Generator

Discussion in 'General Electronics Chat' started by BlueDevry, Jan 4, 2008.

  1. BlueDevry

    Thread Starter Member

    Dec 26, 2007
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    Do we have any information on controlling the output level of a generator using DC voltage ?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    What kind of generator are you referring to?

    hgmjr
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    896
    A generator makes a DC output voltage. The amount of voltage and current depend on its speed and on the load. A voltage regulator can regulate its output voltage to a voltage a little lower than the generator's minimum voltage.
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    Unless, of course, we use a switching regulator that is suited to a very large range of input voltages whilst remaining efficient! Why generate electric power just to make a heater?

    Steve
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Audioguru speaketh the truth ;)

    Old-fashioned automobile generators used a voltage regulator that consisted of a couple of relays and some power resistors. This EM device fed three different levels of current through the rotor depending upon the voltage at the battery. Power was taken from the field winding.

    Power output generated is porportional to the current flowing in the rotor winding and the speed of the rotor.

    This would be a good application for a chopper drive circuit. It could be as simple as a power MOSFET driven by the output of a comparator, using a Zener diode for a reference and a divider network for the input to the comparator. There's a catch though; you wouldn't want to apply power to the rotor windings unless the generator was spinning fast enough to provide useful output. One way that might be tested for in an automotive system would be to check the engine RPM; if it was below idle speed, the output to the field would be disabled.
     
  6. recca02

    Senior Member

    Apr 2, 2007
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    It wud be really helpful if u cud specify the type of generator.
    the alternators(synch type) have a dc excitation. by changing the voltage of dc excitation
    the o/p of generator is changed.
    this can be done by the use of scr etc.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Recca,
    The connection to the rotor windings is generally via slip rings, not a commutator.

    If one attempted to control the rotor windings via an SCR, you would be able to turn the SCR ON, but not OFF.

    You CAN use SCR's for certain types of DC motors, but not generators.
     
  8. BlueDevry

    Thread Starter Member

    Dec 26, 2007
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    The literature I was reading didn’t specify what type of generator but I think there referring to an A/C generator. I was a little confused when they said that " a small DC voltage applied to the field winding " could be used to control the output level of a generator. I believe I may be getting this mixed up with motors.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, I might be guilty of confusing you. It's been a number of years since I've worked on a generator, and I've forgotten whether it's the field or the rotor which receives the exitation voltage. It would seem logical that the rotor would receive the excitation current, since that would mean less current via the slip rings.

    AC generators are usually called "alternators" to differentiate them from "generators". Whereas "generators" produce DC, your typical automotive "alternator" actually produces three-phase AC internally, the exitation voltage is on the rotor, and the field windings are connected to a 3-phase full-wave bridge rectifier to convert the AC to rippled DC.

    After that, the principle is the same though. The more current you feed to the rotor windings, the more power is produced by the field windings (up to the limitations of the thing, or melt-down, whichever occurs first.) Automotive alternators typically have the bridge rectifer and solid-state regulator incorporated in the housing, opposite the pulley.

    DC motors can act as a generator if their rotor is rotated. Those with permanent magnets don't require an exitation voltage to generate power.
     
  10. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    yup, they are with slip rings....or better still they can be brushless excitation.

    using scr we can still control the on time for an scr to conduct after an ac is rectified to dc. this way we control the voltage(rms voltage wud be more correct i think). thus excitation is changed.now, maybe IGBT's etc are used for many advantages..but i know scr are used ..i have seen them in use.

    this type of system is very much used in static excitation system or brushless excitation system for power plant generators..how much will it be practicable for small generators is hard for me to say.

    again using scr is just one way, in earlier times the filed strength of a main excitor(another gen) was changed by changing resistance using AVR .

    edit:OP: DC is used for excitation in alternators.
    since this lets u create fixed magnetic poles on the rotors.(not possible with AC)
     
  11. Externet

    AAC Fanatic!

    Nov 29, 2005
    761
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    All generators, AC or DC produce electricity by magnetism crossing windings.
    Instead of using fixed magnets that would produce an amount of electricity variable on speed and load, the real world uses a field winding electromagnet in the generator.
    By applying more or less -small- amounts of electricity to that electromagnet, can generate a steady output regarding of speed or load.

    That is the example of an automobile generator, whatever be its type, the magnetic field amount is governed by a regulator device to produce constant voltage of 13.8VDC, controlling the output as you ask.

    Miguel
     
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