Electric motors

LAST EDITED ON Dec-09-04 AT 09:00 AM (CST)
 
I need a education on electric motors. The hobby shop is interested in selling only the newest and best thing, very expensive. Motors are rated 12 turns, 14 turns, 18 turns, etc. what does that mean? The hobby shop either doesn't know much or doesn't want to say much about the motors, other than this is what the RC guys use these days. The guys at the hobby shop have absolutely no imagination they only know what they have seen or been told. The only real model airplane motors I have experience with are the Carl Goldberg 550.

There are several motors on ebay. Are these go for anything???

Auctions 5940646359, 5941132296, 5941060100, 5941381194, 5941577663, 5941682364

My good friend Mr. Google had this in part to say when I asked him about 'electric flight motor tutorial'. Mr. Google seems to know a lot about a lot of stuff.

http://www.slewin.clara.net/elec/tmotor.htm

http://loke.as.arizona.edu/~ckulesa/propulsion.html

http://www.astroflight.com/e/env/00014p3C79cmyXseT42n6L6/articles.html?link=articles/elecflt.html

http://www.astroflight.com/e/env/00014p3C79cmyXseT48K1b5/index.html?link=/faq.html

http://www.hobby-lobby.com/ecalc.htm

http://www.motocalc.com/

BTW, Motocalc is one good reason for me to fire up the PC emulator and run WinXP, a good cad/cam laser wood burning design tool would be another.

Hope this helps...

Cy.

The number of turns determins the rpm of the motor. Higher # of turns means lower amp draw and lower rpms. Use these motors for direct drive applications. Lower # of turns means higher amp draw and higher rpm. Use these motors with a gearbox. This does not hold true for all applications, but it is a good starting point.

The motors you mentioned on ebay are all brushed motors and are ok for low power sport flying. You may need to use a gearbox to get any real performance from them. If you want a competitive stunter you should really go with a brushless motor.

They have 3 important properties.

1/ rpm/volt - it gives you unloaded rpm at given voltage. You have to decide what battery you use and match proper rpm/volt range to get wanted prop rpm.

2/ resistency of winding. It gives you how stable are those unload rpm. Higher resistance make more rpm drop at load. You have to know resistance of winging plus cables plus controller plus cells and at known current you know effective voltage for that "no load" rpm to know what rpm it will give at load (and add little reserve to your calculation). That resistance should be minimal because it make smallest variances at load and unload (speedup in wind, slowing uphill) and lower resistance gives better efficiency at wider range of current.

3/ max current. That says how many watts is engine able to radiate. Or how much you load the engine not to burn it. Or it means what power (with your voltage) you can get to prop. It is good to know that engines are not optimized for that current and best efficiency is typically at lower currents, so it is better to use bigger engine to get effective result.

Mike wrote that brushed engines are not good for competitive use. It is because brushed engines are typically with much higher resistance 5-10x higher. That is reason why they do not keep RPM at load, but there are also really good brushed engines with properties similar to brushless. But price is in that case similar, so it really does not make sence. There are also cheap brushles engines (I know minimaly 4 good producers in Czech) so there is no need for price stress. I actually got 70A 12V engine (MEGA) far under 100usd so where is the problem?