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Current drawn from motors and Eddy Currents

  1. Jun 5, 2013 #1
    Hi,

    I am about to sit my Physics A-level paper next week and I have a couple of questions.

    My first question is rather broad, but what are eddy currents? I understand they're formed in transformers and how their effect is reduced, but I don't understand exactly why they're caused. Am I right in thinking they're somewhat related to Lenz's law in the sense that the induced current will flow in such a direction to oppose the change that caused it? If that's the case then surely the eddy currents should be in the secondary coil and not in the iron core, as the induced emf is only in the secondary coil? Argghhh I'm really confused, I've watched countless videos and read so many websites but I still don't get it.

    My second question is a little more concise; why does a motor draw less current as it speeds up?

    Any help would be greatly appreciated, especially to do with the eddy currents - they've been driving me mad for so long!

    Thanks.
     
  2. jcsd
  3. Jun 5, 2013 #2
    Well a transformer and so an electric motor could work without the "armature" or the metal iron cores on which the windings sit upon , but they would be very inefficient especially at small frequencies of AC.Now the metal iron cores are used to make the induced electromagnetic field stronger and give it some direction as in electric motors. but because it is iron it is also a conductor , not as good as copper but still , so as the emf goes from the primary windings it creates the em field which then not only penetrates the iron core makes a a changing magnetic field that induces a emf in the secondary but also the emf is partly induced in the iron core, which results as currents but because the core of a transformer and a motor is usually a loop there is no voltage build up but the currents still travel down the metal sheets , you can't escape it , and that heats up the sheets.

    Anything that even weakly conducts the emf like all kinds of metals etc will get induced currents in it if a changing time varying emf will be near it like the one from mains AC.

    Speaking about the motor starting up , electric motors can take as much as 7x times it's nominal current load under start up especially when it is under load.
    It is actually pretty simple , the em field is doing work in an electric motor , now when it has to get the rotor spinning from standstill it takes more force than it takes to get the rotor to higher rpm from lower ones or similarly.
    This is so with everything like pushing something , once you get it moving it's easier then to build up speed etc.
    Well when the rotor is already spinning it's like a capacitor, only capacitor stores charge , rotor has inertia and that's why putting some load on the motor doesn't result in a huge pwer consumption like it would when the motor has to spin up from standstill because then there is no inertia to help with.
    Do you get it?
     
  4. Jun 5, 2013 #3
    Eddy currents in any conductor experiencing a changing magnetic field. Transformers use AC which produces a changing magnetic field in the iron core. Eddy currents occur in the iron core. They are reduced by constructing the iron core from thin, insulated laminations.
    The current decreases as a motor speeds up due to the 'back emf' generated in the coil as it rotates in the motor magnetic field. At the start there is no ( or very little) back emf so a large current flows into the motor. As the motor speeds up the back emf increases, it opposes the applied emf and so the current decreases.
    In an ideal motor the current would be zero when it reached full speed. (With no load on the motor!!! )
     
    Last edited: Jun 5, 2013
  5. Jun 5, 2013 #4
    Thanks for both of your replies.

    Yes eddy currents make alot more sense now, especially if I think of the iron as a conducting material. In relation to my previous post though, do eddy currents have anything to do with Lenz's law, I take it that they are effectively the 'induced emf', does this mean that they oppose the supplied current in any way?

    Yes I can see how the motor would be easier to spin once it gets moving, would it be possible for you to relate the idea more to electromagnetic induction? The 3 marking points for the question are: 1)Rate of change of flux increases, 2)Induced emf increases, 3) In opposition to current (from supply)

    Would it be possible for you to use these 3 points to generate an answer based on EM induction. I understand how 1 would cause 2, but I don't understand how 1 arises in the first place, surely the rate of change of flux would only increase if the frequency of the AC was increasing?


    Thanks for your help so far, 'Technician' feel free to chime in aswell even though I didn't quote you :)
     
  6. Jun 6, 2013 #5
    A simple motor consists of a coil placed between the poles of a permanent magnet.
    When current flows through the coil a force is experienced by the wires of the coil.
    The coil rotates.
    Now there is a coil spinning in a magnetic field and an emf is induced in the coil. This emf opposes the changing magnetic flux (Lenz'slaw). It is the rotation of the coil that produces the changing magnetic field experienced by the coil.
    The induced emf is called a 'back emf' and means that the current from the supply as the speed increases.
    If there are no energy losses and no work is done by the motor then it will speed up until the back emf equals the applied emf.
    You can see that there is essentially no physical difference between a motor and a generator.
    Pass current into the coil and it spins....a motor.
    Spin the coil and an emf is generated.....a generator
     
  7. Jun 6, 2013 #6

    Ahhh that makes sense now thank you. Just one final thing to make sure I've understood it; you're saying that the motor initially starts as a motor, but when it begins to move it starts to act as a generator, thus creating a new emf which counters the original one, right?

    If it's possible you could please look at my post above and see if I'm along the right lines with eddy currents? Thank you so much :)
     
  8. Jun 7, 2013 #7

    CWatters

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    Yes. Compare a DC permanant magnet motor with a dynamo. They are virtually identical. Both involve wires spinning inside a magnetic field. You can actually use a permanant magnet motor as a dynamo.

    Permanant magnet DC motors with no load will accelerate until the back EMF is almost the same as the supply voltage. At that point they stop accelerating and run at a more or less constant speed.

    This type of motor has a design parameter known as the motor constant. The units are rpm/volt. For model cars and aircraft you can get motors in standard sizes with different numbers of windings on the armature. They might have motor constants of around 500 to 5000 rpm/volt. So for example a motor with a constant of say 1000 rpm/volt connected to a 7V battery should accelerate from rest to around 7000rpm. A motor with fewer turns would have to spin faster to generate the same back emf so a motor with fewer turns would normally be a faster motor than one with more turns of wire.

    Good luck with the exam.
     
  9. Jun 7, 2013 #8
    You are correct to realise that motor and generator are more or less the same.
    In a motor electrical energy is supplied from a battery and mechanical energy is produced.
    In a generator mechanical energy is input and electrical energy is generated.
    You have probably heard of energy recovery systems (in cars) where the electric motor can be used to charge up a battery when it is disconnected and is slowing down. The motor becomes a generator
     
    Last edited: Jun 7, 2013
  10. Jun 7, 2013 #9
    Some good details on eddy currents here:

    http://en.wikipedia.org/wiki/Eddy_currents

    Note that eddy currents can be used to slow down trains...via eddy current brakes....and
    for heating..induction heating...and is a basic cause of skin effects in ac conductors...which makes tubular conductors economical.....
     
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