Alternator tutorial

Discussion in 'General Electronics Chat' started by dlpearce74, Jun 5, 2007.

  1. dlpearce74

    Thread Starter New Member

    Jun 5, 2007
    7
    0
    http://www.allaboutcircuits.com/vol_6/chpt_4/8.html

    This tutorial claims something I suspected: that a car alternator could be run as a low voltage induction motor, by shorting the field to itself.

    I am curious if anyone has tried this, and got a feel for the performance of the alternator running as a motor? It seems torque could be somewhat limited without a re-winding of the rotor, but I could be wrong (I am a CPE not a EE :)).

    Thanks,

    David
     
  2. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Hi..I can just about remember working on something like this years ago.The guy used it to turn over a jet engine..... He could run the alternator up to about 60,000 RPM, but the coils were driven by MOSFETS in a three phase config...... I can't off hand remember the details, and am not sure if I still have the info.... I will have a look....Daniel.
     
  3. sanjeevpandey

    New Member

    Oct 12, 2008
    1
    0
    hi,its a machine which has two parts
    stator,& rotor
    stator which is stationary and rotor is moving part
    stator ,from which we get supply and we give dc supply to rotor
     
  4. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Hey,

    I believe you are correct in that assumption. Why not apply DC to the field coils, then the 3-phase to the motor? You will be able to produce higher amounts of torque. If you put a local 3-phase rectifier plus a bulk capacitor, you will have a supply for the field winding.

    Steve

    LightingMan,

    60KRPM would destroy any car alternator! There are two big problems when spinning up to that kind of speed: natural rotor frequencies, and inertia on field windings. The natural frequency, I am willing to bet, is much lower than 60KRPM. So, you're resonance would probably self-destruct the motor pretty quickly. Secondly, all of the inertia on the windings could cause them to lift out of the rotor, also self-destructing the alternator.

    Don't alternators have bushings instead of bearings??

    Steve
     
  5. vetterick

    Active Member

    Aug 11, 2008
    35
    0
    Steve, by "bushings" do you mean plain bearings, and by "bearings" do you mean ball bearings? Keep in mind that the name bearing was used long before roller or ball bearings were invented.

    Actually I've never seen plain bearings used on an automotive alternator, maybe on earlier generators.

    This peaked my intrest as to how fast these actually spin, so I measured one up, here is my findings;
    Alternator sheave (pulley) is 2 3/4"
    Crank sheave is 7"
    Max engine RPM is 6000
    So 7/2.75*6,000=15,273, that little hummer goes pretty good.

    But 60,000 is very fast for an alternator, maybe he geared up the turbine like 4/1, turbines dont require much torque to start.
     
  6. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Hi,

    That's what I meant, a rotary plain bearing / bushing. I am not an automotive tech, so I don't really know what is in there. Do they have ball-bearings at both ends then? You need to watch the friction coefficient, since as we should know, heat is proportional to both speed and this coefficient. There will be a rated temperature for this, which you shouldn't exceed.

    I would even say that 15000 RPM is pushing it for an alternator. I wouldn't run it past what it would run in a vehicle. It takes precision machining and a PM DC motor to do 60KRPM.

    Steve
     
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