dobber
15-12-2011, 08:09 AM
Ok, i have to admit i nabbed this from another forum but its well written and explains a lot. Its from a Guy Called Matt Murphey who designs and makes brushless motors.
"The smaller the rotor, the more air gap it will have between the rotor and stator. The bigger the air gap, the less torque the motor will have. Also, smaller rotor means less magnet. Less magnet equals less torque.
KV rating in RC Car sensored systems is irrelevant and is a means of falsely marketing the power output capabilities of a motor. Lower KV motors do not have smoother acceleration because of their KV ratings. The rotor is what dictates the feel of the motor. A 17.5 turn motor has many times more torque than does a 5.5 turn motor. However, to get the motors to run optimally, we run MUCH different gear ratios... 17.5 motors have lots of torque, so we gear them for Speed. Modified motors have minimal torque, but excellent top RPM, so we gear them for Torque.
Modified motors typically have a 12.5mm rotor on a 5mm shaft. This = 7.5mm of magnet on the rotor. This is done to make up for the torque lacking in the lower wind motors.
Spec (17.5, 13.5, 10.5) motors typically have a 12.3mm rotor on a 7.1mm shaft. This is 5.2mm of magnet. This is done to move the powerband of the motor more towards the RPM side, because of the extra torque created in a higher wind motor.
The power is in the rotor..... Modified rotors have 45% more magnet than SPEC rotors.
KV rating has absolutely nothing to do with how aggresive the bottom end of a motor is. There is no industry standard for rating the KV of a given motor. I can trick a 17.5 motor to read 3900kv on my equipment, and can detune a 5.5 to read 2500kv. Timing, air gap, rotor size, rotor length, rotor chemistry, shimming, stator design, wire diameter, wire insulation, wire resistance, Input voltage, ambient temperature, and humidity are all variables in motor KV.
ANY wind motor can be setup to be aggressive, and likewise, can be setup to feel smooth.
My recommendation: Find the company you think offers a product you are interested in, contact them, and ask for recommendations for your particular application and needs.
I suggest a 6.5 motor in a 4wd buggy, or a 7.5 if you run on a tight slick track. Less is more on low bite tracks... "
"The smaller the rotor, the more air gap it will have between the rotor and stator. The bigger the air gap, the less torque the motor will have. Also, smaller rotor means less magnet. Less magnet equals less torque.
KV rating in RC Car sensored systems is irrelevant and is a means of falsely marketing the power output capabilities of a motor. Lower KV motors do not have smoother acceleration because of their KV ratings. The rotor is what dictates the feel of the motor. A 17.5 turn motor has many times more torque than does a 5.5 turn motor. However, to get the motors to run optimally, we run MUCH different gear ratios... 17.5 motors have lots of torque, so we gear them for Speed. Modified motors have minimal torque, but excellent top RPM, so we gear them for Torque.
Modified motors typically have a 12.5mm rotor on a 5mm shaft. This = 7.5mm of magnet on the rotor. This is done to make up for the torque lacking in the lower wind motors.
Spec (17.5, 13.5, 10.5) motors typically have a 12.3mm rotor on a 7.1mm shaft. This is 5.2mm of magnet. This is done to move the powerband of the motor more towards the RPM side, because of the extra torque created in a higher wind motor.
The power is in the rotor..... Modified rotors have 45% more magnet than SPEC rotors.
KV rating has absolutely nothing to do with how aggresive the bottom end of a motor is. There is no industry standard for rating the KV of a given motor. I can trick a 17.5 motor to read 3900kv on my equipment, and can detune a 5.5 to read 2500kv. Timing, air gap, rotor size, rotor length, rotor chemistry, shimming, stator design, wire diameter, wire insulation, wire resistance, Input voltage, ambient temperature, and humidity are all variables in motor KV.
ANY wind motor can be setup to be aggressive, and likewise, can be setup to feel smooth.
My recommendation: Find the company you think offers a product you are interested in, contact them, and ask for recommendations for your particular application and needs.
I suggest a 6.5 motor in a 4wd buggy, or a 7.5 if you run on a tight slick track. Less is more on low bite tracks... "