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  • zaveri

    zaveri

    MemberDec 8, 2013

    Questions about DC voltage and current

    What is the relationship between "voltage", "potential difference" and "e.m.f" ? Are they the same ?

    Second: In case of a DC current source, how do we determine the polarity of the current source ?

    Third: If the outputs of DC generators are connected in parallel, what will happen ?, will it give out a voltage , equal to the sum total of the individual voltage output of the generators, or will it result in a explosion ? !
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  • lal

    MemberDec 9, 2013

    1. Voltage and potential difference are the same. Emf is the potential difference accross a source when no current is drawn from it. Every source will be having an internal resistance and when some current is drawn from it, a drop occurs accross this resistance which reduces the terminal voltage by a small value.

    Emf = voltage drop across the internal resistance + terminal voltage
    Or
    Emf = (Current drawn x internal resistance) + terminal voltage

    2. Current flows from the positive to negative terminal. A galvanometer may be used to determine the direction of flow of current.

    3. When connecting two dc generators in parallel, the voltages of both should be exactly the same. Otherwise the machine with lower voltage will act as a dc motor and draw power from the other which generates higher voltage.

    But, when the two generators output exactly the same terminal voltage, the machines can be connected in parallel and the total current capacity will be the sum of two. That is, voltage will remain the same, but the maximum power/current that can be drawn will be the sum of two. (Just like connecting two batteries in parallel)
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  • Ramani Aswath

    MemberDec 9, 2013

    Continuing on @#-Link-Snipped-#'s good answer:
    1. The voltage drop across the internal resistance appears only when a current is drawn.

    2. A solenoid coil and a magnet with known North-South poles can be used to identify the terminals.


    3. It is advisable to put a forward biased diode on each source to protect damage from mismatched voltages.
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  • zaveri

    MemberDec 9, 2013

    Thank you guys. @#-Link-Snipped-# and @#-Link-Snipped-#

    Now if two generators of different output voltages were to be connected in series, what would happen, would it be the same as connecting two batteries in series ?

    What exactly is "terminal voltage" ?
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  • lal

    MemberDec 10, 2013

    When connecting DC generators in series, as far as the voltage drop across the internal resistance of any generator doesn't rise beyond its generated voltage's value, it should be theoreticaly fine to run so. Otherwise, the generator which generates lover voltage than the voltage drop will start drawing power and run as a motor rather than supplying.

    Terminal voltage is the voltage that appears accross the terminals (of the generator in this case).
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  • Ramani Aswath

    MemberDec 10, 2013

    zaveri
    Thank you guys. @#-Link-Snipped-# and @#-Link-Snipped-#

    Now if two generators of different output voltages were to be connected in series, what would happen, would it be the same as connecting two batteries in series ?

    What exactly is "terminal voltage" ?
    Ordinarily the voltages add up. I am a bit hazy about various types of DC generators.
    Terminal voltage is what a voltmeter will measure if connected across the terminals of the generator.
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  • lal

    MemberDec 10, 2013

    Oh yes, I should have mentioned that I was talking about a permanent magnet dc generator or an externally excited dc generator.

    As Ramani sir said above, it will be different for different types of generators.
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  • Ramani Aswath

    MemberDec 10, 2013

    General info on DC generators:
    #-Link-Snipped-#
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