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Alternating poles in the stator of an alternator

  1. Jun 22, 2010 #1
    I have run into something i dont understand while working with an alternator. I purchased a magnetic pole detector to identify the poles of some magnets and tried to see if i could detect the changing magnetic field of the alternator in my car while it was running by placing the pole detector on a specific point at the stator, but the pole detector didnt do anything, didnt indicate a changing field. So i tried the pole detector on a couple of direct current coils i built some time ago and the pole detector identified the poles of the coils. I bought a alternator stator to run tests on and i ran direct current through one phase of the 3 phases of the stator and got the same results as when the alternator in my car was running producing an alternating current, the pole detector didnt identify any poles, i also connected the stator to household wall current through a variable AC transformer at low voltage but with a high current flowing through the winding of the one phase of the stator and got the same results, no pole detection. Why does the detector not identify poles in the stators iron core? direct current or alternating current, the results are the same, the pole detector doesnt identify either pole. Can anyone offer some explaination why?
  2. jcsd
  3. Jun 23, 2010 #2
    Your magnetic flux density is probably not high enough. Permeability of free space is quite low compared to that of a iron core.

    A motor/generator don't need that high magnetic flux density since the airgap between rotor and stator is so small.
    Your car alternator is also more likely a 3 phase synchrounos generator, which generates a rotating magnetic field. The rotor (poles) changes direction as the shaft turns, so you dont have any stationary magnetic poles.
  4. Jun 24, 2010 #3
    I'm pretty sure that the magnetic pole detector is used to identify poles of a steady state field. The alternator does not have a steady state field in it. It generates AC voltage then rectifies it. If the detector is sensitive enough you should be able to hold it up to a single wire carrying current and it should detect the field around the wire if the current is strong enough and you hold the detector in the correct orientation. If you do this near a pos and neg wires the fields will tend to cancel each other out. L
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