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Forum Index : Electronics : Simple experiment re induction and F&Ps
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| KiwiJohn Guru  Joined: 01/12/2005 Location: New ZealandPosts: 691 |
Inductance is something that is hard to see, not easy to measure but very significant in our F&P generator circuits. Inductance is the effect that limits current flow as frequency increases. As our F&Ps turn faster inductance gets more and more significant. Inductance is the opposite to capacitance, it resists change in current flow where capacitance resists change in voltage. I did a simple test to see what effect inductance has at the frequencies we might encounter in a typical windmill situation. My rig consists of a F&P mounted on an exercycle, I have 3 phase rectification and a resistive load. A multimeter was used to measure frequency, at a modest pedalling rate the frequency showed as 100Hz, the output voltage was 10 volts and the current 4.5 Amps. Doubling my pedal speed to give a frequency of 200 Hz the voltage rose to 13volts and the current rose to 6 Amps. Low speed, 10 volts x 4.5 = 45 watts. High speed 13 volts x 6 = 78 watts. At double the RPM one might have expected double the power out but not so! In fact the power has only increased by about 70%. In the absence of any other explanation and considering the simplicity of the test rig I can only conclude that increasing inductance and eddy currents in the stator cores have limited the current flow and hence the power output. The F&P I use has only 18 coils the rest having been removed to suit the power available from me sitting on the bike. Doubtless the same experiment with an entire stator would show different figures but I am confident the ratios would be similiar. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I measured the inductance for the F&P a few weeks ago, they might help in your experiments. Details here. I'm pretty sure I got the math right. Yeah the inductance would play a part in output, especially at higher RPM's Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
I just went out and measured 3 values: Phase to star point for unmodified machine 24.3mH Phase to Phase for unmodified machine 70.1mH One coil in isolation 1.87mH  |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I got 2.5mH for a single pole of a 80 series ( 35 mH from star center to point ), so thats pretty close to your meter readings, though I would say your readings would be more accurate than my long winded way of measuring inductance. I dont suppose you have a 60 and 100 series floating around you could measure? Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Sadly, no. And it wouldn't be worth the postage to send a couple to me just to test.... unless someone has a ruined stator and wants to just send one coil? (but that could be pretty hit-and-miss, chopping up the laminations) |
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