dirty_d wrote:eP, i don't really understand how those motors work, are there any more links that have better pictures or something to help explain how they work?
its a cheap set for do-it-and-convert-your-bike yourself
the motor is something like a Speed700 used in RC-hobbies (sold for 15 Euro)
Miles wrote:Maybe these people would give us something closer to a finished product?
johnrobholmes wrote:If the construction is changed to a 14 mag pole/12 slot design the shaft rotation slows to 8.57 degrees per switch and takes 42 switches per revolution. The motor is now 7 times slower but can still produce the same power.
Miles wrote:To do that it would need to produce 7 times the torque, no?
I can't see how, in this respect, changing the number of poles is any different to changing the number of turns in the winding.
johnrobholmes wrote:Correct. Since the shaft is moving 7x less distance per phase switch, the 14pole/12slot motor will produce 7x the torque at 1/7 the speed as compared to the 2pole/3slot motor. Basic physics of a torque arm really. Use a 1 foot wrench to twist a bolt vs a 7 foot wrench. You get more torque with the 7 foot lever, and with a constant hand speed the bolt turns 7x slower with the same net work done overall.
johnrobholmes wrote: The 14 pole travels 7x slower and needs 1/7 the geardown of the 2 pole motor. Therefore to produce the same power it must produce 7x the torque to keep final wheelspeed the same.
johnrobholmes wrote:It is the equivalent of mechanical gearing, but don't say that to an engineer.
Really it isn't a longer lever, it is a shorter distance traveled for the same work if we take into account the motor speed and final gearing differences. The 14 pole travels 7x slower and needs 1/7 the geardown of the 2 pole motor. Therefore to produce the same power it must produce 7x the torque to keep final wheelspeed the same.
Basically a motor can be built for torque or speed that is dependent on the shaft rotation per phase switch. A shorter movement per switch yields more torque at a sacrifice of top speed without further design changes, as a motor can only be switched so fast before switching losses begin to produce more heat than the motor can dissipate.
As a devil's advocate i'm asking some questions:
Who (or what physics law) told you it must produce the same power ?
Maybe 14 poles motor need 7 times faster coils energizing to reach the same power ?
Which physics law telling us the latter cannot be the truth ?
dirty_d wrote:john, it seems that only using 1/3rd of the stator windings will lower the resistance by 3, so you would get 3X the current, but since there is 1/3rd the active stator poles, you get the same torque, the backemf would also be 1/3rd its original value, so the rpms can increase much higher, the motor seems to actually be more powerful in this arrangement. power is proportional to resistance for a given voltage.
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