A Lower (vs higher) Kv Motor Produces More Torque
It is a common misconception that a lower Kv motor automatically produces more torque then a higher Kv motor in the same size class. What we do know is that the Kv constant of the motor is directly related to the torque constant of the motor, Kt. The relationship of Kt is the inverse of Kv. The lower a motors Kv value, the higher its Kt value will be. The torque constant of the motor tells us the amount of torque output per amp that we can get out of the motor.
Lower Kv motors do have a higher torque constant. However, by nature they are not able to sustain the same amount of current output as a higher Kv motor. Therefore a higher Kv motor can actually develop the exact same amount of torque as a lower Kv motor just by pulling more current.
Let’s take a look at an example.
Motor A – 1200Kv, 60A Max Current, Kt = 7.974 mNm/A
Motor B – 1650Kv, 83A Max Current, Kt = 5.799 mNm/A
The Maximum amount of torque output for motor A will be the Kt value multiplied by the max current. Motor A will be able to deliver 60A x 7.974 mNm/A = 478 mNm. Maximum torque output for Motor B is 83A x 5.799 mNm/A = 481 mNm. The difference of 3 mNm is negligible and discovered through rounding error.
Other Considerations when using a lower Kv motor in place of a higher Kv motor
If you are comparing a high vs low Kv motor operating at the same input voltage, the lower Kv motor will have a harder time making the same power output. This is due to the maximum current being lower on the lower Kv motor. The way that you would be able to make up for the loss in max current is to increase the voltage.
Cogging in a Brushless Motor
Cogging of a brushless motor is commonly regarded as the relationship of the motor and ESC being out of sync when starting from zero RPM. However, this is not the definition of cogging.
Cogging is a result of the iron core in a brushless motor reacting with the permanent magnet within the motor. As you rotate a motors output shaft, you can feel the “steps” of the motor through one full revolution. These steps require a certain amount of torque to overcome the resistance. Once the resistance is overcome, the rotor begins to accelerate as the torque requirement swings in the opposite direction. This creates an unbalance of torque at low speeds. Luckily for us, we can not feel the unbalance or even really notice is when operating our RC vehicles at low speeds. The only disadvantage of cogging is that it does tend to make the initial ESC to motor synchronization process a bit more difficult. To eliminate cogging, one can simply use a slotless rotor. A slotless rotor does not utilize an iron core. Visit this link to learn more on how to improve cogging.
Voltage Destroys Electrical Components
Voltage does indeed destroy electrical components. This is definitely not the misconception here. If you simply have a 6 cell LiPo ESC and run 7 cells to it, surely it will have smoke pouring out of it in no time. This misconception comes from the idea of adding a cell to a power system and having a speed control or motor blow, thinking it is directly related to the voltage increase.
Consider an example where we use the same 6 cell ESC as above and we go from using 5s LiPo to 6s LiPo. We then discover that our motor burned out. What caused the issue? Well when adding a cell extra of voltage, we are expecting our motor to turn quicker as RPM output is equal to Kv x voltage. Since our motor has the potential to spin faster and our gearing nor other RC car specs has not changed, load has increased. We know load must increase as it will take more torque for the motor to maintain a higher output speed and top speed of the car. Torque is directly related to the amount of current that our motor will pull. Increasing the voltage of the motor without decreasing the load or gearing of the RC car, will increase current. Since current is exactly what contributes to an increase in heat, this is how our motor will overheat and burn out.
Surely, Voltage Will Contribute to Motor Burnout?
When you look at why a motor would fail electrically, it really only is because of current. Not voltage. Now, a motor can fail mechanically. A mechanical motor failure can occur in the following areas. The first being the bearings that support the shaft of the motor. Mechanically over spinning the bearings can lead to bearing failure. At the same time mechanically over spinning the rotor of the motor can lead to a very similar failure. In fact a rotor failure is usually catastrophic. All motors have a rated maximum RPM and that is based on the limiting factor between either the rotor and the bearings.
The maximum RPM can be broken down in to Kv and Voltage. If you increase the voltage high enough, you will over spin the motor causing mechanical damage. The formula to determine the maximum voltage of a motor is:
Vmax = Max Motor RPM / Kv
Cavitation in RC Boat Propellers
Woops, yep, this is a fourth. We will call this a bonus!
I hear this misconception all the time not for just RC boats but even for full scale boats as well. Cavitation is the general term used to describe slippage that occurs when the prop sucks in a bunch of air causing the engine or motor to unload. However, this is not what cavitation actually represents.
Cavitation occurs when a rapid change of pressure in a liquid leads to formation of small vapor pockets in places where the pressure is relatively low. When subjected to higher pressure, the vapor pockets collapse and can generate a large shockwave. This shockwave can lead to pits found on the surface of the propeller ultimately damaging the propeller.
Now the way to reduce cavitation is by simply introducing aeration. Aeration is the process where you introduce air in to the propeller as defined in the actual misconceptions definition above. Introduction of air in to the propeller will simply not allow low pressures to build eliminating the potential for cavitation to occur.
This means that in our RC boats utilizing a surface driven prop, we will never really see the potential for cavitation to occur. Our more significant problem is we get too much aeration and are unable to get the boat on to plane quickly. The simple solution here is to move up to a larger propeller. Just make sure the larger propeller does not place too much stress on to your power system.