Home
JAQForum Ver 20.06
Log In or Join  
Active Topics
Local Time 21:40 29 Mar 2024 Privacy Policy
Jump to

Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.

Forum Index : Electronics : Simple experiment re induction and F&Ps

Author Message
KiwiJohn
Guru

Joined: 01/12/2005
Location: New Zealand
Posts: 691
Posted: 10:43pm 31 Dec 2006
Copy link to clipboard 
Print this post

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.Edited by KiwiJohn 2007-01-02
 
Gizmo

Admin Group

Joined: 05/06/2004
Location: Australia
Posts: 5004
Posted: 12:12am 01 Jan 2007
Copy link to clipboard 
Print this post

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
 
RossW
Guru

Joined: 25/02/2006
Location: Australia
Posts: 495
Posted: 06:17am 01 Jan 2007
Copy link to clipboard 
Print this post

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

 
Gizmo

Admin Group

Joined: 05/06/2004
Location: Australia
Posts: 5004
Posted: 11:34am 01 Jan 2007
Copy link to clipboard 
Print this post

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
 
RossW
Guru

Joined: 25/02/2006
Location: Australia
Posts: 495
Posted: 07:38pm 01 Jan 2007
Copy link to clipboard 
Print this post

  Gizmo said   I got 2.5mH for a single pole of a 80 series
....
I dont suppose you have a 60 and 100 series floating around you could measure?


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)
 
Print this page


To reply to this topic, you need to log in.

© JAQ Software 2024