Brushless Motor's Rotor length affect the performance?

[petersi]

Please check the attached picture. It is a brushless motor rotors. Rotor A has longer magnet then sample B. Others are the same. Want to know what would the performance on the longer and shorter rotor different thanks.

 

[mikekilroy]

torque goes up by the square of the diam and linearly on length: hence your longer magnets, assuming a correspondingly longer matching rotor, will provide that ratio more torque.

Inertia of motor goes up by the 4rth power of dia and again linearly with length: hence the longer motor will accel the SAME in either case.

You can ignore the winding differences as the small incremental length will have little bearing on ele performance, and if this was too much, then you can change the number of turns to anything you want to compensate.

 

[mikekilroy]

correspondingly longer matching rotor

meant to write

correspondingly longer matching stator

 

[petersi]

Mike, Thanks so much.

I was expecting the torque will go up too.

I can't change the setting or winding on the stator. The only thing I can change is the rotor. So, according to your say is longer magnet has more torque. Also I know thicker magnet will increase torque too. As both methods make the torque go up, what would the different in the other performances. Such as temperature, rmp, amp draw.........

I have experienced that the thicker magnet has more torque, less rpm, more drag break effect(motor slows down faster when no power input), Motor runs at lower temperature (this one I'm not 100% confirmed).

Please help for my learning. Thanks a lot.

 

[mikekilroy]

longer magnets without corresponding longer stator to SEE them gives NO more torque.

Thicker magnets do NOT give more torque capability in a motor!

Thicker, if it changes the magnetic field, will change the motor Kt & BEMF ratings and thus you will get more or less torque PER AMP and more or less RPM PER VOLT. The thermal capability of the motor will NOT change so you cannot get 'more torque.' If you only want more peak torque, then larger magnetic field might do it, but that does not make the motor capable of more continuous torque rating.