AC current for coil

Discussion in 'The Projects Forum' started by kuba777, Feb 16, 2011.

  1. kuba777

    Thread Starter New Member

    Feb 16, 2011
    2
    0
    Hello,

    I need to excite big coil with 250 mA, 400 Hz AC current. Impedance of coil at 400 Hz is 85 ohm, DC R is 5 ohm. Do I need special AC power supply for this?
    What would be best way to make sure the current has right value?
    I would appreciate any comments or suggestions.

    Regards
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,120
    3,046
    Yes. A DC motor spun at 400 rps might work.
    Easiest way is to measure AC voltage across a low ohms resistor in series with your coil. Not all meters will work at 400Hz, though. Measuring peak voltage could be achieved with a diode (or a bridge rectifier) and a capacitor configured as a peak detector. The best way would be an oscilloscope that would allow you to see the waveform.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Unless I'm missing something that calculates to 22.5VAC (Sine) input V.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    Wouldn't the output freq be dependent on RPM x Pn where Pn is the number of poles? Also, wouldn't the output from a DC motor be pulsating DC?
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Yes, the number of poles matters. Not sure how many poles are in a small DC motor?
    I think without a rectifier, the current goes back and forth, meaning it is true AC. I suppose this depends on the internal configuration. The ones I play with make AC.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    A DC motor may not give useful AC if it is a commutator type, as a lot of small and medium sized DC motors are. A permanent-magnet commutator motor would definitely give (rough) DC.

    Permanent-magnet stepper motors should give AC, but I would not like to make any bets about their efficiency used in this way.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    Using a motor as a generator for this application doesn't seem like a practical solution to me. A more realistic solution would be a 400Hz audio oscillator feeding an audio amplifier. 400Hz is well within the passband of same and a 10W amp will do the job with some power to spare.

    What is this going to be used for? If my memory isn't escaping me I remember 400Hz as being the freq for the HV power in military aircraft.
     
  8. kuba777

    Thread Starter New Member

    Feb 16, 2011
    2
    0
    The coil is used to check sensitivity of microphone to an external ac magnetic field. I have to compare voltage from microphone and from calibrating coil after they are placed inside big coil.

    This microphone is going to be used in aircraft. From what I know 400 Hz is used in military aircraft.

    Thanks for suggesting audio amplifier, I don't now why but I haven't thought about it before.
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,120
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    +1
    I just threw it out there as an "outside the box" idea. Audio amp is a good idea and you can buy pre-made circuits or kits for just about any level of mono or stereo power you want.
     
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