Induction Motor, AC Circuits, Project

Discussion in 'General Electronics Chat' started by John L, Mar 19, 2008.

  1. John L

    Thread Starter New Member

    Mar 19, 2008
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    I tried setting up the simple induction motor as described in vol 6, chapter 4.
    For some reason I cannot get the rotor to spin. I have checked the wiring to match the diagram over and over. In calculating the equavalent resistance of a 25 watt lamp I calculate 571 ohms. not 820 ohms as the project describes. However, I used a 820 ohm resistor as the project describes. I also used three 10 microfarad capacitors wired in series. The three capacitors wired in series gives me an equalivant of a 3.3 mirofarad capacitor. and an inpedance of approximately 820 ohms. Thus the resistor and the capacitor, have the same resistance/impedance. Wiring the project as shown in Vol 6, chapter 4 and measuring the current thru the coil in series with the resistor I get .14 amps. Measuring the current thru the other coil in series with the capacitor I get .14 amps. Thus both coils have the same amperage. The rotor will not spin. Any ideals as to why?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The project actually gives a range of 15 to 25 Watts. 571 Ohms for 25 Watts is OK, but 15 Watts is 960 Ohms. 820 Ohms is a value midway between the two; roughly 20W.

    The three 10uF caps wired in series may be causing a problem due to additional phase shift. You should really use a single 3.3uF cap.
     
  3. John L

    Thread Starter New Member

    Mar 19, 2008
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    Thank you for your insight. I tried your suggestion with one 3.3 micro farad 360 volt capacitor and it still did not work. The rotor did not spin and the capacitor overheated and blow after about 1 to 2 minutes. The capacitor was polorized. Any more suggestions?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Ack! :eek: You can't use polarized caps for AC. :( When they have current flowing through them in reverse, they will conduct, overheat and blow as you are now witness to.
    Non-polarized electrolytics are (conceptually) two polarized electrolytics connected back-to-back to prevent that.

    OK, you have 0.14A flowing through your coils. What's the voltage across them?

    Also, are your coil's turns on the forms closely spaced and in four layers (or closer with more layers) as described in the experiment?

    Can your rotor be spun freely by just blowing on one edge of it? (This motor is very low-powered, and won't be able to overcome very much friction at all.)

    Will a magnet stick to the edge of your rotor? (If not, it's non-ferrous. Try using something like a Mason jar lid, or a lid from a large pickle jar.)
     
  5. John L

    Thread Starter New Member

    Mar 19, 2008
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    Again, I appreciate your help. Thank you for the insight on poliarized caps. The motor rotro is magnitic and spins freely. To the best of my ability the coils are formed closely spaced. Four or five layer high, one inch wide. 440 (plus or minus a few) turns per coil. I will get a non polirized cap, rewire the circuit and measure the voltage across the coils. This will probably not happen until sometime next week or so. I will keep you informed. Thanks again for sharing your wisdom.
     
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