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B Motors & electromagnetics

  1. Apr 19, 2017 #1
    Motors are supposed to rotate inside because of changing magnetic field.. but when looking at high speed fan.. i'm thinking how can the changing magnetic field responds so quickly... is the rotation or changing magnetic/electric field the same speed as the fan?
     
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  3. Apr 19, 2017 #2

    russ_watters

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    The answer is yes, but there is a different cause/effect depending on what type of motor it is. 3 phase AC induction motors are essentially sync'd to a multiple of the grid frequency (60x30=1800rpm in the USA) by the alternation of the grid current changing the magnetic field. DC motors or non synchronous AC motors sync their own magnetic fields to their RPMs. The easiest to understand is probably the brushed DC motor, which changes the magnetic field by touching the electrical leads to different parts of the motor as it rotates.
     
  4. Apr 19, 2017 #3
    Watching several youtube videos now.. I think we can say motors are powered by Fleming's Left hand rule. I though the north pole and south pole just attracted each other in the rotor and stator (casing) and wondered how this could keep rotating. But let's say in a universe that didn't have Fleming's left hand rule.. are there electric motors which can also rotate simply based on the attracting north and south poles only?
     
  5. Apr 19, 2017 #4
    So basically one change the speed (or make the motor) go faster by simply increasing the current or by shifting the alternating north and south pole in certain way?
     
  6. Apr 20, 2017 #5

    CWatters

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    Not quite. If you connect a motor to a volt meter and spin the shaft you will see it can generate a voltage. This voltage is called the motor Back EMF and it depends on how fast you spin it. When power is applied to such a motor the motor accelerates until the back EMF is roughly equal to the supply voltage. The back EMF also depends on the number of turns on the windings and the strength of the magnets. This relationship between voltage and speed is called the Motor velocity constant or back EMF constant and is given the symbol Kv. It has the units rpm/volt. If the voltage is increased the motor will spin faster.

    The current drawn depends mainly on the load on the motor. If the motor was an ideal motor and had no load then once up to speed the current would fall to zero. Adding a load increases the current drawn by the motor. If the motor is an ideal motor the speed would remain constant despite increased load. Real world motors have losses caused by the resistance of the windings, friction and air resistance etc. These all conspire to mean the speed isn't constant and will vary with increasing load. As the load is increased the motor will slow down until at some point it cannot turn the load and it stalls. If you want the motor to maintain speed you will have to increase the voltage.
     
  7. Apr 20, 2017 #6
    In turbojets in airplane. Do they use fuel to power the motors directly, how.. maybe they use stroke engine like in cars to rotate the rotors in the turbojets motors or maybe they charge using battery to power the motors? When riding planes.. I always wonder if the motors or turbojet would suddenly stop turning and how fuel power it or via battery.
     
  8. Apr 20, 2017 #7

    CWatters

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    A turbojet is essentially the original form of the jet engine. All the air goes through the engine. Turbojets are still used on military jets but on civilian passenger aircraft the have been mostly replaced by high bypass turbofan and turboprop engines. No electric motors involved ( except for starting and perhaps fuel pumps).

    https://en.m.wikipedia.org/wiki/Turbojet
     
  9. Apr 21, 2017 #8
    Whenever a turbojet powered commercial airplane is powered on.. the fans begin to slowly rotate.. are you saying it is jet fuel that started the turbo jet fan rotations? It is very slowly at first.. it's controlled by fuel flow?

    Is there any incident where the fuel flow becomes maximum at beginning so the airplane that is parked suddenly accelerate into the surrounding?
     
  10. Apr 21, 2017 #9

    sophiecentaur

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    Electromagnetic forces can change a lot quicker than mechanical perturbations so the speed of a motor is not limited by the rate at which you can apply a changing field to the mechanical parts. (That's a bit too glib because you would need to design different coils ion order to produce very rapid changes.)
    But the bottom line is that EM waves can travel and change faster than mechanical disturbances, which depend on simple electric forces (molecular attraction) and molecular masses (which slow things down)
     
  11. Apr 21, 2017 #10

    CWatters

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    No they use an electric motor to spin up the turbine. It has to be going quite fast before they can turn on the fuel and even faster before they can switch off the starter and the turbine becomes self sustaining.
     
  12. Apr 21, 2017 #11
    How do they initially start up the electric motor? Do they use battery or is it directly from fuel to power the electric motor? I think it's the latter. How do you power electric motor directly from fuel like in house power generator? In the airplane case.. it's not gasoline or diesel but directly aviation fuel to power the electric motor?
     
  13. Apr 21, 2017 #12

    Drakkith

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    The have to use either a battery or another source of electricity to power the motor. You can't get electricity directly from any source of fuel without using it to power an engine that drives a generator of some type (except perhaps fuel cells).
     
  14. Apr 21, 2017 #13
    How does home power generator work then? is there piston inside like in car to drive the motor rotors?
     
  15. Apr 21, 2017 #14

    Drakkith

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    Yes, there is a combustion engine inside that's connected to the generator portion.
     
  16. Apr 22, 2017 #15

    CWatters

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    I believe most aircraft use a ground power connection for normal starting but many have a battery as well for when ground power isn't available. You might remember the problem with the Boeing 787 battery..

    https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_problems

    The battery is charged by generators on the jet engines.
     
  17. Apr 23, 2017 #16

    David Lewis

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    If the units are rpm/volt then it's a velocity constant (Kω).
    Whereas the back EMF constant (Ke) = 1/Kω.
     
  18. Apr 23, 2017 #17
    "by generators on the jet engines" you mean there is a small combustion engine inside the jet engines that would convert the aviation fuel to power the rotors in the generators? But it can't be on the jet engines themselves or the combustion engine would melt from the heat.. maybe just beside the wing? but aviation fuel is not like gasoline or diesel.. won't the piston just explode from the pressure? What kind of combustion engine can use aviation fuel?
     
  19. Apr 23, 2017 #18

    Drakkith

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    No, the generators are powered by the jet engine itself, though large aircraft are typically equipped with one or more auxiliary power units (APU's) in case of engine failure.
     
  20. Apr 23, 2017 #19
    How can the jet engine power the generators? The jet engine is pure flame and after burner.. how does it power the generator? what kind of connection does it use?
     
  21. Apr 23, 2017 #20

    Drakkith

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    I don't know the exact connections, but I assume the shaft running down the middle of the engine is connected to a gearbox which is then connected to the generator. So the shaft turns, turning the generator, producing electricity. The F107 jet engine in the missiles I used to perform maintenance on works in this manner:

    From here: https://www.forecastinternational.com/archive/disp_pdf.cfm?DACH_RECNO=1047
     
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