Torque Vs. speed

[menace]

I know in brushed motors that the faster you go, the less torque the motor can develop. Just wondering if the same relationship holds in brushless motors? Might install a novak HV pro 4.5 in my summit versus the 6.5 thats in it now--just curious what the differences will be...

[honcho1.9]

From what I understand its the same situation, think of like this, a 17.5t brushless has comparable torque to a 27t brushed, so your 4.5t brushless motor will have the torque of a 10 - 12t brushed motor, thats how my dad explained it to me years ago

[skinner]

As quoted above I believe true...my low turn or high kv brushless motors are faster and less torque than say a high turn/low kv motor....

[JohnRobHolmes]

It doesn't really hold the same for brushless, if we ignore battery and ESC resistance. To say that a 17.5 has more torque than a 10.5 is not true. The slower motor does have a higher torque constant (Kt), so it does produce more torque per amp. The 10.5 will actually produce more power and torque on a given voltage if you are not saturating the motor or overamping the battery, and you can use the extra rpm to gain even more torque at the wheel if you gear down further. Since the faster motor also has less resistance, it can pull the amperage needed to produce the same torque as compared to the slower motor. Drive both motors the same speed and you will basically have the same torque and power curves for both, ignoring copper density changes or bad designs like a 1.5 turn. In reality there are certain combinations that will have better performance because of discreet battery voltages, wire resistance, ESC losses, and other factors I probably don't even know. Until you need that last % of power, it doesn't really matter.

Brushed motors reach a point of current saturation in the brushes and commutator before the magnets or rotors saturate. The first order effect is that amperage is limited. The second order effect is that torque is limited, because torque is produced with amperage (Kt x A = torque). The third order effect is that higher KV brushed motors produce less torque beyond a certain point, as they need more amperage. The unchanging resistance of the brush to commutator interface does not allow for the higher amperage to compensate for the lower Kt value. Basically, either volting a brushed motor too high or winding it too fast will make you start hitting a brick wall with torque while you use RPM to gain extra power.