all small motors (think phone size and pager size) are armature-less.

Give me a little help: explain why an armature needs to be conductive metal (other than for heat-transfer).
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Old 12-05-2012, 01:05 PM   #2
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I'm not quite sure if I'm understanding you correctly, but if you're talking about the laminated metal cores that the wire actually wraps around, the answer is that the armature inductance goes way up with the presence of a ferrous core. The core is laminated to cut down on the eddy currents you're referring to. If you're talking about the shaft that the armatures are mounted to, they could be made of silly putty if it were strong enough.
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Old 12-05-2012, 01:30 PM   #3
Mark R
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The use of a ferromagnetic material (more specifically a material with high magnetic permeability) increases the magnetic flux density generated by the armature coil.

Magnetic flux is circular - i.e. it must form a circuit. There is analogy between electrical and magnetic circuits. Potential difference (volts) is the driving force for a current (amps), in the same way that magnetomotive force (ampere-turns) is teh driving force for magnetic flux (Webers); and the relation between potential difference and current is given by the resistance (ohms) of an electrical surface, and in the relation between MMF and Flux is given by the magnetic reluctance.

Iron has a very high permeability, which means a very low reluctance, and therefore a much larger magnetic flux for the same current-turns product,

As it is the interaction of the magnetic field of the armature with that or the stator that generates the torque, the stronger the field, the more torque.

You can make motors without a metal core, but you get very poor torque, and because of the high currents and huge turns number required, you get a lot of electrical resistance and poor efficiency. However, this technique has been used on experimental superconducting motors operating at super-strong magnetic field strengths (iron experiences "magnetic saturation" where once the magnetic field strength reaches a critical level, it cannot increase further - without iron you can just keep increasing the current/turns. With regular copper wire you'll never match an iron core without burning out the coil, but with superconductor you can crank up the current and turns to ridiculous levels and get fields 5x higher than that achievable with iron).

Last edited by Mark R; 12-05-2012 at 01:36 PM.
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Old 12-05-2012, 07:51 PM   #4
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Quote:
Originally Posted by Mark R View Post
The use of a ferromagnetic material (more specifically a material with high magnetic permeability) increases the magnetic flux density generated by the armature coil.

Magnetic flux is circular - i.e. it must form a circuit. There is analogy between electrical and magnetic circuits. Potential difference (volts) is the driving force for a current (amps), in the same way that magnetomotive force (ampere-turns) is teh driving force for magnetic flux (Webers); and the relation between potential difference and current is given by the resistance (ohms) of an electrical surface, and in the relation between MMF and Flux is given by the magnetic reluctance.

Iron has a very high permeability, which means a very low reluctance, and therefore a much larger magnetic flux for the same current-turns product,

As it is the interaction of the magnetic field of the armature with that or the stator that generates the torque, the stronger the field, the more torque.
Still not following. Are the magnets opposing the field from the coils, the field from the armature, or both? It seems to me that the field generated from the coils necessarily cancels out, and you're left with the field generated from the armature to turn the rotor.
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Old 12-06-2012, 02:25 PM   #5
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The field from the armature coils interacts with the stator field, producing a force.

Imagine a stator which produces an even magnetic field running from left to right.

You have an armature with 1 turn; on your left, the wire current flows directly away from you. on your right, the current runs directly at you.

The wire on the left creats a field that runs left-to-right above the wire, and right-to-left below the wire. This leads to an area of increased left-to-right magnetic field strength above the wire, and an area of weakened field strength below the wire.

Magnetic fields are a bit like springs, and will exert forces on the objects creating the field, so as to reduce stresses and stored energy. The unbalanced magnetic field above and below the wire, creates a force that pushes the wire down.

The same thing happens if you have two wires next to each other, carrying current in opposite directions (e.g. in a power cord). The magnetic field from the two wires interacts, and creates a zone of stronger magnetic field between the two conductors, and this creates a repulsive force on the conductors.

On the right, the current is flowing the opposite direction, so the imbalance in the field is reversed, and this results in a force that pushes the wire up.

The two forces acting on wires on opposite sides of the spindle creates a torque which causes the rotor to spin.
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Old 12-06-2012, 09:16 PM   #6
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Excellent explanation Mark!

Let me give it a try:

The metal core is like wire for magnetism. Sure electricity can flow in air (lightning) but wire makes it much easier. Same goes for the metal core. Sure you can make a motor without the metal core but the metal makes it better.

Also you need a sturdy member to push on. Even if you didn't need for the metal to act like a conductor for magnetism you would still need to attach the wire to something sturdy (but that really isn't the issue we are talking about).
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Old 12-07-2012, 01:33 AM   #7
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Quote:
Originally Posted by Pulsar View Post
Still not following. Are the magnets opposing the field from the coils, the field from the armature, or both? It seems to me that the field generated from the coils necessarily cancels out, and you're left with the field generated from the armature to turn the rotor.
It seems to me like you are talking about the eddy currents stormkroe referred to.
The reason i think this is that you are mentioning the "field generated from the coils necessarily cancels out" which is something similar to what eddy currents do.

So here's the lowdown:
The armature is not supposed to be conductive like you are saying, that is simply a side effect. And like you have noticed, it is NOT a good side effect.

The reason they choose iron is not because it is an electrical conductor (again, like you have noticed, this is actually bad). The reason they choose iron, like mark mentioned, is because it has a high magnetic permeability. That means its a very good material for containing and guiding magnetic fields.

I don't know if you ever noticed, but usually motors will have laminated sheets of iron to create the armature. Part of the reason for these laminated sheets is to counter-act the eddy currents that you are referring to. By using laminated sheets there is never a large single conductive path for large eddy currents to form, this way you have an armature that guides magnetic field lines very well (because the plates allign that way) and doesn't allow for the creation of large eddy currents (because the plates are alligned incorrectly for such currents to be created).
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