Author Topic: Cut in speed factors?  (Read 1870 times)

0 Members and 1 Guest are viewing this topic.

Michael G

  • Newbie
  • *
  • Posts: 9
Cut in speed factors?
« on: August 29, 2006, 11:00:09 AM »
  I checked FAQs and searched the board but I havevn't found the answer to this Question.  What determines cut in speed?  Magnet size?  Number of turns in coil?  The air gap?  Also is there a mathemtical point that governs cut in speed like 50% of maximum output?


Michael

« Last Edit: August 29, 2006, 11:00:09 AM by (unknown) »

elvin1949

  • Hero Member
  • *****
  • Posts: 645
Re: Cut in speed factors?
« Reply #1 on: August 29, 2006, 06:08:06 AM »
  Cut-in speed is controled mostly by number of turns in coil.  More turns lower cut-in.

later

elvin
« Last Edit: August 29, 2006, 06:08:06 AM by elvin1949 »

Flux

  • Super Hero Member Plus
  • *******
  • Posts: 6275
Re: Cut in speed factors?
« Reply #2 on: August 29, 2006, 06:46:14 AM »
Cut in speed depends on the number of turns, flux per pole and frequency ( depends on number of poles and rotational speed).


The flux per pole is dependent on the magnet material, area and air gap in proportional; to magnet length.


You need to choose a cut in speed where there is enough power in the wind to supply the machine losses. If yo go too low the blades will not come out of stall and you will get nothing.


If you make your cut in too high you will not extract useful power that may be available in lower winds.


Typically a low loss alternator can be expected to cut in at about 7 mph, if you have significant iron or other loss you may do better to cut in at up to 10 mph.


Without clever things you can only track a useful band of wind speed so if you are in a particularly windy area you will do better with a higher cut in and higher maximum power. For a poor wind area you need to extract what you can in low winds and accept poor efficiency in high winds that happen infrequently.

Flux

« Last Edit: August 29, 2006, 06:46:14 AM by Flux »

finnsawyer

  • Hero Member
  • *****
  • Posts: 1565
Re: Cut in speed factors?
« Reply #3 on: August 29, 2006, 10:12:48 AM »
What you're asking is tough to answer.  It's a matter of trade offs.  A large number of the 3:4 alternators have been built.  Each one had a cut-in rpm.  Unfortunately no one has ever bothered to put the data all in one place, so one could see the results of the various trade offs, such as the items you mention.  I've recently muddied the waters more by suggesting an alternator design that has three coils to two magnets (3:2).  It too will have it's unique relationship between the cut-in rpm and such things as magnet size, coil size, number of turns, rotor size, air gaps, and use of iron cores.  Right now it is untested and I don't have the resources to build 100 or so different versions of it to wring out all the relationships.  Even if people do start building that type of alternator you are not likely to see the data in any one place.  
« Last Edit: August 29, 2006, 10:12:48 AM by finnsawyer »