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Emf induction

  1. Dec 14, 2004 #1
    emf induction....

    Why does a moving conductor becomes induced with an emf? Well I know it has to do something with changing magnetic flux. The thing I want to know is why the changing magnetic flux leads to e.m.f. induction?

    Secondly, we have an A.C. generator in which the coil is rotated to produce the current. I'm given the task to connect a fuse to one of the wires leading out of the generator. How would I which one is live wire and which one is neutral considering that the direction of the currect is continuosly changing?
    Thanks in advance for any help. :approve:
    Last edited: Dec 14, 2004
  2. jcsd
  3. Dec 14, 2004 #2


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    Why is not a real good question in physics, unless you are able accept Maxwells equations as a reason why. The understanding of how a current is induced will come with an understanding of Maxwell. In the truest sense of the word WHY, we can only say, because it does.

    As for the problem with your fuse, there is now way that question can be answered without a schematic diagram of the wireing of the geneator. How can we possibly answer a question about specific hardware without documentation? You will need to ask the designer or the plant engineer about that.
  4. Dec 14, 2004 #3
    the change of B-field lead EMF is like the change of momentum leads force
  5. Dec 14, 2004 #4


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    One way to understand how an emf is induced in a moving conductor (IN A MAGNETIC FIELD!) is to recognize that the conductor is made up of electrons and ions. Charges that move in a magnetic field are subject to the Lorentz force which tends to push electrons in one direction and ions in the other direction. The ions are essentially fixed (lattice) while the electrons are mobile. In effect this causes a charge separation thereby inducing a potential difference across the conductor with the resulting electric field eventually balancing the Lorentz force.
  6. Dec 15, 2004 #5
    A stand alone generator (or alternator) with all leads isolated is considered a "floating" power supply. If the output voltage is 115 you can ground either one and fuse the other. If it's 230 volts, there should be a center tap (neutral) that can be grounded, then fuse the other 2 leads.
    The reason for grounding is to limit the potential to ground; an ungrounded 115 volt alternator could read 1000 volts or more to actual ground, creating a dangerous condition.
  7. Dec 18, 2004 #6
    Well, Here is a simple generator with wiring. What do you say now?

    Attached Files:

    Last edited: Dec 18, 2004
  8. Dec 18, 2004 #7


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    I am not going to say what I am thinking.

    Put the fuse in the load line.
  9. Dec 18, 2004 #8
    Is it too complicated or is my question too dumb?
    Last edited: Dec 18, 2004
  10. Dec 18, 2004 #9


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    The drawing you provided is not a schematic of an operating generator. In that drawing there is only one possible place for a fuse. It must go in the load line. But that does not answer the question you asked. Which seemed, to me, to be a specific quesition about a specific generator. A fuse must be placed in a current carrying load line is the general answer.
  11. Dec 18, 2004 #10
    Incorrect as written. To be dangerous, there must be a complete current path from the voltage source, through the load (person), and back to the voltage source. Additionally, what would create the 1000 volt potential?
  12. Dec 19, 2004 #11
    It's written from experience. A defective tool or appliance can have exposed parts live. Simply holding one while standing on concrete with leather soled shoes will complete the circuit. Many people have felt an electrical shock this way at one time or other.
    An ungrounded power source, be it a Y connected transformer secondary or a portable generator, can have the potential float to an unsafe level (compared to actual ground) if there's no physical connection through a resistor or lamp to reference it.
    Many years ago bathroom receptacles were fed through a 115/115 volt isolating transformer to limit the risk of electric shock. Touching one side or the other would pull that side to zero volts (if touching the sink or taps).
    One more example, you run a portable generator (which has rubber vibration mounts and feet). With a high input resistance meter, measure between a real ground (steel fence post etc.) and the hot lead of the generator. It might read 115 volts, or 60, or 0, or 500. There's no real reference.
  13. Dec 19, 2004 #12
    If the system is referenced to ground that is indeed the case. If the system was floating, the circuit path is not complete, thus near zero current flow through the person. The small current is a result of small capacitance coupling to the earth but never the less; the current would be limited to a few micro amps and not hazardous. This is why a floating system is safer than a ground referenced system as in your example…
    In the above example, to receive a perceptible shock, it is necessary for one or the other terminals to be connected to ground (converting it to a ground referenced system) before one can receive a perceptible shock by contacting the other terminal and ground.
    Linemen routinely service multi-kilovolt power distribution systems and do so safely as long as they are not in contact with either of the other two phases or ground and not within the arc-over distance. Again, if there is not a complete circuit, no perceptual current will flow; just ask the birds.
    Electricians I’ve spoken to sometimes refer to the voltage readings as “phantom” voltage and they realize the measurement is of no consequence. Many electricians prefer the old analog, low input impedance voltmeters so as not to be confused by these readings. If one places a 10kohm resistor across the terminals of the modern DVM, the phantom reading will disappear. The 10kohm resistor across the terminals makes the system neither more nor less safe than it was previously, it simply lessens the value of an irrelevant reading. Lastly and again, a complete circuit must be realized before perceptual current can flow.
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