Making a DC motor speed question

[33639]

I've made a simple DC motor like this image:
http://farm4.static.flickr.com/3156/2864655414_234d5d3207.jpg

I'm just confused why initially when i increase the number of loops (about 15) it spins faster.But after this limit the speed of the rotor is no longer as fast as it was before.
Why is that?

 

[TSny]

There could be several factors involved. One thing to think about is what happens to the current in the coil as you increase the number of loops N? Would you expect the current to remain constant, or would you expect the current to be roughly proportional to N, or perhaps inversely proportional to N? Here I'm just thinking of the effect due to the fact that the resistance of a wire increases with length.

Another factor is the "back emf" that is induced in the coil due to its rotation in the B field of the magnet. How does that depend on N? How does that depend on the rotation rate? This will also affect the amount of current.

 

[CWatters]

As TSny said, there are a lot of factors.

If you made several motors all with same number of turns I wouldn't be surprised if they rotated at different speeds. For example the commutator is formed by scraping the enamel off the wire and it will be hard to ensure that's done identically on all motors. I suspect the phase (timing) only has to be slightly out to make a difference? Vibration may also effect how the commutation works. The load on the motor also isn't well controlled - it's mostly air resistance and friction. Changing the number of turns will change the air drag load on the motor and that may also be a factor.

Note the question Tsny makes about backemf. Ask yourself what would you expect to happen to the rpm of a motor if you replace the magnets on a motor with stronger ones. Would you expect the rpm to increase or decrease? Ditto changing the number of turns.

To properly characterise a motor the best way is to hook it up to a dyno, load it up and plot torque vs RPM so you can calulate the efficiency = power out/power in. It's possible to achieve efficiencies of >95% with some types of motor. Typically motor efficiency peaks at a particular rpm/load and falls off either side of that. Given your motor has nothing connected to the shaft (no useful mechanical load) how would you estimate the efficiency?