The internal rotor usually is with less poles,high speed,small torque, while outer rotor is with much more poles,low speed and big torque.
The brushless motor's speed is related to the voltage,the external rotor and internal rotor with same idle speed,the external rotor is significantly stronger than then inner rotor comparing load capacity.It means if you want the motor to drive something heavy,you have to choose the outer rotor, otherwise if you only consider the speed and cost,don't require much on loading, you can choose an inner rotor.
Contact: Shanghai Wind Automation Equipment Co.,Ltd
Skype: motor.machinery
Mobile: +86-18067425780
Website: http://www.motor-machinery.com; http://www.coilswinding.com
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Emily