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Three questions: Motors, Generators, Batteries

  1. May 7, 2004 #1
    Question 1: Why use an iron core armature?
    I took an old electric motor apart to study it. I noticed the armature has windings around an iron core. Iron is used to concentrate the magnetic flux. But iron is also attracted to the electromagnets used to repell the armature windings! It seems to be both a push and pull situation. Granted it is a greater push than pull or the motor would not spin. So, to avoid the iron core being attracted to the electromagnets, why not use a non-ferrous metal for the core? Is the benefit gained from using iron to concentrate the magnetic flux that significant?

    Question 2: Why does it become more difficult to turn a generator when a load is placed on it?
    I've read that this is the case, but I haven't been able to find out why. What happens inside the generator to cause the drag?

    Question 3: How do I safely connect a battery to a coil/electromagnet?
    I've read that connecting a coil/electromagnet to a car battery could cause the battery acid to heat and expand and possibly cause the battery to rupture. What is the typical safe way to power a coil/electromagnet using a battery? Potentiometer? Or is it more complex?

    Thanks in advance.

  2. jcsd
  3. May 8, 2004 #2


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    I'm not sure on #1, or #3 really.

    #2, if the generator is loaded, it requires X amount of torque to produce the required amount of power by the load. In a non loaded situation, it may only require a small amount of input energy to spin the generator, whereas once you've hooked up a device that is using the output energy, it requires more input to produce.

    A good example of this can be seen with a gasoline welder. My friend has one, inline 4 cyl which doubles as a generator. With no load, it idles at about 800 rpm, but as soon as you start welding, or running something off the generator, the idle increases. The greater the demand for power, the rpm of the engine increases.

    That damned thermodynamics I guess.

    And taking a guess at #3, I've actually played with this a bit trying to make my own electromagnet. I did the ole wrap wire around a nail trick, where you have a single piece of wire wrapped around a nail, with each end of the wire going to battery terminals. The problem I had was the resistance was to small, which made the wire, nail and battery heat up rather quickly.

    I was thinking perhaps sticking a resistor inline somewhere would help, but I'm not really sure.
  4. May 8, 2004 #3


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    Greetings !

    1. As for you first question, Iron doesn't just concentrate magnetic
    flux - it multiplies it MANY times, so you get a much stronger
    magnetic field. Without it, electromagnets consisting of coils alone
    could never lift cars, trains and more.

    3. Like megashawn said, in general you need a resistor because
    coils ussualy have low resistance and you basicly end up short
    circuiting your source. As for any numbers, you have to look
    at the actual equipement you're using, of course.

    Live long and prosper.
  5. May 8, 2004 #4


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    First you have to understand why Iron is attracted to a magnet. The atom that make up Iron form magnetic domains (essentially microscopic magnets). In their normal state they are all pointing in random directions and thus cancel each other out.

    When you place Iron near a magnet, the lines of flux from the magnet pass through the iron and cause the domains to align. This now, in turn, makes the hunk of Iron a magnet itself. And since the poles of this new magnet rely on the direction the domains align, and they align according to the original magnet's line of flux, The two magnets will always be attracted to each other.

    In the case of the motor, the armature windings generate lines of flux which pass through the core which aligns the domains in the core with them. This is what reinforces the magnetic flux, and turns the core into a magnet with the same poles as the armature windings. In this case the iron core repels the electromagnets . (It cannot be attracted. For that to happen, the domains would have to realign themsleves, and they are being held in their present alignment by the flux of the armature coils.
    When you place a load on a generator it draws a current. This means that this current is also flowing thorugh the windings of the generator. This turns them into electromagnets, electromagnets that have a polarity such that the magnetic force they create works in opposition to the rotation of the armature. The greater the load on the generator, the more the current draw, the greater the current flow through the coils, and the stronger the force opposing the rotation.
  6. May 9, 2004 #5
    You must be talking about a universal motor, not an induction motor. (The metal used is almost certainly soft steel and not pure iron, incidently) At any rate BOTH the rotor and stator are soft steel wound with copper coils. In other words: both the rotor and stator are electromagnets. They are arranged such that, at any given time each electromagnet is repelled by the one it is directly facing and attracted to the nearest one in the desired direction of rotation. The switching occurs through the wiring of the commutator.

    If the rotor on the motor you took apart had no copper windings on it, then it was an induction motor. This works on a different principle, and requires alternating current to operate. A universal motor can operate on either direct current or alternating current, it doesn't matter because what ever you put into it, the geometry of the rotor and stator assure that the fields generated will be in opposition to each other and rotation will happen.

    An induction motor relies on the switching of polarity of alternating current to operate. Imbedded in the apparently smooth drum shaped rotor you may be able to see lines of a different metal (sometimes copper, sometimes zinc) that connect to rings of the same different metal at each end. This is what is known as the "squirrel cage". Actually, if it were freestanding it might look more like a hampster running wheel. There might be two or four stator or armature coils, depending on the speed the motor is designed for. The more poles, the slower it goes.

    The armature coils (electromagnets) induce current to run through the squirrel cage. There is no direct electrical connection to the rotor. This induced current is amplified by the steel of the core and if the armature magnet is, say, of a south polarity the part of the rotor nearest it will take on a north polarity. Just when it is settling into that situation of attraction the armature magnet switches polarity (because the current has alternated) and also becomes north. The rotor doesn't catch up instantaneously. There is a lag time during which both are now north and they repell each other: the rotor turns to present its south face to the north of the armature. Just as it does this the armature changes again. It continues like this indefinitely: the armature induces a pole in the rotor and then suddenly changes to the same pole, repelling it.

    The induction motor was a brilliant invention that did away with the need for the complex commutator, and the carbon brushes that wear out, start to spark, and need replacement. It's main drawback is that it is never as powerful as a universal motor of the same size. It also does not make a very good variable speed motor. Never-the-less it is now the motor of choice whenever alot of starting torque isn't needed and service-free long term operation is desirable. You will find an induction motor in just about all the fans in your house. For a universal motor look in your food blenders and your electric drill and vacuum cleaner. If it's loud and noisy, it's probably a universal motor. If it's quiet: an induction motor.
  7. May 9, 2004 #6
    I've played with wrapped nails alot and never had this problem. Someone else told me they recently burned their fingers while trying this.

    I think the reason mine may not have heated up is that I never wrapped just one nail. I usually bunch together about ten or more of those finishing nails with very small heads, alternating the heads and the points.

    I also wrap at least several layers of wire, not just one layer. The more wire you wrap the stronger the magnet because you have more "amp turns". Since you are using what amounts to a longer piece of wire the more turns you put, the more the coil acts as its own resisitor.

    As a rough rule of thumb, if you wrap your core so that the thickness of the windings triples the diameter of the whole, you will have a great electromagnet. In other words, if your core is a half inch in diameter, keep winding layers of wire on till the whole thing is one and a half inches in diameter.

    Another good tip is to cut the heads and tips of the nails off altogether, wrap them with masking tape to keep them together and grind the ends smooth on a bench grinder or disc sander. Then wrap your coil over this (leave the masking tape on). You can make a surprisingly powerful electromagnet this way that will run off a couple of flashlight batteries.
  8. May 10, 2004 #7
    Thank you all for your replies.
    1. I see that the iron takes on the polarity of the windings on it and repell with a great force.
    2. I see that the windings become electromagnets when current is generated in them, and cause the 'drag'.
    3. Still not clear. Is a resistor (fixed or variable) the only component required? How would I go about selecting a resistor for say a coil of 100 feet of 18-guage magnet wire connected to a 6-volt lantern battery? What about a 12-volt car battery? What about 100 feet of 20-guage magnet wire for 6-volt and for 12-volt?
  9. May 10, 2004 #8


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    For your resistor to reduce the current flowing in your electromagnet, you need to find the resistance of your wire and its current handling ability.

    A car battery is a very large battery and will melt wires and cause fires if used improperly, be careful!!!

    We could calculate the resistance and find the current (max) that could be handled, but then the battery life will be short since the more current you pull the faster they discharge on an curve.

    Instead, you could get like an 8-ohm 25-watt resistor and wire it in series and that should keep things in check for the examples you listed.

  10. May 10, 2004 #9
    As Drag said, the core greatly amplifies the magnetic field of the coil. The resulting electromagnet has two poles, north and south. It does not automatically always repell: like poles repell and unlike poles attract.
    I'm not sure what you mean by "drag" here.
    The strength of an electromagnet is in proportion to the amperage, not the voltage.

    Increasing the amperage of the power source will increase the strength of the magnet. But it will also make it get hotter faster. Using a more powerful battery is counter productive.

    The trick to increasing the amperage without the magnet getting hotter is to wind more low amperage turns onto it.
    Each additional turn you wind on a coil increases the amperage of the coil and increases the strength of the magnetic field.
    If you double the number of windings, you double the amperage contributing to the magnetic field, and double the strength of your magnet.
    The larger the diameter of the wire you use the more amps it will allow to pass. Therefore you want to start with wire that is on the small size, and make up for the lower amperage by putting more "amp turns" on the coil. Because the wire is smaller, more amp turns will fit.

    It is difficult to give you exact instructions because everything depends on the dimensions of the core you use, whether or not it is solid or laminated or wire, and the exact metal you use. Solid cores are generally a bad idea. They heat up faster and give the worst magnetic fields, because the magnetic fields just swirl around internally in what are called "eddy currents". Cores made of thin laminations are the best.

    However, for any given core I am sure that smaller diameter wire with more amp turns is superior to large wire with few amp turns.

    If you take a large, powerful battery, like a car battery, and put a big resistor on it, all you're doing is effectively turning it into a smaller battery.

    Go here:


    and scroll down to the section titled Battery Powered Electromagnet Capable Of Lifting 500 lbs!. This magnet runs off of two "D" cells. I believe part of the power can be attributed to the fact that the rear pole is brought forward in the outer ring so that a complete magnetic circuit is made. In addition you can see that the coil is quite a large diameter for the length - many amp turns per unit of length.
    Last edited: May 10, 2004
  11. May 10, 2004 #10
    For your resistor to reduce the current flowing in your electromagnet, you need to find the resistance of your wire and its current handling ability.

    I found that 18-guage copper wire has 6.385 ohms per 1000 feet, and can conservatively carry 2.3 amps for power transmission. I also found a power rating formula for resistors: P = (I*I)R, where P is in watts, I is in amps and R is in ohms. So I think I need to find out how many amps each type of battery is capable of outputting, then any battery I want to use which outputs over 2.3 amps would need a resistor. Right? Then using the formula I can determine the resulting current from various resistors. Correct?


    I'm not sure what you mean by "drag" here.

    I'm referring to the increased difficulty in turning a generator when a load is placed on it.

    The strength of an electromagnet is in proportion to the amperage, not the voltage.

    Yes. I just assumed all you knowledgable people would know how many amps are output by a 6-volt lantern battery and a 12-volt car battery. :)

    If you take a large, powerful battery, like a car battery, and put a big resistor on it, all you're doing is effectively turning it into a smaller battery.

    But longer lasting, right?

    Cores made of thin laminations are the best.

    Thin laminations? Is that the stack of metal-sheets I see sandwiched together inside the motor I took apart? They produce better magnetic fields? Interesting! Less heat too? Hmmm! Who would-a-thunk it! :)
  12. May 10, 2004 #11


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    Use ohm's law. Voltage = Current * Resistance

    If you know two, you can find the other. So if you have 12V and want 2A of current, you need a total of 6 ohms.

    And its more than just the voltage, you need another to determine if the current/resistance will be ok. My suggestion of an 8ohm resistor is to simply limit the amount of current to keep everything reasonable, but you could easily use ohm's law and find the exact number for the resistor (and then find the wattage rating you'd need as well).

  13. May 10, 2004 #12
    You should be able to find the cranking amps of any given car battery on the battery itself. Mine, which isn't a particularly big one, has a rating of 815 cranking amps at 32 degrees and above.

    That's alot of amps.
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